From: Kefu Chai <kchai@redhat.com>
Date: Tue, 15 May 2018 15:55:09 +0000 (+0800)
Subject: mds: use const_iterator for decode
X-Git-Tag: v14.0.0~97^2~1
X-Git-Url: http://git.apps.os.sepia.ceph.com/?a=commitdiff_plain;h=94e4928f178ea67f8a733187df26d7364438d5dd;p=ceph-ci.git

mds: use const_iterator for decode

Signed-off-by: Kefu Chai <kchai@redhat.com>
---

diff --git a/src/mds/Anchor.cc b/src/mds/Anchor.cc
index 99f1504ea74..02cc2d2b46c 100644
--- a/src/mds/Anchor.cc
+++ b/src/mds/Anchor.cc
@@ -26,7 +26,7 @@ void Anchor::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void Anchor::decode(bufferlist::iterator &bl)
+void Anchor::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START(1, bl);
   decode(ino, bl);
diff --git a/src/mds/Anchor.h b/src/mds/Anchor.h
index eb2d65b18e2..49b592b9f0e 100644
--- a/src/mds/Anchor.h
+++ b/src/mds/Anchor.h
@@ -40,7 +40,7 @@ public:
     ino(i), dirino(di), d_name(str), d_type(tp) {}
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<Anchor*>& ls);
 };
diff --git a/src/mds/CDentry.cc b/src/mds/CDentry.cc
index ce409c5f131..f21e5b2f83c 100644
--- a/src/mds/CDentry.cc
+++ b/src/mds/CDentry.cc
@@ -400,7 +400,7 @@ bool CDentry::is_freezing() const
   return dir->is_freezing();
 }
 
-void CDentry::decode_replica(bufferlist::iterator& p, bool is_new)
+void CDentry::decode_replica(bufferlist::const_iterator& p, bool is_new)
 {
   __u32 nonce;
   decode(nonce, p);
@@ -459,7 +459,7 @@ void CDentry::encode_lock_state(int type, bufferlist& bl)
 
 void CDentry::decode_lock_state(int type, bufferlist& bl)
 {  
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
 
   snapid_t newfirst;
   decode(newfirst, p);
diff --git a/src/mds/CDentry.h b/src/mds/CDentry.h
index 5de3db1b59f..c417bb261ac 100644
--- a/src/mds/CDentry.h
+++ b/src/mds/CDentry.h
@@ -256,7 +256,7 @@ public:
     lock.encode_state_for_replica(bl);
     encode(need_recover, bl);
   }
-  void decode_replica(bufferlist::iterator& p, bool is_new);
+  void decode_replica(bufferlist::const_iterator& p, bool is_new);
 
   // -- exporting
   // note: this assumes the dentry already exists.  
@@ -282,7 +282,7 @@ public:
   void abort_export() {
     put(PIN_TEMPEXPORTING);
   }
-  void decode_import(bufferlist::iterator& blp, LogSegment *ls) {
+  void decode_import(bufferlist::const_iterator& blp, LogSegment *ls) {
     decode(first, blp);
     __u32 nstate;
     decode(nstate, blp);
diff --git a/src/mds/CDir.cc b/src/mds/CDir.cc
index e6796ee37c8..fc16586675a 100644
--- a/src/mds/CDir.cc
+++ b/src/mds/CDir.cc
@@ -1696,7 +1696,7 @@ CDentry *CDir::_load_dentry(
     const std::set<snapid_t> *snaps,
     bool *force_dirty)
 {
-  bufferlist::iterator q = bl.begin();
+  auto q = bl.cbegin();
 
   snapid_t first;
   decode(first, q);
@@ -1909,7 +1909,7 @@ void CDir::_omap_fetched(bufferlist& hdrbl, map<string, bufferlist>& omap,
 
   fnode_t got_fnode;
   {
-    bufferlist::iterator p = hdrbl.begin();
+    auto p = hdrbl.cbegin();
     try {
       decode(got_fnode, p);
     } catch (const buffer::error &err) {
@@ -2512,7 +2512,7 @@ void CDir::finish_export(utime_t now)
   dirty_old_rstat.clear();
 }
 
-void CDir::decode_import(bufferlist::iterator& blp, utime_t now, LogSegment *ls)
+void CDir::decode_import(bufferlist::const_iterator& blp, utime_t now, LogSegment *ls)
 {
   decode(first, blp);
   decode(fnode, blp);
diff --git a/src/mds/CDir.h b/src/mds/CDir.h
index 5739e907ba5..2691bf174fa 100644
--- a/src/mds/CDir.h
+++ b/src/mds/CDir.h
@@ -573,7 +573,7 @@ private:
     encode(dir_rep, bl);
     encode(dir_rep_by, bl);
   }
-  void _decode_base(bufferlist::iterator& p) {
+  void _decode_base(bufferlist::const_iterator& p) {
     decode(first, p);
     decode(fnode, p);
     decode(dir_rep, p);
@@ -584,7 +584,7 @@ private:
     encode(nonce, bl);
     _encode_base(bl);
   }
-  void decode_replica(bufferlist::iterator& p) {
+  void decode_replica(bufferlist::const_iterator& p) {
     __u32 nonce;
     decode(nonce, p);
     replica_nonce = nonce;
@@ -704,7 +704,7 @@ public:
   void abort_export() {
     put(PIN_TEMPEXPORTING);
   }
-  void decode_import(bufferlist::iterator& blp, utime_t now, LogSegment *ls);
+  void decode_import(bufferlist::const_iterator& blp, utime_t now, LogSegment *ls);
   void abort_import(utime_t now);
 
   // -- auth pins --
diff --git a/src/mds/CInode.cc b/src/mds/CInode.cc
index 4b9379cc781..cd763927738 100644
--- a/src/mds/CInode.cc
+++ b/src/mds/CInode.cc
@@ -1174,11 +1174,11 @@ void CInode::fetch(MDSInternalContextBase *fin)
 void CInode::_fetched(bufferlist& bl, bufferlist& bl2, Context *fin)
 {
   dout(10) << __func__ << " got " << bl.length() << " and " << bl2.length() << dendl;
-  bufferlist::iterator p;
+  bufferlist::const_iterator p;
   if (bl2.length()) {
-    p = bl2.begin();
+    p = bl2.cbegin();
   } else if (bl.length()) {
-    p = bl.begin();
+    p = bl.cbegin();
   } else {
     derr << "No data while reading inode " << ino() << dendl;
     fin->complete(-ENOENT);
@@ -1442,7 +1442,7 @@ void CInode::encode_store(bufferlist& bl, uint64_t features)
 			 &snap_blob);
 }
 
-void InodeStoreBase::decode_bare(bufferlist::iterator &bl,
+void InodeStoreBase::decode_bare(bufferlist::const_iterator &bl,
 			      bufferlist& snap_blob, __u8 struct_v)
 {
   using ceph::decode;
@@ -1480,14 +1480,14 @@ void InodeStoreBase::decode_bare(bufferlist::iterator &bl,
 }
 
 
-void InodeStoreBase::decode(bufferlist::iterator &bl, bufferlist& snap_blob)
+void InodeStoreBase::decode(bufferlist::const_iterator &bl, bufferlist& snap_blob)
 {
   DECODE_START_LEGACY_COMPAT_LEN(5, 4, 4, bl);
   decode_bare(bl, snap_blob, struct_v);
   DECODE_FINISH(bl);
 }
 
-void CInode::decode_store(bufferlist::iterator& bl)
+void CInode::decode_store(bufferlist::const_iterator& bl)
 {
   bufferlist snap_blob;
   InodeStoreBase::decode(bl, snap_blob);
@@ -1668,7 +1668,7 @@ void CInode::encode_lock_state(int type, bufferlist& bl)
 
 void CInode::decode_lock_state(int type, bufferlist& bl)
 {
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
   utime_t tm;
 
   snapid_t newfirst;
@@ -2789,7 +2789,7 @@ void CInode::decode_snap_blob(bufferlist& snapbl)
   if (snapbl.length()) {
     open_snaprealm();
     auto old_flags = snaprealm->srnode.flags;
-    bufferlist::iterator p = snapbl.begin();
+    auto p = snapbl.cbegin();
     decode(snaprealm->srnode, p);
     if (is_base()) {
       bool ok = snaprealm->_open_parents(NULL);
@@ -2816,7 +2816,7 @@ void CInode::encode_snap(bufferlist& bl)
   encode(oldest_snap, bl);
 }
 
-void CInode::decode_snap(bufferlist::iterator& p)
+void CInode::decode_snap(bufferlist::const_iterator& p)
 {
   using ceph::decode;
   bufferlist snapbl;
@@ -3686,7 +3686,7 @@ void CInode::_encode_base(bufferlist& bl, uint64_t features)
   encode(damage_flags, bl);
   encode_snap(bl);
 }
-void CInode::_decode_base(bufferlist::iterator& p)
+void CInode::_decode_base(bufferlist::const_iterator& p)
 {
   using ceph::decode;
   decode(first, p);
@@ -3718,7 +3718,7 @@ void CInode::_encode_locks_full(bufferlist& bl)
 
   encode(loner_cap, bl);
 }
-void CInode::_decode_locks_full(bufferlist::iterator& p)
+void CInode::_decode_locks_full(bufferlist::const_iterator& p)
 {
   using ceph::decode;
   decode(authlock, p);
@@ -3764,7 +3764,7 @@ void CInode::_encode_locks_state_for_rejoin(bufferlist& bl, int rep)
   policylock.encode_state_for_replica(bl);
 }
 
-void CInode::_decode_locks_state(bufferlist::iterator& p, bool is_new)
+void CInode::_decode_locks_state(bufferlist::const_iterator& p, bool is_new)
 {
   authlock.decode_state(p, is_new);
   linklock.decode_state(p, is_new);
@@ -3793,7 +3793,7 @@ void CInode::_decode_locks_state(bufferlist::iterator& p, bool is_new)
     policylock.mark_need_recover();
   }
 }
-void CInode::_decode_locks_rejoin(bufferlist::iterator& p, list<MDSInternalContextBase*>& waiters,
+void CInode::_decode_locks_rejoin(bufferlist::const_iterator& p, list<MDSInternalContextBase*>& waiters,
 				  list<SimpleLock*>& eval_locks, bool survivor)
 {
   authlock.decode_state_rejoin(p, waiters, survivor);
@@ -3867,7 +3867,7 @@ void CInode::finish_export(utime_t now)
   put(PIN_TEMPEXPORTING);
 }
 
-void CInode::decode_import(bufferlist::iterator& p,
+void CInode::decode_import(bufferlist::const_iterator& p,
 			   LogSegment *ls)
 {
   DECODE_START(5, p);
@@ -3897,7 +3897,7 @@ void CInode::decode_import(bufferlist::iterator& p,
   // decode fragstat info on bounding cdirs
   bufferlist bounding;
   decode(bounding, p);
-  bufferlist::iterator q = bounding.begin();
+  auto q = bounding.cbegin();
   while (!q.end()) {
     frag_t fg;
     decode(fg, q);
@@ -4101,7 +4101,7 @@ void CInode::validate_disk_state(CInode::validated_data *results,
 
       // extract the backtrace, and compare it to a newly-constructed one
       try {
-        bufferlist::iterator p = bl.begin();
+        auto p = bl.cbegin();
 	using ceph::decode;
         decode(results->backtrace.ondisk_value, p);
         dout(10) << "decoded " << bl.length() << " bytes of backtrace successfully" << dendl;
diff --git a/src/mds/CInode.h b/src/mds/CInode.h
index 4e067f01954..99308d1b6ce 100644
--- a/src/mds/CInode.h
+++ b/src/mds/CInode.h
@@ -98,11 +98,11 @@ public:
 
   /* Full serialization for use in ".inode" root inode objects */
   void encode(bufferlist &bl, uint64_t features, const bufferlist *snap_blob=NULL) const;
-  void decode(bufferlist::iterator &bl, bufferlist& snap_blob);
+  void decode(bufferlist::const_iterator &bl, bufferlist& snap_blob);
 
   /* Serialization without ENCODE_START/FINISH blocks for use embedded in dentry */
   void encode_bare(bufferlist &bl, uint64_t features, const bufferlist *snap_blob=NULL) const;
-  void decode_bare(bufferlist::iterator &bl, bufferlist &snap_blob, __u8 struct_v=5);
+  void decode_bare(bufferlist::const_iterator &bl, bufferlist &snap_blob, __u8 struct_v=5);
 
   /* For test/debug output */
   void dump(Formatter *f) const;
@@ -120,13 +120,13 @@ public:
   void encode(bufferlist &bl, uint64_t features) const {
     InodeStoreBase::encode(bl, features, &snap_blob);
   }
-  void decode(bufferlist::iterator &bl) {
+  void decode(bufferlist::const_iterator &bl) {
     InodeStoreBase::decode(bl, snap_blob);
   }
   void encode_bare(bufferlist &bl, uint64_t features) const {
     InodeStoreBase::encode_bare(bl, features, &snap_blob);
   }
-  void decode_bare(bufferlist::iterator &bl) {
+  void decode_bare(bufferlist::const_iterator &bl) {
     InodeStoreBase::decode_bare(bl, snap_blob);
   }
 
@@ -140,7 +140,7 @@ public:
   void encode(bufferlist &bl, uint64_t features) const {
     InodeStore::encode_bare(bl, features);
   }
-  void decode(bufferlist::iterator &bl) {
+  void decode(bufferlist::const_iterator &bl) {
     InodeStore::decode_bare(bl);
   }
   static void generate_test_instances(std::list<InodeStoreBare*>& ls);
@@ -633,7 +633,7 @@ protected:
     if (has_flock_locks)
       encode(*flock_locks, bl);
   }
-  void _decode_file_locks(bufferlist::iterator& p) {
+  void _decode_file_locks(bufferlist::const_iterator& p) {
     using ceph::decode;
     bool has_fcntl_locks;
     decode(has_fcntl_locks, p);
@@ -816,7 +816,7 @@ public:
   void encode_snap_blob(bufferlist &bl);
   void decode_snap_blob(bufferlist &bl);
   void encode_store(bufferlist& bl, uint64_t features);
-  void decode_store(bufferlist::iterator& bl);
+  void decode_store(bufferlist::const_iterator& bl);
 
   void encode_replica(mds_rank_t rep, bufferlist& bl, uint64_t features, bool need_recover) {
     assert(is_auth());
@@ -832,7 +832,7 @@ public:
     _encode_base(bl, features);
     _encode_locks_state_for_replica(bl, need_recover);
   }
-  void decode_replica(bufferlist::iterator& p, bool is_new) {
+  void decode_replica(bufferlist::const_iterator& p, bool is_new) {
     using ceph::decode;
     __u32 nonce;
     decode(nonce, p);
@@ -856,13 +856,13 @@ public:
 
   // -- encode/decode helpers --
   void _encode_base(bufferlist& bl, uint64_t features);
-  void _decode_base(bufferlist::iterator& p);
+  void _decode_base(bufferlist::const_iterator& p);
   void _encode_locks_full(bufferlist& bl);
-  void _decode_locks_full(bufferlist::iterator& p);
+  void _decode_locks_full(bufferlist::const_iterator& p);
   void _encode_locks_state_for_replica(bufferlist& bl, bool need_recover);
   void _encode_locks_state_for_rejoin(bufferlist& bl, int rep);
-  void _decode_locks_state(bufferlist::iterator& p, bool is_new);
-  void _decode_locks_rejoin(bufferlist::iterator& p, std::list<MDSInternalContextBase*>& waiters,
+  void _decode_locks_state(bufferlist::const_iterator& p, bool is_new);
+  void _decode_locks_rejoin(bufferlist::const_iterator& p, std::list<MDSInternalContextBase*>& waiters,
 			    std::list<SimpleLock*>& eval_locks, bool survivor);
 
   // -- import/export --
@@ -874,7 +874,7 @@ public:
     state_clear(STATE_EXPORTINGCAPS);
     put(PIN_EXPORTINGCAPS);
   }
-  void decode_import(bufferlist::iterator& p, LogSegment *ls);
+  void decode_import(bufferlist::const_iterator& p, LogSegment *ls);
   
 
   // for giving to clients
@@ -945,7 +945,7 @@ public:
   void close_snaprealm(bool no_join=false);
   SnapRealm *find_snaprealm() const;
   void encode_snap(bufferlist& bl);
-  void decode_snap(bufferlist::iterator& p);
+  void decode_snap(bufferlist::const_iterator& p);
 
   // -- caps -- (new)
   // client caps
diff --git a/src/mds/Capability.cc b/src/mds/Capability.cc
index 3cd408937b3..3d7c498bc52 100644
--- a/src/mds/Capability.cc
+++ b/src/mds/Capability.cc
@@ -36,7 +36,7 @@ void Capability::Export::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void Capability::Export::decode(bufferlist::iterator &p)
+void Capability::Export::decode(bufferlist::const_iterator &p)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, p);
   decode(cap_id, p);
@@ -83,7 +83,7 @@ void Capability::Import::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void Capability::Import::decode(bufferlist::iterator &bl)
+void Capability::Import::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START(1, bl);
   decode(cap_id, bl);
@@ -112,7 +112,7 @@ void Capability::revoke_info::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void Capability::revoke_info::decode(bufferlist::iterator& bl)
+void Capability::revoke_info::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(before, bl);
@@ -165,7 +165,7 @@ void Capability::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void Capability::decode(bufferlist::iterator &bl)
+void Capability::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl)
   decode(last_sent, bl);
diff --git a/src/mds/Capability.h b/src/mds/Capability.h
index 4f5d7a9848f..690dd1ebdc4 100644
--- a/src/mds/Capability.h
+++ b/src/mds/Capability.h
@@ -84,7 +84,7 @@ public:
       cap_id(id), wanted(w), issued(i), pending(p), client_follows(cf),
       seq(s), mseq(m), last_issue_stamp(lis) {}
     void encode(bufferlist &bl) const;
-    void decode(bufferlist::iterator &p);
+    void decode(bufferlist::const_iterator &p);
     void dump(Formatter *f) const;
     static void generate_test_instances(list<Export*>& ls);
   };
@@ -95,7 +95,7 @@ public:
     Import() : cap_id(0), issue_seq(0), mseq(0) {}
     Import(int64_t i, ceph_seq_t s, ceph_seq_t m) : cap_id(i), issue_seq(s), mseq(m) {}
     void encode(bufferlist &bl) const;
-    void decode(bufferlist::iterator &p);
+    void decode(bufferlist::const_iterator &p);
     void dump(Formatter *f) const;
   };
   struct revoke_info {
@@ -104,7 +104,7 @@ public:
     revoke_info() : before(0), seq(0), last_issue(0) {}
     revoke_info(__u32 b, ceph_seq_t s, ceph_seq_t li) : before(b), seq(s), last_issue(li) {}
     void encode(bufferlist& bl) const;
-    void decode(bufferlist::iterator& bl);
+    void decode(bufferlist::const_iterator& bl);
     void dump(Formatter *f) const;
     static void generate_test_instances(list<revoke_info*>& ls);
   };
@@ -316,7 +316,7 @@ public:
 
   // serializers
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<Capability*>& ls);
   
diff --git a/src/mds/FSMap.cc b/src/mds/FSMap.cc
index 10ef3fae65f..9923392fc53 100644
--- a/src/mds/FSMap.cc
+++ b/src/mds/FSMap.cc
@@ -424,7 +424,7 @@ void FSMap::encode(bufferlist& bl, uint64_t features) const
   }
 }
 
-void FSMap::decode(bufferlist::iterator& p)
+void FSMap::decode(bufferlist::const_iterator& p)
 {
   // The highest MDSMap encoding version before we changed the
   // MDSMonitor to store an FSMap instead of an MDSMap was
@@ -627,13 +627,13 @@ void Filesystem::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void Filesystem::decode(bufferlist::iterator& p)
+void Filesystem::decode(bufferlist::const_iterator& p)
 {
   DECODE_START(1, p);
   decode(fscid, p);
   bufferlist mdsmap_bl;
   decode(mdsmap_bl, p);
-  bufferlist::iterator mdsmap_bl_iter = mdsmap_bl.begin();
+  auto mdsmap_bl_iter = mdsmap_bl.cbegin();
   mds_map.decode(mdsmap_bl_iter);
   DECODE_FINISH(p);
 }
diff --git a/src/mds/FSMap.h b/src/mds/FSMap.h
index 6450edecf1f..050ed114b50 100644
--- a/src/mds/FSMap.h
+++ b/src/mds/FSMap.h
@@ -54,7 +54,7 @@ public:
   }
 
   void encode(bufferlist& bl, uint64_t features) const;
-  void decode(bufferlist::iterator& p);
+  void decode(bufferlist::const_iterator& p);
 
   void dump(Formatter *f) const;
   void print(std::ostream& out) const;
@@ -484,9 +484,9 @@ public:
   void sanity() const;
 
   void encode(bufferlist& bl, uint64_t features) const;
-  void decode(bufferlist::iterator& p);
+  void decode(bufferlist::const_iterator& p);
   void decode(bufferlist& bl) {
-    bufferlist::iterator p = bl.begin();
+    auto p = bl.cbegin();
     decode(p);
   }
   void sanitize(const std::function<bool(int64_t pool)>& pool_exists);
diff --git a/src/mds/FSMapUser.cc b/src/mds/FSMapUser.cc
index 7dd5afe585b..47d5f19c76b 100644
--- a/src/mds/FSMapUser.cc
+++ b/src/mds/FSMapUser.cc
@@ -12,7 +12,7 @@ void FSMapUser::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void FSMapUser::decode(bufferlist::iterator& p)
+void FSMapUser::decode(bufferlist::const_iterator& p)
 {
   DECODE_START(1, p);
   decode(epoch, p);
@@ -33,7 +33,7 @@ void FSMapUser::fs_info_t::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void FSMapUser::fs_info_t::decode(bufferlist::iterator& p)
+void FSMapUser::fs_info_t::decode(bufferlist::const_iterator& p)
 {
   DECODE_START(1, p);
   decode(cid, p);
diff --git a/src/mds/FSMapUser.h b/src/mds/FSMapUser.h
index 2ae0792ffd0..23af84731a6 100644
--- a/src/mds/FSMapUser.h
+++ b/src/mds/FSMapUser.h
@@ -27,7 +27,7 @@ public:
     std::string name;
     fs_info_t() : cid(FS_CLUSTER_ID_NONE) {}
     void encode(bufferlist& bl, uint64_t features) const;
-    void decode(bufferlist::iterator &bl);
+    void decode(bufferlist::const_iterator &bl);
   };
 
   epoch_t epoch;
@@ -48,7 +48,7 @@ public:
   }
 
   void encode(bufferlist& bl, uint64_t features) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
 
   void print(ostream& out) const;
   void print_summary(Formatter *f, ostream *out);
diff --git a/src/mds/InoTable.h b/src/mds/InoTable.h
index 431bc1121a5..cc228797ae3 100644
--- a/src/mds/InoTable.h
+++ b/src/mds/InoTable.h
@@ -53,7 +53,7 @@ class InoTable : public MDSTable {
     encode(free, bl);
     ENCODE_FINISH(bl);
   }
-  void decode_state(bufferlist::iterator& bl) override {
+  void decode_state(bufferlist::const_iterator& bl) override {
     DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
     decode(free, bl);
     projected_free = free;
@@ -65,7 +65,7 @@ class InoTable : public MDSTable {
   void encode(bufferlist& bl) const {
     encode_state(bl);
   }
-  void decode(bufferlist::iterator& bl) {
+  void decode(bufferlist::const_iterator& bl) {
     decode_state(bl);
   }
   void dump(Formatter *f) const;
diff --git a/src/mds/JournalPointer.cc b/src/mds/JournalPointer.cc
index fdf6a3d9f60..64d8620f285 100644
--- a/src/mds/JournalPointer.cc
+++ b/src/mds/JournalPointer.cc
@@ -56,7 +56,7 @@ int JournalPointer::load(Objecter *objecter)
 
   // Construct JournalPointer result, null or decoded data
   if (r == 0) {
-    bufferlist::iterator q = data.begin();
+    auto q = data.cbegin();
     try {
       decode(q);
     } catch (const buffer::error &e) {
diff --git a/src/mds/JournalPointer.h b/src/mds/JournalPointer.h
index b81ee54c26e..0f423266054 100644
--- a/src/mds/JournalPointer.h
+++ b/src/mds/JournalPointer.h
@@ -45,7 +45,7 @@ class JournalPointer {
     ENCODE_FINISH(bl);
   }
 
-  void decode(bufferlist::iterator &bl) {
+  void decode(bufferlist::const_iterator &bl) {
     DECODE_START(1, bl);
     decode(front, bl);
     decode(back, bl);
diff --git a/src/mds/Locker.cc b/src/mds/Locker.cc
index e8955697ff1..e0df6018f59 100644
--- a/src/mds/Locker.cc
+++ b/src/mds/Locker.cc
@@ -3154,7 +3154,7 @@ void Locker::_do_snap_update(CInode *in, snapid_t snap, int dirty, snapid_t foll
     dout(7) << " xattrs v" << i->xattr_version << " -> " << m->head.xattr_version
 	    << " len " << m->xattrbl.length() << dendl;
     i->xattr_version = m->head.xattr_version;
-    bufferlist::iterator p = m->xattrbl.begin();
+    auto p = m->xattrbl.cbegin();
     decode(*px, p);
   }
 
@@ -3361,7 +3361,7 @@ bool Locker::_do_cap_update(CInode *in, Capability *cap,
 
   if (m->flockbl.length()) {
     int32_t num_locks;
-    bufferlist::iterator bli = m->flockbl.begin();
+    auto bli = m->flockbl.cbegin();
     decode(num_locks, bli);
     for ( int i=0; i < num_locks; ++i) {
       ceph_filelock decoded_lock;
@@ -3429,7 +3429,7 @@ bool Locker::_do_cap_update(CInode *in, Capability *cap,
   if (xattr) {
     dout(7) << " xattrs v" << pi.inode.xattr_version << " -> " << m->head.xattr_version << dendl;
     pi.inode.xattr_version = m->head.xattr_version;
-    bufferlist::iterator p = m->xattrbl.begin();
+    auto p = m->xattrbl.cbegin();
     decode(*pi.xattrs, p);
     wrlock_force(&in->xattrlock, mut);
   }
diff --git a/src/mds/LogEvent.cc b/src/mds/LogEvent.cc
index 19e09049066..494aa112fab 100644
--- a/src/mds/LogEvent.cc
+++ b/src/mds/LogEvent.cc
@@ -44,7 +44,7 @@
 LogEvent *LogEvent::decode(bufferlist& bl)
 {
   // parse type, length
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
   EventType type;
   LogEvent *event = NULL;
   using ceph::decode;
@@ -126,7 +126,7 @@ LogEvent::EventType LogEvent::str_to_type(std::string_view str)
 }
 
 
-LogEvent *LogEvent::decode_event(bufferlist& bl, bufferlist::iterator& p, LogEvent::EventType type)
+LogEvent *LogEvent::decode_event(bufferlist& bl, bufferlist::const_iterator& p, LogEvent::EventType type)
 {
   int length = bl.length() - p.get_off();
   generic_dout(15) << "decode_log_event type " << type << ", size " << length << dendl;
diff --git a/src/mds/LogEvent.h b/src/mds/LogEvent.h
index 681a957e506..7ded7d7a14c 100644
--- a/src/mds/LogEvent.h
+++ b/src/mds/LogEvent.h
@@ -58,7 +58,7 @@ public:
 private:
   EventType _type;
   uint64_t _start_off;
-  static LogEvent *decode_event(bufferlist& bl, bufferlist::iterator& p, EventType type);
+  static LogEvent *decode_event(bufferlist& bl, bufferlist::const_iterator& p, EventType type);
 
 protected:
   utime_t stamp;
@@ -85,7 +85,7 @@ public:
 
   // encoding
   virtual void encode(bufferlist& bl, uint64_t features) const = 0;
-  virtual void decode(bufferlist::iterator &bl) = 0;
+  virtual void decode(bufferlist::const_iterator &bl) = 0;
   static LogEvent *decode(bufferlist &bl);
   virtual void dump(Formatter *f) const = 0;
 
diff --git a/src/mds/MDCache.cc b/src/mds/MDCache.cc
index a7e4d4b6731..4697278bfa9 100644
--- a/src/mds/MDCache.cc
+++ b/src/mds/MDCache.cc
@@ -3226,7 +3226,7 @@ void MDCache::handle_resolve(MMDSResolve *m)
 
 	  inodeno_t ino;
 	  map<client_t,Capability::Export> cap_exports;
-	  bufferlist::iterator q = p->second.inode_caps.begin();
+	  auto q = p->second.inode_caps.cbegin();
 	  decode(ino, q);
 	  decode(cap_exports, q);
 
@@ -5115,13 +5115,13 @@ void MDCache::handle_cache_rejoin_ack(MMDSCacheRejoin *ack)
        ++p) {
     CDir *dir = get_dirfrag(p->first);
     assert(dir);
-    bufferlist::iterator q = p->second.begin();
+    auto q = p->second.cbegin();
     dir->_decode_base(q);
     dout(10) << " got dir replica " << *dir << dendl;
   }
 
   // full inodes
-  bufferlist::iterator p = ack->inode_base.begin();
+  auto p = ack->inode_base.cbegin();
   while (!p.end()) {
     inodeno_t ino;
     snapid_t last;
@@ -5131,7 +5131,7 @@ void MDCache::handle_cache_rejoin_ack(MMDSCacheRejoin *ack)
     decode(basebl, p);
     CInode *in = get_inode(ino, last);
     assert(in);
-    bufferlist::iterator q = basebl.begin();
+    auto q = basebl.cbegin();
     snapid_t sseq = 0;
     if (in->snaprealm)
       sseq = in->snaprealm->srnode.seq;
@@ -5144,7 +5144,7 @@ void MDCache::handle_cache_rejoin_ack(MMDSCacheRejoin *ack)
   }
 
   // inodes
-  p = ack->inode_locks.begin();
+  p = ack->inode_locks.cbegin();
   //dout(10) << "inode_locks len " << ack->inode_locks.length() << " is " << ack->inode_locks << dendl;
   while (!p.end()) {
     inodeno_t ino;
@@ -5159,7 +5159,7 @@ void MDCache::handle_cache_rejoin_ack(MMDSCacheRejoin *ack)
     CInode *in = get_inode(ino, last);
     assert(in);
     in->set_replica_nonce(nonce);
-    bufferlist::iterator q = lockbl.begin();
+    auto q = lockbl.cbegin();
     in->_decode_locks_rejoin(q, rejoin_waiters, rejoin_eval_locks, survivor);
     in->state_clear(CInode::STATE_REJOINING);
     dout(10) << " got inode locks " << *in << dendl;
@@ -5170,7 +5170,7 @@ void MDCache::handle_cache_rejoin_ack(MMDSCacheRejoin *ack)
   assert(isolated_inodes.empty());
 
   map<inodeno_t,map<client_t,Capability::Import> > peer_imported;
-  bufferlist::iterator bp = ack->imported_caps.begin();
+  auto bp = ack->imported_caps.cbegin();
   decode(peer_imported, bp);
 
   for (map<inodeno_t,map<client_t,Capability::Import> >::iterator p = peer_imported.begin();
@@ -9673,7 +9673,7 @@ void MDCache::handle_snap_update(MMDSSnapUpdate *m)
     assert(!in->is_auth());
     if (mds->get_state() > MDSMap::STATE_REJOIN ||
 	(mds->is_rejoin() && !in->is_rejoining())) {
-      bufferlist::iterator p = m->snap_blob.begin();
+      auto p = m->snap_blob.cbegin();
       in->decode_snap(p);
 
       if (!notify_clients) {
@@ -10269,7 +10269,7 @@ void MDCache::handle_discover_reply(MDiscoverReply *m)
 
   // starting point
   CInode *cur = get_inode(m->get_base_ino());
-  bufferlist::iterator p = m->trace.begin();
+  auto p = m->trace.cbegin();
 
   int next = m->starts_with;
 
@@ -10431,7 +10431,7 @@ void MDCache::replicate_inode(CInode *in, mds_rank_t to, bufferlist& bl,
   in->encode_replica(to, bl, features, mds->get_state() < MDSMap::STATE_ACTIVE);
 }
 
-CDir *MDCache::add_replica_dir(bufferlist::iterator& p, CInode *diri, mds_rank_t from,
+CDir *MDCache::add_replica_dir(bufferlist::const_iterator& p, CInode *diri, mds_rank_t from,
 			       list<MDSInternalContextBase*>& finished)
 {
   dirfrag_t df;
@@ -10473,7 +10473,7 @@ CDir *MDCache::add_replica_dir(bufferlist::iterator& p, CInode *diri, mds_rank_t
   return dir;
 }
 
-CDentry *MDCache::add_replica_dentry(bufferlist::iterator& p, CDir *dir, list<MDSInternalContextBase*>& finished)
+CDentry *MDCache::add_replica_dentry(bufferlist::const_iterator& p, CDir *dir, list<MDSInternalContextBase*>& finished)
 {
   string name;
   snapid_t last;
@@ -10497,7 +10497,7 @@ CDentry *MDCache::add_replica_dentry(bufferlist::iterator& p, CDir *dir, list<MD
   return dn;
 }
 
-CInode *MDCache::add_replica_inode(bufferlist::iterator& p, CDentry *dn, list<MDSInternalContextBase*>& finished)
+CInode *MDCache::add_replica_inode(bufferlist::const_iterator& p, CDentry *dn, list<MDSInternalContextBase*>& finished)
 {
   inodeno_t ino;
   snapid_t last;
@@ -10545,7 +10545,7 @@ void MDCache::replicate_stray(CDentry *straydn, mds_rank_t who, bufferlist& bl)
 CDentry *MDCache::add_replica_stray(bufferlist &bl, mds_rank_t from)
 {
   list<MDSInternalContextBase*> finished;
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
 
   CInode *mdsin = add_replica_inode(p, NULL, finished);
   CDir *mdsdir = add_replica_dir(p, mdsin, from, finished);
@@ -10708,7 +10708,7 @@ void MDCache::handle_dentry_link(MDentryLink *m)
     }
   }
 
-  bufferlist::iterator p = m->bl.begin();
+  auto p = m->bl.cbegin();
   list<MDSInternalContextBase*> finished;
   if (dn) {
     if (m->get_is_primary()) {
@@ -11765,7 +11765,7 @@ void MDCache::handle_fragment_notify(MMDSFragmentNotify *notify)
       diri->take_dir_waiting((*p)->get_frag(), waiters);
 
     // add new replica dirs values
-    bufferlist::iterator p = notify->basebl.begin();
+    auto p = notify->basebl.cbegin();
     while (!p.end())
       add_replica_dir(p, diri, mds_rank_t(notify->get_source().num()), waiters);
 
@@ -11863,7 +11863,7 @@ void MDCache::rollback_uncommitted_fragments()
       list<MDSInternalContextBase*> waiters;
       adjust_dir_fragments(diri, p->first.frag, -uf.bits, resultfrags, waiters, true);
     } else {
-      bufferlist::iterator bp = uf.rollback.begin();
+      auto bp = uf.rollback.cbegin();
       for (list<frag_t>::iterator q = uf.old_frags.begin(); q != uf.old_frags.end(); ++q) {
 	CDir *dir = force_dir_fragment(diri, *q);
 	resultfrags.push_back(dir);
diff --git a/src/mds/MDCache.h b/src/mds/MDCache.h
index b952c7fe946..63f608f21da 100644
--- a/src/mds/MDCache.h
+++ b/src/mds/MDCache.h
@@ -1088,9 +1088,9 @@ public:
   void replicate_inode(CInode *in, mds_rank_t to, bufferlist& bl,
 		       uint64_t features);
   
-  CDir* add_replica_dir(bufferlist::iterator& p, CInode *diri, mds_rank_t from, list<MDSInternalContextBase*>& finished);
-  CDentry *add_replica_dentry(bufferlist::iterator& p, CDir *dir, list<MDSInternalContextBase*>& finished);
-  CInode *add_replica_inode(bufferlist::iterator& p, CDentry *dn, list<MDSInternalContextBase*>& finished);
+  CDir* add_replica_dir(bufferlist::const_iterator& p, CInode *diri, mds_rank_t from, list<MDSInternalContextBase*>& finished);
+  CDentry *add_replica_dentry(bufferlist::const_iterator& p, CDir *dir, list<MDSInternalContextBase*>& finished);
+  CInode *add_replica_inode(bufferlist::const_iterator& p, CDentry *dn, list<MDSInternalContextBase*>& finished);
 
   void replicate_stray(CDentry *straydn, mds_rank_t who, bufferlist& bl);
   CDentry *add_replica_stray(bufferlist &bl, mds_rank_t from);
diff --git a/src/mds/MDSDaemon.cc b/src/mds/MDSDaemon.cc
index f5c024a1089..eb74750329c 100644
--- a/src/mds/MDSDaemon.cc
+++ b/src/mds/MDSDaemon.cc
@@ -1385,7 +1385,7 @@ bool MDSDaemon::ms_verify_authorizer(Connection *con, int peer_type,
       // Flag for auth providers that don't provide cap strings
       s->auth_caps.set_allow_all();
     } else {
-      bufferlist::iterator p = caps_info.caps.begin();
+      auto p = caps_info.caps.cbegin();
       string auth_cap_str;
       try {
         decode(auth_cap_str, p);
diff --git a/src/mds/MDSMap.cc b/src/mds/MDSMap.cc
index b9b16d3e02d..fc12a739bea 100644
--- a/src/mds/MDSMap.cc
+++ b/src/mds/MDSMap.cc
@@ -544,7 +544,7 @@ void MDSMap::mds_info_t::encode_unversioned(bufferlist& bl) const
   encode(export_targets, bl);
 }
 
-void MDSMap::mds_info_t::decode(bufferlist::iterator& bl)
+void MDSMap::mds_info_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(7, 4, 4, bl);
   decode(global_id, bl);
@@ -710,7 +710,7 @@ void MDSMap::sanitize(const std::function<bool(int64_t pool)>& pool_exists)
   }
 }
 
-void MDSMap::decode(bufferlist::iterator& p)
+void MDSMap::decode(bufferlist::const_iterator& p)
 {
   std::map<mds_rank_t,int32_t> inc;  // Legacy field, parse and drop
 
diff --git a/src/mds/MDSMap.h b/src/mds/MDSMap.h
index 362f4b303e7..878f0e87a9a 100644
--- a/src/mds/MDSMap.h
+++ b/src/mds/MDSMap.h
@@ -155,7 +155,7 @@ public:
       if ((features & CEPH_FEATURE_MDSENC) == 0 ) encode_unversioned(bl);
       else encode_versioned(bl, features);
     }
-    void decode(bufferlist::iterator& p);
+    void decode(bufferlist::const_iterator& p);
     void dump(Formatter *f) const;
     void print_summary(ostream &out) const;
 
@@ -675,9 +675,9 @@ public:
     return -1;
   }
   void encode(bufferlist& bl, uint64_t features) const;
-  void decode(bufferlist::iterator& p);
+  void decode(bufferlist::const_iterator& p);
   void decode(bufferlist& bl) {
-    bufferlist::iterator p = bl.begin();
+    auto p = bl.cbegin();
     decode(p);
   }
   void sanitize(const std::function<bool(int64_t pool)>& pool_exists);
diff --git a/src/mds/MDSTable.cc b/src/mds/MDSTable.cc
index be12dbfc0d6..81d140f1a29 100644
--- a/src/mds/MDSTable.cc
+++ b/src/mds/MDSTable.cc
@@ -173,7 +173,7 @@ void MDSTable::load_2(int r, bufferlist& bl, Context *onfinish)
   }
 
   dout(10) << "load_2 got " << bl.length() << " bytes" << dendl;
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
 
   try {
     decode(version, p);
diff --git a/src/mds/MDSTable.h b/src/mds/MDSTable.h
index 13e9bf55f96..52221102c8b 100644
--- a/src/mds/MDSTable.h
+++ b/src/mds/MDSTable.h
@@ -85,7 +85,7 @@ public:
 
   // child must overload these
   virtual void reset_state() = 0;
-  virtual void decode_state(bufferlist::iterator& p) = 0;
+  virtual void decode_state(bufferlist::const_iterator& p) = 0;
   virtual void encode_state(bufferlist& bl) const = 0;
 };
 
diff --git a/src/mds/MDSTableServer.h b/src/mds/MDSTableServer.h
index a9edead457c..5ace13f93ce 100644
--- a/src/mds/MDSTableServer.h
+++ b/src/mds/MDSTableServer.h
@@ -97,13 +97,13 @@ public:
   void do_server_update(bufferlist& bl);
 
   virtual void encode_server_state(bufferlist& bl) const = 0;
-  virtual void decode_server_state(bufferlist::iterator& bl) = 0;
+  virtual void decode_server_state(bufferlist::const_iterator& bl) = 0;
 
   void encode_state(bufferlist& bl) const override {
     encode_server_state(bl);
     encode(pending_for_mds, bl);
   }
-  void decode_state(bufferlist::iterator& bl) override {
+  void decode_state(bufferlist::const_iterator& bl) override {
     decode_server_state(bl);
     decode(pending_for_mds, bl);
   }
diff --git a/src/mds/Migrator.cc b/src/mds/Migrator.cc
index 3c6019d972e..6de08525790 100644
--- a/src/mds/Migrator.cc
+++ b/src/mds/Migrator.cc
@@ -1745,7 +1745,7 @@ void Migrator::handle_export_ack(MExportDirAck *m)
   assert(it->second.state == EXPORT_EXPORTING);
   assert(it->second.tid == m->get_tid());
 
-  bufferlist::iterator bp = m->imported_caps.begin();
+  auto bp = m->imported_caps.cbegin();
   decode(it->second.peer_imported, bp);
 
   it->second.state = EXPORT_LOGGINGFINISH;
@@ -2252,7 +2252,7 @@ void Migrator::handle_export_prep(MExportDirPrep *m)
     assert(it->second.peer == oldauth);
     diri = cache->get_inode(m->get_dirfrag().ino);
     assert(diri);
-    bufferlist::iterator p = m->basedir.begin();
+    auto p = m->basedir.cbegin();
     dir = cache->add_replica_dir(p, diri, oldauth, finished);
     dout(7) << "handle_export_prep on " << *dir << " (first pass)" << dendl;
   } else {
@@ -2308,7 +2308,7 @@ void Migrator::handle_export_prep(MExportDirPrep *m)
     for (list<bufferlist>::iterator p = m->traces.begin();
 	 p != m->traces.end();
 	 ++p) {
-      bufferlist::iterator q = p->begin();
+      auto q = p->cbegin();
       dirfrag_t df;
       decode(df, q);
       char start;
@@ -2495,14 +2495,14 @@ void Migrator::handle_export_dir(MExportDir *m)
 
   // new client sessions, open these after we journal
   // include imported sessions in EImportStart
-  bufferlist::iterator cmp = m->client_map.begin();
+  auto cmp = m->client_map.cbegin();
   map<client_t,entity_inst_t> client_map;
   decode(client_map, cmp);
   assert(cmp.end());
   le->cmapv = mds->server->prepare_force_open_sessions(client_map, onlogged->imported_session_map);
   encode(client_map, le->client_map, mds->mdsmap->get_up_features());
 
-  bufferlist::iterator blp = m->export_data.begin();
+  auto blp = m->export_data.cbegin();
   int num_imported_inodes = 0;
   while (!blp.end()) {
     num_imported_inodes += 
@@ -2981,7 +2981,7 @@ void Migrator::import_finish(CDir *dir, bool notify, bool last)
 }
 
 
-void Migrator::decode_import_inode(CDentry *dn, bufferlist::iterator& blp,
+void Migrator::decode_import_inode(CDentry *dn, bufferlist::const_iterator& blp,
 				   mds_rank_t oldauth, LogSegment *ls,
 				   map<CInode*, map<client_t,Capability::Export> >& peer_exports,
 				   list<ScatterLock*>& updated_scatterlocks)
@@ -3050,7 +3050,7 @@ void Migrator::decode_import_inode(CDentry *dn, bufferlist::iterator& blp,
 }
 
 void Migrator::decode_import_inode_caps(CInode *in, bool auth_cap,
-					bufferlist::iterator &blp,
+					bufferlist::const_iterator &blp,
 					map<CInode*, map<client_t,Capability::Export> >& peer_exports)
 {
   map<client_t,Capability::Export> cap_map;
@@ -3111,7 +3111,7 @@ void Migrator::finish_import_inode_caps(CInode *in, mds_rank_t peer, bool auth_c
   }
 }
 
-int Migrator::decode_import_dir(bufferlist::iterator& blp,
+int Migrator::decode_import_dir(bufferlist::const_iterator& blp,
 				mds_rank_t oldauth,
 				CDir *import_root,
 				EImportStart *le,
@@ -3357,7 +3357,7 @@ void Migrator::handle_export_caps(MExportCaps *ex)
   version_t pv = mds->server->prepare_force_open_sessions(client_map,
 							  finish->imported_session_map);
   // decode new caps
-  bufferlist::iterator blp = ex->cap_bl.begin();
+  auto blp = ex->cap_bl.cbegin();
   decode_import_inode_caps(in, false, blp, finish->peer_exports);
   assert(!finish->peer_exports.empty());   // thus, inode is pinned.
 
diff --git a/src/mds/Migrator.h b/src/mds/Migrator.h
index 148b2fb4fd2..b9aaa3a19f6 100644
--- a/src/mds/Migrator.h
+++ b/src/mds/Migrator.h
@@ -326,17 +326,17 @@ public:
 
   void export_caps(CInode *in);
 
-  void decode_import_inode(CDentry *dn, bufferlist::iterator& blp,
+  void decode_import_inode(CDentry *dn, bufferlist::const_iterator& blp,
 			   mds_rank_t oldauth, LogSegment *ls,
 			   map<CInode*, map<client_t,Capability::Export> >& cap_imports,
 			   list<ScatterLock*>& updated_scatterlocks);
-  void decode_import_inode_caps(CInode *in, bool auth_cap, bufferlist::iterator &blp,
+  void decode_import_inode_caps(CInode *in, bool auth_cap, bufferlist::const_iterator &blp,
 				map<CInode*, map<client_t,Capability::Export> >& cap_imports);
   void finish_import_inode_caps(CInode *in, mds_rank_t from, bool auth_cap,
 				const map<client_t,pair<Session*,uint64_t> >& smap,
 				const map<client_t,Capability::Export> &export_map,
 				map<client_t,Capability::Import> &import_map);
-  int decode_import_dir(bufferlist::iterator& blp,
+  int decode_import_dir(bufferlist::const_iterator& blp,
 			mds_rank_t oldauth,
 			CDir *import_root,
 			EImportStart *le, 
diff --git a/src/mds/OpenFileTable.cc b/src/mds/OpenFileTable.cc
index 0e30e975cf5..44a6c2cdf97 100644
--- a/src/mds/OpenFileTable.cc
+++ b/src/mds/OpenFileTable.cc
@@ -682,7 +682,7 @@ void OpenFileTable::_load_finish(int op_r, int header_r, int values_r,
   int err = -EINVAL;
 
   auto decode_func = [this](unsigned idx, inodeno_t ino, bufferlist &bl) {
-    bufferlist::iterator p = bl.begin();
+    auto p = bl.cbegin();
 
     size_t count = loaded_anchor_map.size();
     auto it = loaded_anchor_map.emplace_hint(loaded_anchor_map.end(),
@@ -712,7 +712,7 @@ void OpenFileTable::_load_finish(int op_r, int header_r, int values_r,
 
   try {
     if (first) {
-      bufferlist::iterator p = header_bl.begin();
+      auto p = header_bl.cbegin();
       version_t version;
       unsigned num_objs;
       __u8 jstate;
@@ -793,7 +793,7 @@ void OpenFileTable::_load_finish(int op_r, int header_r, int values_r,
 	for (auto& it : loaded_journal) {
 	  if (journal_state != JOURNAL_FINISH)
 	    continue;
-	  bufferlist::iterator p = it.second.begin();
+	  auto p = it.second.cbegin();
 	  version_t version;
 	  std::map<string, bufferlist> to_update;
 	  std::set<string> to_remove;
diff --git a/src/mds/PurgeQueue.cc b/src/mds/PurgeQueue.cc
index e01e46e4ca4..097e473dcd8 100644
--- a/src/mds/PurgeQueue.cc
+++ b/src/mds/PurgeQueue.cc
@@ -41,7 +41,7 @@ void PurgeItem::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void PurgeItem::decode(bufferlist::iterator &p)
+void PurgeItem::decode(bufferlist::const_iterator &p)
 {
   DECODE_START(1, p);
   decode((uint8_t&)action, p);
@@ -383,7 +383,7 @@ bool PurgeQueue::_consume()
 
     dout(20) << " decoding entry" << dendl;
     PurgeItem item;
-    bufferlist::iterator q = bl.begin();
+    auto q = bl.cbegin();
     try {
       decode(item, q);
     } catch (const buffer::error &err) {
diff --git a/src/mds/PurgeQueue.h b/src/mds/PurgeQueue.h
index 079f3763165..096a58f7acf 100644
--- a/src/mds/PurgeQueue.h
+++ b/src/mds/PurgeQueue.h
@@ -49,7 +49,7 @@ public:
   {}
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator &p);
+  void decode(bufferlist::const_iterator &p);
 };
 WRITE_CLASS_ENCODER(PurgeItem)
 
diff --git a/src/mds/ScatterLock.h b/src/mds/ScatterLock.h
index f7483877587..8665b93e8a9 100644
--- a/src/mds/ScatterLock.h
+++ b/src/mds/ScatterLock.h
@@ -194,7 +194,7 @@ public:
     encode(s, bl);
   }
 
-  void decode_state_rejoin(bufferlist::iterator& p, list<MDSInternalContextBase*>& waiters, bool survivor) {
+  void decode_state_rejoin(bufferlist::const_iterator& p, list<MDSInternalContextBase*>& waiters, bool survivor) {
     SimpleLock::decode_state_rejoin(p, waiters, survivor);
     if (is_flushing()) {
       set_dirty();
diff --git a/src/mds/Server.cc b/src/mds/Server.cc
index d68f863c872..8a4828e5a9b 100644
--- a/src/mds/Server.cc
+++ b/src/mds/Server.cc
@@ -1098,7 +1098,7 @@ void Server::recover_filelocks(CInode *in, bufferlist locks, int64_t client)
   if (!locks.length()) return;
   int numlocks;
   ceph_filelock lock;
-  bufferlist::iterator p = locks.begin();
+  auto p = locks.cbegin();
   decode(numlocks, p);
   for (int i = 0; i < numlocks; ++i) {
     decode(lock, p);
@@ -2697,7 +2697,7 @@ CInode* Server::prepare_new_inode(MDRequestRef& mdr, CDir *dir, inodeno_t useino
 
   MClientRequest *req = mdr->client_request;
   if (req->get_data().length()) {
-    bufferlist::iterator p = req->get_data().begin();
+    auto p = req->get_data().cbegin();
 
     // xattrs on new inode?
     CInode::mempool_xattr_map xattrs;
@@ -5790,7 +5790,7 @@ void Server::handle_slave_link_prep(MDRequestRef& mdr)
     pi.inode.nlink--;
     if (targeti->is_projected_snaprealm_global()) {
       assert(mdr->slave_request->desti_snapbl.length());
-      auto p = mdr->slave_request->desti_snapbl.begin();
+      auto p = mdr->slave_request->desti_snapbl.cbegin();
 
       sr_t *newsnap = targeti->project_snaprealm();
       decode(*newsnap, p);
@@ -5936,7 +5936,7 @@ struct C_MDS_LoggedLinkRollback : public ServerLogContext {
 void Server::do_link_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef& mdr)
 {
   link_rollback rollback;
-  bufferlist::iterator p = rbl.begin();
+  auto p = rbl.cbegin();
   decode(rollback, p);
 
   dout(10) << "do_link_rollback on " << rollback.reqid 
@@ -5984,7 +5984,7 @@ void Server::do_link_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef&
   map<client_t,MClientSnap*> splits;
   if (rollback.snapbl.length() && in->snaprealm) {
     bool hadrealm;
-    bufferlist::iterator p = rollback.snapbl.begin();
+    auto p = rollback.snapbl.cbegin();
     decode(hadrealm, p);
     if (hadrealm) {
       if (!mds->is_resolve()) {
@@ -6660,7 +6660,7 @@ void Server::do_rmdir_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef&
   // the file system are taking place here, so there is no Mutation.
 
   rmdir_rollback rollback;
-  bufferlist::iterator p = rbl.begin();
+  auto p = rbl.cbegin();
   decode(rollback, p);
   
   dout(10) << "do_rmdir_rollback on " << rollback.reqid << dendl;
@@ -6686,7 +6686,7 @@ void Server::do_rmdir_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef&
 
   if (rollback.snapbl.length() && in->snaprealm) {
     bool hadrealm;
-    bufferlist::iterator p = rollback.snapbl.begin();
+    auto p = rollback.snapbl.cbegin();
     decode(hadrealm, p);
     if (hadrealm) {
       decode(in->snaprealm->srnode, p);
@@ -7388,7 +7388,7 @@ version_t Server::_rename_prepare_import(MDRequestRef& mdr, CDentry *srcdn, buff
   CDentry::linkage_t *srcdnl = srcdn->get_linkage();
 
   /* import node */
-  bufferlist::iterator blp = mdr->more()->inode_import.begin();
+  auto blp = mdr->more()->inode_import.cbegin();
 	  
   // imported caps
   map<client_t,entity_inst_t> client_map;
@@ -7660,7 +7660,7 @@ void Server::_rename_prepare(MDRequestRef& mdr,
 	if (mdr->slave_request) {
 	  if (mdr->slave_request->desti_snapbl.length() > 0) {
 	    new_srnode = new sr_t();
-	    auto p = mdr->slave_request->desti_snapbl.begin();
+	    auto p = mdr->slave_request->desti_snapbl.cbegin();
 	    decode(*new_srnode, p);
 	  }
 	} else if (auto& desti_srnode = mdr->more()->desti_srnode) {
@@ -7696,7 +7696,7 @@ void Server::_rename_prepare(MDRequestRef& mdr,
       if (mdr->slave_request) {
 	if (mdr->slave_request->srci_snapbl.length() > 0) {
 	  sr_t *new_srnode = new sr_t();
-	  auto p = mdr->slave_request->srci_snapbl.begin();
+	  auto p = mdr->slave_request->srci_snapbl.cbegin();
 	  decode(*new_srnode, p);
 	  srci->project_snaprealm(new_srnode);
 	}
@@ -8351,7 +8351,7 @@ void Server::_commit_slave_rename(MDRequestRef& mdr, int r,
       }
 
       map<client_t,Capability::Import> peer_imported;
-      bufferlist::iterator bp = mdr->more()->inode_import.begin();
+      auto bp = mdr->more()->inode_import.cbegin();
       decode(peer_imported, bp);
 
       dout(10) << " finishing inode export on " << *destdnl->get_inode() << dendl;
@@ -8477,7 +8477,7 @@ void Server::do_rename_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef
 				bool finish_mdr)
 {
   rename_rollback rollback;
-  bufferlist::iterator p = rbl.begin();
+  auto p = rbl.cbegin();
   decode(rollback, p);
 
   dout(10) << "do_rename_rollback on " << rollback.reqid << dendl;
@@ -8594,7 +8594,7 @@ void Server::do_rename_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef
 
     if (rollback.srci_snapbl.length() && in->snaprealm) {
       bool hadrealm;
-      bufferlist::iterator p = rollback.srci_snapbl.begin();
+      auto p = rollback.srci_snapbl.cbegin();
       decode(hadrealm, p);
       if (hadrealm) {
 	if (projected && !mds->is_resolve()) {
@@ -8671,7 +8671,7 @@ void Server::do_rename_rollback(bufferlist &rbl, mds_rank_t master, MDRequestRef
 
     if (rollback.desti_snapbl.length() && target->snaprealm) {
       bool hadrealm;
-      bufferlist::iterator p = rollback.desti_snapbl.begin();
+      auto p = rollback.desti_snapbl.cbegin();
       decode(hadrealm, p);
       if (hadrealm) {
 	if (projected && !mds->is_resolve()) {
@@ -9138,7 +9138,7 @@ void Server::handle_client_mksnap(MDRequestRef& mdr)
 
   version_t stid = mdr->more()->stid;
   snapid_t snapid;
-  bufferlist::iterator p = mdr->more()->snapidbl.begin();
+  auto p = mdr->more()->snapidbl.cbegin();
   decode(snapid, p);
   dout(10) << " stid " << stid << " snapid " << snapid << dendl;
 
@@ -9279,7 +9279,7 @@ void Server::handle_client_rmsnap(MDRequestRef& mdr)
     return;
   }
   version_t stid = mdr->more()->stid;
-  bufferlist::iterator p = mdr->more()->snapidbl.begin();
+  auto p = mdr->more()->snapidbl.cbegin();
   snapid_t seq;
   decode(seq, p);  
   dout(10) << " stid is " << stid << ", seq is " << seq << dendl;
@@ -9315,7 +9315,7 @@ void Server::_rmsnap_finish(MDRequestRef& mdr, CInode *diri, snapid_t snapid)
 {
   dout(10) << "_rmsnap_finish " << *mdr << " " << snapid << dendl;
   snapid_t stid = mdr->more()->stid;
-  bufferlist::iterator p = mdr->more()->snapidbl.begin();
+  auto p = mdr->more()->snapidbl.cbegin();
   snapid_t seq;
   decode(seq, p);  
 
diff --git a/src/mds/SessionMap.cc b/src/mds/SessionMap.cc
index dff223cf611..43464a92979 100644
--- a/src/mds/SessionMap.cc
+++ b/src/mds/SessionMap.cc
@@ -109,7 +109,7 @@ public:
 void SessionMapStore::decode_header(
       bufferlist &header_bl)
 {
-  bufferlist::iterator q = header_bl.begin();
+  auto q = header_bl.cbegin();
   DECODE_START(1, q)
   decode(version, q);
   DECODE_FINISH(q);
@@ -143,7 +143,7 @@ void SessionMapStore::decode_values(std::map<std::string, bufferlist> &session_v
     Session *s = get_or_add_session(inst);
     if (s->is_closed())
       s->set_state(Session::STATE_OPEN);
-    bufferlist::iterator q = i->second.begin();
+    auto q = i->second.cbegin();
     s->decode(q);
   }
 }
@@ -303,7 +303,7 @@ void SessionMap::load_legacy()
 
 void SessionMap::_load_legacy_finish(int r, bufferlist &bl)
 { 
-  bufferlist::iterator blp = bl.begin();
+  auto blp = bl.cbegin();
   if (r < 0) {
     derr << "_load_finish got " << cpp_strerror(r) << dendl;
     assert(0 == "failed to load sessionmap");
@@ -450,7 +450,7 @@ void SessionMap::_save_finish(version_t v)
 /**
  * Deserialize sessions, and update by_state index
  */
-void SessionMap::decode_legacy(bufferlist::iterator &p)
+void SessionMap::decode_legacy(bufferlist::const_iterator &p)
 {
   // Populate `sessions`
   SessionMapStore::decode_legacy(p);
@@ -478,7 +478,7 @@ uint64_t SessionMap::set_state(Session *session, int s) {
   return session->get_state_seq();
 }
 
-void SessionMapStore::decode_legacy(bufferlist::iterator& p)
+void SessionMapStore::decode_legacy(bufferlist::const_iterator& p)
 {
   utime_t now = ceph_clock_now();
   uint64_t pre;
@@ -508,7 +508,7 @@ void SessionMapStore::decode_legacy(bufferlist::iterator& p)
     decode(n, p);
     
     while (n-- && !p.end()) {
-      bufferlist::iterator p2 = p;
+      auto p2 = p;
       Session *s = new Session;
       s->info.decode(p);
       if (session_map.count(s->info.inst.name)) {
@@ -882,7 +882,7 @@ void Session::_update_human_name()
   }
 }
 
-void Session::decode(bufferlist::iterator &p)
+void Session::decode(bufferlist::const_iterator &p)
 {
   info.decode(p);
 
diff --git a/src/mds/SessionMap.h b/src/mds/SessionMap.h
index b9714eeb010..7ae0ed28e67 100644
--- a/src/mds/SessionMap.h
+++ b/src/mds/SessionMap.h
@@ -123,7 +123,7 @@ public:
       state_seq++;
     }
   }
-  void decode(bufferlist::iterator &p);
+  void decode(bufferlist::const_iterator &p);
   void set_client_metadata(std::map<std::string, std::string> const &meta);
   std::string get_human_name() const {return human_name;}
 
@@ -401,7 +401,7 @@ public:
   virtual void encode_header(bufferlist *header_bl);
   virtual void decode_header(bufferlist &header_bl);
   virtual void decode_values(std::map<std::string, bufferlist> &session_vals);
-  virtual void decode_legacy(bufferlist::iterator& blp);
+  virtual void decode_legacy(bufferlist::const_iterator& blp);
   void dump(Formatter *f) const;
 
   void set_rank(mds_rank_t r)
@@ -494,7 +494,7 @@ public:
   }
 
   // sessions
-  void decode_legacy(bufferlist::iterator& blp) override;
+  void decode_legacy(bufferlist::const_iterator& blp) override;
   bool empty() const { return session_map.empty(); }
   const ceph::unordered_map<entity_name_t, Session*> &get_sessions() const
   {
diff --git a/src/mds/SimpleLock.h b/src/mds/SimpleLock.h
index 35ddd006442..0342e0cf113 100644
--- a/src/mds/SimpleLock.h
+++ b/src/mds/SimpleLock.h
@@ -569,7 +569,7 @@ public:
       encode(empty_gather_set, bl);
     ENCODE_FINISH(bl);
   }
-  void decode(bufferlist::iterator& p) {
+  void decode(bufferlist::const_iterator& p) {
     DECODE_START(2, p);
     decode(state, p);
     set<__s32> g;
@@ -583,14 +583,14 @@ public:
     using ceph::encode;
     encode(s, bl);
   }
-  void decode_state(bufferlist::iterator& p, bool is_new=true) {
+  void decode_state(bufferlist::const_iterator& p, bool is_new=true) {
     using ceph::decode;
     __s16 s;
     decode(s, p);
     if (is_new)
       state = s;
   }
-  void decode_state_rejoin(bufferlist::iterator& p, list<MDSInternalContextBase*>& waiters, bool survivor) {
+  void decode_state_rejoin(bufferlist::const_iterator& p, list<MDSInternalContextBase*>& waiters, bool survivor) {
     __s16 s;
     using ceph::decode;
     decode(s, p);
diff --git a/src/mds/SnapClient.cc b/src/mds/SnapClient.cc
index 82b1111cb22..ca49e2219bf 100644
--- a/src/mds/SnapClient.cc
+++ b/src/mds/SnapClient.cc
@@ -46,7 +46,7 @@ void SnapClient::handle_query_result(MMDSTableRequest *m)
 
   char type;
   using ceph::decode;
-  bufferlist::iterator p = m->bl.begin();
+  auto p = m->bl.cbegin();
   decode(type, p);
 
   switch (type) {
diff --git a/src/mds/SnapServer.cc b/src/mds/SnapServer.cc
index b7ae4519a16..ddfd3d07319 100644
--- a/src/mds/SnapServer.cc
+++ b/src/mds/SnapServer.cc
@@ -71,7 +71,7 @@ void SnapServer::_prepare(bufferlist &bl, uint64_t reqid, mds_rank_t bymds)
 {
   using ceph::decode;
   using ceph::encode;
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
   __u32 op;
   decode(op, p);
 
@@ -234,7 +234,7 @@ void SnapServer::_rollback(version_t tid)
 void SnapServer::_server_update(bufferlist& bl)
 {
   using ceph::decode;
-  bufferlist::iterator p = bl.begin();
+  auto p = bl.cbegin();
   map<int, vector<snapid_t> > purge;
   decode(purge, p);
 
@@ -277,7 +277,7 @@ void SnapServer::handle_query(MMDSTableRequest *req)
   using ceph::encode;
   using ceph::decode;
   char op;
-  bufferlist::iterator p = req->bl.begin();
+  auto p = req->bl.cbegin();
   decode(op, p);
 
   MMDSTableRequest *reply = new MMDSTableRequest(table, TABLESERVER_OP_QUERY_REPLY, req->reqid, version);
diff --git a/src/mds/SnapServer.h b/src/mds/SnapServer.h
index 7b9c4af9e47..9c88fbec83d 100644
--- a/src/mds/SnapServer.h
+++ b/src/mds/SnapServer.h
@@ -52,7 +52,7 @@ protected:
     encode(snaprealm_v2_since, bl);
     ENCODE_FINISH(bl);
   }
-  void decode_server_state(bufferlist::iterator& bl) override {
+  void decode_server_state(bufferlist::const_iterator& bl) override {
     DECODE_START_LEGACY_COMPAT_LEN(5, 3, 3, bl);
     decode(last_snap, bl);
     decode(snaps, bl);
@@ -133,7 +133,7 @@ public:
   void encode(bufferlist& bl) const {
     encode_server_state(bl);
   }
-  void decode(bufferlist::iterator& bl) {
+  void decode(bufferlist::const_iterator& bl) {
     decode_server_state(bl);
   }
 
diff --git a/src/mds/events/ECommitted.h b/src/mds/events/ECommitted.h
index 19868414fc9..0459f9d0fae 100644
--- a/src/mds/events/ECommitted.h
+++ b/src/mds/events/ECommitted.h
@@ -31,7 +31,7 @@ public:
   }
 
   void encode(bufferlist &bl, uint64_t features) const override;
-  void decode(bufferlist::iterator &bl) override;
+  void decode(bufferlist::const_iterator &bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<ECommitted*>& ls);
 
diff --git a/src/mds/events/EExport.h b/src/mds/events/EExport.h
index 1de05449c18..94e39a84816 100644
--- a/src/mds/events/EExport.h
+++ b/src/mds/events/EExport.h
@@ -47,7 +47,7 @@ public:
   EMetaBlob *get_metablob() override { return &metablob; }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator &bl) override;
+  void decode(bufferlist::const_iterator &bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EExport*>& ls);
   void replay(MDSRank *mds) override;
diff --git a/src/mds/events/EFragment.h b/src/mds/events/EFragment.h
index fc17922ffa8..6204fcb7777 100644
--- a/src/mds/events/EFragment.h
+++ b/src/mds/events/EFragment.h
@@ -22,7 +22,7 @@ struct dirfrag_rollback {
   fnode_t fnode;
   dirfrag_rollback() { }
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
 };
 WRITE_CLASS_ENCODER(dirfrag_rollback)
 
@@ -71,7 +71,7 @@ public:
   EMetaBlob *get_metablob() override { return &metablob; }
 
   void encode(bufferlist &bl, uint64_t features) const override;
-  void decode(bufferlist::iterator &bl) override;
+  void decode(bufferlist::const_iterator &bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EFragment*>& ls);
   void replay(MDSRank *mds) override;
diff --git a/src/mds/events/EImportFinish.h b/src/mds/events/EImportFinish.h
index 16c4d931dcd..699c0527c5b 100644
--- a/src/mds/events/EImportFinish.h
+++ b/src/mds/events/EImportFinish.h
@@ -41,7 +41,7 @@ class EImportFinish : public LogEvent {
   }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator &bl) override;
+  void decode(bufferlist::const_iterator &bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EImportFinish*>& ls);
   
diff --git a/src/mds/events/EImportStart.h b/src/mds/events/EImportStart.h
index 970b35faab9..c53dfa269ec 100644
--- a/src/mds/events/EImportStart.h
+++ b/src/mds/events/EImportStart.h
@@ -48,7 +48,7 @@ public:
   EMetaBlob *get_metablob() override { return &metablob; }
   
   void encode(bufferlist &bl, uint64_t features) const override;
-  void decode(bufferlist::iterator &bl) override;
+  void decode(bufferlist::const_iterator &bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EImportStart*>& ls);
   
diff --git a/src/mds/events/EMetaBlob.h b/src/mds/events/EMetaBlob.h
index c38c2ccb528..94ff9899e00 100644
--- a/src/mds/events/EMetaBlob.h
+++ b/src/mds/events/EMetaBlob.h
@@ -96,14 +96,14 @@ public:
 	old_inodes = *oi;
       snapbl = sbl;
     }
-    explicit fullbit(bufferlist::iterator &p) {
+    explicit fullbit(bufferlist::const_iterator &p) {
       decode(p);
     }
     fullbit() {}
     ~fullbit() {}
 
     void encode(bufferlist& bl, uint64_t features) const;
-    void decode(bufferlist::iterator &bl);
+    void decode(bufferlist::const_iterator &bl);
     void dump(Formatter *f) const;
     static void generate_test_instances(list<EMetaBlob::fullbit*>& ls);
 
@@ -147,12 +147,12 @@ public:
 
     remotebit(std::string_view d, snapid_t df, snapid_t dl, version_t v, inodeno_t i, unsigned char dt, bool dr) : 
       dn(d), dnfirst(df), dnlast(dl), dnv(v), ino(i), d_type(dt), dirty(dr) { }
-    explicit remotebit(bufferlist::iterator &p) { decode(p); }
+    explicit remotebit(bufferlist::const_iterator &p) { decode(p); }
     remotebit(): dnfirst(0), dnlast(0), dnv(0), ino(0),
 	d_type('\0'), dirty(false) {}
 
     void encode(bufferlist& bl) const;
-    void decode(bufferlist::iterator &bl);
+    void decode(bufferlist::const_iterator &bl);
     void print(ostream& out) const {
       out << " remotebit dn " << dn << " [" << dnfirst << "," << dnlast << "] dnv " << dnv
 	  << " ino " << ino
@@ -174,11 +174,11 @@ public:
 
     nullbit(std::string_view d, snapid_t df, snapid_t dl, version_t v, bool dr) :
       dn(d), dnfirst(df), dnlast(dl), dnv(v), dirty(dr) { }
-    explicit nullbit(bufferlist::iterator &p) { decode(p); }
+    explicit nullbit(bufferlist::const_iterator &p) { decode(p); }
     nullbit(): dnfirst(0), dnlast(0), dnv(0), dirty(false) {}
 
     void encode(bufferlist& bl) const;
-    void decode(bufferlist::iterator &bl);
+    void decode(bufferlist::const_iterator &bl);
     void dump(Formatter *f) const;
     static void generate_test_instances(list<nullbit*>& ls);
     void print(ostream& out) {
@@ -275,7 +275,7 @@ public:
     void _decode_bits() const { 
       using ceph::decode;
       if (dn_decoded) return;
-      bufferlist::iterator p = dnbl.begin();
+      auto p = dnbl.cbegin();
       decode(dfull, p);
       decode(dremote, p);
       decode(dnull, p);
@@ -283,7 +283,7 @@ public:
     }
 
     void encode(bufferlist& bl, uint64_t features) const;
-    void decode(bufferlist::iterator &bl);
+    void decode(bufferlist::const_iterator &bl);
     void dump(Formatter *f) const;
     static void generate_test_instances(list<dirlump*>& ls);
   };
@@ -323,7 +323,7 @@ private:
 
  public:
   void encode(bufferlist& bl, uint64_t features) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void get_inodes(std::set<inodeno_t> &inodes) const;
   void get_paths(std::vector<std::string> &paths) const;
   void get_dentries(std::map<dirfrag_t, std::set<std::string> > &dentries) const;
diff --git a/src/mds/events/ENoOp.h b/src/mds/events/ENoOp.h
index 9a150704305..1bf5161e805 100644
--- a/src/mds/events/ENoOp.h
+++ b/src/mds/events/ENoOp.h
@@ -25,7 +25,7 @@ public:
   explicit ENoOp(uint32_t size_) : LogEvent(EVENT_NOOP), pad_size(size_){ }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override {}
 
   void replay(MDSRank *mds) override;
diff --git a/src/mds/events/EOpen.h b/src/mds/events/EOpen.h
index 655f97841fd..192745d9bf0 100644
--- a/src/mds/events/EOpen.h
+++ b/src/mds/events/EOpen.h
@@ -49,7 +49,7 @@ public:
   }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EOpen*>& ls);
 
diff --git a/src/mds/events/EResetJournal.h b/src/mds/events/EResetJournal.h
index be14b608524..3004978a22b 100644
--- a/src/mds/events/EResetJournal.h
+++ b/src/mds/events/EResetJournal.h
@@ -25,7 +25,7 @@ class EResetJournal : public LogEvent {
   ~EResetJournal() override {}
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EResetJournal*>& ls);
   void print(ostream& out) const override {
diff --git a/src/mds/events/ESession.h b/src/mds/events/ESession.h
index 53f320b4263..4cf348017c6 100644
--- a/src/mds/events/ESession.h
+++ b/src/mds/events/ESession.h
@@ -52,7 +52,7 @@ class ESession : public LogEvent {
     inos(i), inotablev(iv) { }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<ESession*>& ls);
 
diff --git a/src/mds/events/ESessions.h b/src/mds/events/ESessions.h
index f784d63eb0c..0b29b53794b 100644
--- a/src/mds/events/ESessions.h
+++ b/src/mds/events/ESessions.h
@@ -39,9 +39,9 @@ public:
   void mark_old_encoding() { old_style_encode = true; }
 
   void encode(bufferlist &bl, uint64_t features) const override;
-  void decode_old(bufferlist::iterator &bl);
-  void decode_new(bufferlist::iterator &bl);
-  void decode(bufferlist::iterator &bl) override {
+  void decode_old(bufferlist::const_iterator &bl);
+  void decode_new(bufferlist::const_iterator &bl);
+  void decode(bufferlist::const_iterator &bl) override {
     if (old_style_encode) decode_old(bl);
     else decode_new(bl);
   }
diff --git a/src/mds/events/ESlaveUpdate.h b/src/mds/events/ESlaveUpdate.h
index 541294a1f9f..193f4a08409 100644
--- a/src/mds/events/ESlaveUpdate.h
+++ b/src/mds/events/ESlaveUpdate.h
@@ -35,7 +35,7 @@ struct link_rollback {
   link_rollback() : ino(0), was_inc(false) {}
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<link_rollback*>& ls);
 };
@@ -56,7 +56,7 @@ struct rmdir_rollback {
   bufferlist snapbl;
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<rmdir_rollback*>& ls);
 };
@@ -75,7 +75,7 @@ struct rename_rollback {
     drec() : remote_d_type((char)S_IFREG) {}
 
     void encode(bufferlist& bl) const;
-    void decode(bufferlist::iterator& bl);
+    void decode(bufferlist::const_iterator& bl);
     void dump(Formatter *f) const;
     static void generate_test_instances(list<drec*>& ls);
   };
@@ -89,7 +89,7 @@ struct rename_rollback {
   bufferlist desti_snapbl;
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<rename_rollback*>& ls);
 };
@@ -144,7 +144,7 @@ public:
   EMetaBlob *get_metablob() override { return &commit; }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<ESlaveUpdate*>& ls);
 
diff --git a/src/mds/events/ESubtreeMap.h b/src/mds/events/ESubtreeMap.h
index def22a7916f..08d4a581176 100644
--- a/src/mds/events/ESubtreeMap.h
+++ b/src/mds/events/ESubtreeMap.h
@@ -37,7 +37,7 @@ public:
   EMetaBlob *get_metablob() override { return &metablob; }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<ESubtreeMap*>& ls);
 
diff --git a/src/mds/events/ETableClient.h b/src/mds/events/ETableClient.h
index 7633d9e066f..bf3e752d5d4 100644
--- a/src/mds/events/ETableClient.h
+++ b/src/mds/events/ETableClient.h
@@ -32,7 +32,7 @@ struct ETableClient : public LogEvent {
     table(t), op(o), tid(ti) { }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<ETableClient*>& ls);
 
diff --git a/src/mds/events/ETableServer.h b/src/mds/events/ETableServer.h
index b8dd9dbacfc..0005b132466 100644
--- a/src/mds/events/ETableServer.h
+++ b/src/mds/events/ETableServer.h
@@ -37,7 +37,7 @@ struct ETableServer : public LogEvent {
     table(t), op(o), reqid(ri), bymds(m), tid(ti), version(v) { }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<ETableServer*>& ls);
 
diff --git a/src/mds/events/EUpdate.h b/src/mds/events/EUpdate.h
index 0710917027b..9984070597d 100644
--- a/src/mds/events/EUpdate.h
+++ b/src/mds/events/EUpdate.h
@@ -41,7 +41,7 @@ public:
   EMetaBlob *get_metablob() override { return &metablob; }
 
   void encode(bufferlist& bl, uint64_t features) const override;
-  void decode(bufferlist::iterator& bl) override;
+  void decode(bufferlist::const_iterator& bl) override;
   void dump(Formatter *f) const override;
   static void generate_test_instances(list<EUpdate*>& ls);
 
diff --git a/src/mds/flock.h b/src/mds/flock.h
index d013972740b..ef1793f46aa 100644
--- a/src/mds/flock.h
+++ b/src/mds/flock.h
@@ -255,7 +255,7 @@ public:
     encode(held_locks, bl);
     encode(client_held_lock_counts, bl);
   }
-  void decode(bufferlist::iterator& bl) {
+  void decode(bufferlist::const_iterator& bl) {
     using ceph::decode;
     decode(held_locks, bl);
     decode(client_held_lock_counts, bl);
diff --git a/src/mds/inode_backtrace.cc b/src/mds/inode_backtrace.cc
index 4eae542ddf6..50e986a10d9 100644
--- a/src/mds/inode_backtrace.cc
+++ b/src/mds/inode_backtrace.cc
@@ -16,7 +16,7 @@ void inode_backpointer_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void inode_backpointer_t::decode(bufferlist::iterator& bl)
+void inode_backpointer_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(dirino, bl);
@@ -25,7 +25,7 @@ void inode_backpointer_t::decode(bufferlist::iterator& bl)
   DECODE_FINISH(bl);
 }
 
-void inode_backpointer_t::decode_old(bufferlist::iterator& bl)
+void inode_backpointer_t::decode_old(bufferlist::const_iterator& bl)
 {
   using ceph::decode;
   decode(dirino, bl);
@@ -64,7 +64,7 @@ void inode_backtrace_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void inode_backtrace_t::decode(bufferlist::iterator& bl)
+void inode_backtrace_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(5, 4, 4, bl);
   if (struct_v < 3)
diff --git a/src/mds/inode_backtrace.h b/src/mds/inode_backtrace.h
index 87b988f8282..7c60865c22d 100644
--- a/src/mds/inode_backtrace.h
+++ b/src/mds/inode_backtrace.h
@@ -30,8 +30,8 @@ struct inode_backpointer_t {
   inode_backpointer_t(inodeno_t i, std::string_view d, version_t v) : dirino(i), dname(d), version(v) {}
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator &bl);
-  void decode_old(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
+  void decode_old(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<inode_backpointer_t*>& ls);
 };
@@ -60,7 +60,7 @@ struct inode_backtrace_t {
   inode_backtrace_t() : pool(-1) {}
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<inode_backtrace_t*>& ls);
 
diff --git a/src/mds/journal.cc b/src/mds/journal.cc
index 23c2f15a54b..e67c3858c17 100644
--- a/src/mds/journal.cc
+++ b/src/mds/journal.cc
@@ -417,7 +417,7 @@ void EMetaBlob::fullbit::encode(bufferlist& bl, uint64_t features) const {
   ENCODE_FINISH(bl);
 }
 
-void EMetaBlob::fullbit::decode(bufferlist::iterator &bl) {
+void EMetaBlob::fullbit::decode(bufferlist::const_iterator &bl) {
   DECODE_START_LEGACY_COMPAT_LEN(7, 5, 5, bl);
   decode(dn, bl);
   decode(dnfirst, bl);
@@ -596,7 +596,7 @@ void EMetaBlob::remotebit::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void EMetaBlob::remotebit::decode(bufferlist::iterator &bl)
+void EMetaBlob::remotebit::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(dn, bl);
@@ -660,7 +660,7 @@ void EMetaBlob::nullbit::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void EMetaBlob::nullbit::decode(bufferlist::iterator &bl)
+void EMetaBlob::nullbit::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(dn, bl);
@@ -703,7 +703,7 @@ void EMetaBlob::dirlump::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void EMetaBlob::dirlump::decode(bufferlist::iterator &bl)
+void EMetaBlob::dirlump::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl)
   decode(fnode, bl);
@@ -794,7 +794,7 @@ void EMetaBlob::encode(bufferlist& bl, uint64_t features) const
   encode(client_flushes, bl);
   ENCODE_FINISH(bl);
 }
-void EMetaBlob::decode(bufferlist::iterator &bl)
+void EMetaBlob::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(7, 5, 5, bl);
   decode(lump_order, bl);
@@ -805,7 +805,7 @@ void EMetaBlob::decode(bufferlist::iterator &bl)
     bufferlist rootbl;
     decode(rootbl, bl);
     if (rootbl.length()) {
-      bufferlist::iterator p = rootbl.begin();
+      auto p = rootbl.cbegin();
       roots.push_back(ceph::shared_ptr<fullbit>(new fullbit(p)));
     }
   }
@@ -1732,7 +1732,7 @@ void ESession::encode(bufferlist &bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void ESession::decode(bufferlist::iterator &bl)
+void ESession::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(4, 3, 3, bl);
   if (struct_v >= 2)
@@ -1780,7 +1780,7 @@ void ESessions::encode(bufferlist &bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void ESessions::decode_old(bufferlist::iterator &bl)
+void ESessions::decode_old(bufferlist::const_iterator &bl)
 {
   using ceph::decode;
   decode(client_map, bl);
@@ -1789,7 +1789,7 @@ void ESessions::decode_old(bufferlist::iterator &bl)
     decode(stamp, bl);
 }
 
-void ESessions::decode_new(bufferlist::iterator &bl)
+void ESessions::decode_new(bufferlist::const_iterator &bl)
 {
   DECODE_START(1, bl);
   decode(client_map, bl);
@@ -1856,7 +1856,7 @@ void ETableServer::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void ETableServer::decode(bufferlist::iterator &bl)
+void ETableServer::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
@@ -1952,7 +1952,7 @@ void ETableClient::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void ETableClient::decode(bufferlist::iterator &bl)
+void ETableClient::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
@@ -2040,7 +2040,7 @@ void EUpdate::encode(bufferlist &bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
  
-void EUpdate::decode(bufferlist::iterator &bl)
+void EUpdate::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(4, 4, 4, bl);
   if (struct_v >= 2)
@@ -2105,7 +2105,7 @@ void EUpdate::replay(MDSRank *mds)
 	       << " < " << cmapv << dendl;
       // open client sessions?
       map<client_t,entity_inst_t> cm;
-      bufferlist::iterator blp = client_map.begin();
+      auto blp = client_map.cbegin();
       using ceph::decode;
       decode(cm, blp);
       mds->sessionmap.replay_open_sessions(cm);
@@ -2128,7 +2128,7 @@ void EOpen::encode(bufferlist &bl, uint64_t features) const {
   ENCODE_FINISH(bl);
 } 
 
-void EOpen::decode(bufferlist::iterator &bl) {
+void EOpen::decode(bufferlist::const_iterator &bl) {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
     decode(stamp, bl);
@@ -2211,7 +2211,7 @@ void ECommitted::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 } 
 
-void ECommitted::decode(bufferlist::iterator& bl)
+void ECommitted::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
@@ -2249,7 +2249,7 @@ void link_rollback::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void link_rollback::decode(bufferlist::iterator &bl)
+void link_rollback::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 2, 2, bl);
   decode(reqid, bl);
@@ -2290,7 +2290,7 @@ void rmdir_rollback::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void rmdir_rollback::decode(bufferlist::iterator& bl)
+void rmdir_rollback::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 2, 2, bl);
   decode(reqid, bl);
@@ -2331,7 +2331,7 @@ void rename_rollback::drec::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void rename_rollback::drec::decode(bufferlist::iterator &bl)
+void rename_rollback::drec::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(dirfrag, bl);
@@ -2388,7 +2388,7 @@ void rename_rollback::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void rename_rollback::decode(bufferlist::iterator &bl)
+void rename_rollback::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 2, 2, bl);
   decode(reqid, bl);
@@ -2440,7 +2440,7 @@ void ESlaveUpdate::encode(bufferlist &bl, uint64_t features) const
   ENCODE_FINISH(bl);
 } 
 
-void ESlaveUpdate::decode(bufferlist::iterator &bl)
+void ESlaveUpdate::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
@@ -2530,7 +2530,7 @@ void ESubtreeMap::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
  
-void ESubtreeMap::decode(bufferlist::iterator &bl)
+void ESubtreeMap::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(6, 5, 5, bl);
   if (struct_v >= 2)
@@ -2774,7 +2774,7 @@ void EFragment::encode(bufferlist &bl, uint64_t features) const {
   ENCODE_FINISH(bl);
 }
 
-void EFragment::decode(bufferlist::iterator &bl) {
+void EFragment::decode(bufferlist::const_iterator &bl) {
   DECODE_START_LEGACY_COMPAT_LEN(5, 4, 4, bl);
   if (struct_v >= 2)
     decode(stamp, bl);
@@ -2818,7 +2818,7 @@ void dirfrag_rollback::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void dirfrag_rollback::decode(bufferlist::iterator &bl)
+void dirfrag_rollback::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START(1, bl);
   decode(fnode, bl);
@@ -2866,7 +2866,7 @@ void EExport::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void EExport::decode(bufferlist::iterator &bl)
+void EExport::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
@@ -2944,7 +2944,7 @@ void EImportStart::replay(MDSRank *mds)
     dout(10) << "EImportStart.replay sessionmap " << mds->sessionmap.get_version() 
 	     << " < " << cmapv << dendl;
     map<client_t,entity_inst_t> cm;
-    bufferlist::iterator blp = client_map.begin();
+    auto blp = client_map.cbegin();
     using ceph::decode;
     decode(cm, blp);
     mds->sessionmap.replay_open_sessions(cm);
@@ -2972,7 +2972,7 @@ void EImportStart::encode(bufferlist &bl, uint64_t features) const {
   ENCODE_FINISH(bl);
 }
 
-void EImportStart::decode(bufferlist::iterator &bl) {
+void EImportStart::decode(bufferlist::const_iterator &bl) {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
     decode(stamp, bl);
@@ -3040,7 +3040,7 @@ void EImportFinish::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void EImportFinish::decode(bufferlist::iterator &bl)
+void EImportFinish::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 3, 3, bl);
   if (struct_v >= 2)
@@ -3073,7 +3073,7 @@ void EResetJournal::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
  
-void EResetJournal::decode(bufferlist::iterator &bl)
+void EResetJournal::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(stamp, bl);
@@ -3123,7 +3123,7 @@ void ENoOp::encode(bufferlist &bl, uint64_t features) const
 }
 
 
-void ENoOp::decode(bufferlist::iterator &bl)
+void ENoOp::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START(2, bl);
   decode(pad_size, bl);
diff --git a/src/mds/mdstypes.cc b/src/mds/mdstypes.cc
index a641f2812bf..492a845b415 100644
--- a/src/mds/mdstypes.cc
+++ b/src/mds/mdstypes.cc
@@ -24,7 +24,7 @@ void frag_info_t::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void frag_info_t::decode(bufferlist::iterator &bl)
+void frag_info_t::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 2, 2, bl);
   decode(version, bl);
@@ -91,7 +91,7 @@ void nest_info_t::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void nest_info_t::decode(bufferlist::iterator &bl)
+void nest_info_t::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 2, 2, bl);
   decode(version, bl);
@@ -185,7 +185,7 @@ void client_writeable_range_t::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void client_writeable_range_t::decode(bufferlist::iterator& bl)
+void client_writeable_range_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(range.first, bl);
@@ -229,7 +229,7 @@ void inline_data_t::encode(bufferlist &bl) const
   else
     encode(bufferlist(), bl);
 }
-void inline_data_t::decode(bufferlist::iterator &p)
+void inline_data_t::decode(bufferlist::const_iterator &p)
 {
   using ceph::decode;
   decode(version, p);
@@ -262,7 +262,7 @@ void fnode_t::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void fnode_t::decode(bufferlist::iterator &bl)
+void fnode_t::decode(bufferlist::const_iterator &bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(3, 2, 2, bl);
   decode(version, bl);
@@ -334,7 +334,7 @@ void old_rstat_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void old_rstat_t::decode(bufferlist::iterator& bl)
+void old_rstat_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(first, bl);
@@ -381,7 +381,7 @@ void session_info_t::encode(bufferlist& bl, uint64_t features) const
   ENCODE_FINISH(bl);
 }
 
-void session_info_t::decode(bufferlist::iterator& p)
+void session_info_t::decode(bufferlist::const_iterator& p)
 {
   DECODE_START_LEGACY_COMPAT_LEN(6, 2, 2, p);
   decode(inst, p);
@@ -478,7 +478,7 @@ void string_snap_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void string_snap_t::decode(bufferlist::iterator& bl)
+void string_snap_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(name, bl);
@@ -517,7 +517,7 @@ void MDSCacheObjectInfo::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void MDSCacheObjectInfo::decode(bufferlist::iterator& p)
+void MDSCacheObjectInfo::decode(bufferlist::const_iterator& p)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, p);
   decode(ino, p);
@@ -562,7 +562,7 @@ void mds_table_pending_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void mds_table_pending_t::decode(bufferlist::iterator& bl)
+void mds_table_pending_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(reqid, bl);
@@ -600,7 +600,7 @@ void inode_load_vec_t::encode(bufferlist &bl) const
   ENCODE_FINISH(bl);
 }
 
-void inode_load_vec_t::decode(const utime_t &t, bufferlist::iterator &p)
+void inode_load_vec_t::decode(const utime_t &t, bufferlist::const_iterator &p)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, p);
   for (auto &i : vec) {
@@ -671,7 +671,7 @@ void mds_load_t::encode(bufferlist &bl) const {
   ENCODE_FINISH(bl);
 }
 
-void mds_load_t::decode(const utime_t &t, bufferlist::iterator &bl) {
+void mds_load_t::decode(const utime_t &t, bufferlist::const_iterator &bl) {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(auth, t, bl);
   decode(all, t, bl);
@@ -720,7 +720,7 @@ void cap_reconnect_t::encode_old(bufferlist& bl) const {
   encode_nohead(flockbl, bl);
 }
 
-void cap_reconnect_t::decode(bufferlist::iterator& bl) {
+void cap_reconnect_t::decode(bufferlist::const_iterator& bl) {
   DECODE_START(1, bl);
   decode_old(bl); // extract out when something changes
   if (struct_v >= 2)
@@ -728,7 +728,7 @@ void cap_reconnect_t::decode(bufferlist::iterator& bl) {
   DECODE_FINISH(bl);
 }
 
-void cap_reconnect_t::decode_old(bufferlist::iterator& bl) {
+void cap_reconnect_t::decode_old(bufferlist::const_iterator& bl) {
   using ceph::decode;
   decode(path, bl);
   decode(capinfo, bl);
diff --git a/src/mds/mdstypes.h b/src/mds/mdstypes.h
index aa93b52b65f..f4da1643035 100644
--- a/src/mds/mdstypes.h
+++ b/src/mds/mdstypes.h
@@ -206,7 +206,7 @@ struct frag_info_t : public scatter_info_t {
   }
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<frag_info_t*>& ls);
 };
@@ -269,7 +269,7 @@ struct nest_info_t : public scatter_info_t {
   }
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<nest_info_t*>& ls);
 };
@@ -296,7 +296,7 @@ struct vinodeno_t {
     encode(ino, bl);
     encode(snapid, bl);
   }
-  void decode(bufferlist::iterator& p) {
+  void decode(bufferlist::const_iterator& p) {
     using ceph::decode;
     decode(ino, p);
     decode(snapid, p);
@@ -329,7 +329,7 @@ struct quota_info_t
     encode(max_files, bl);
     ENCODE_FINISH(bl);
   }
-  void decode(bufferlist::iterator& p) {
+  void decode(bufferlist::const_iterator& p) {
     DECODE_START_LEGACY_COMPAT_LEN(1, 1, 1, p);
     decode(max_bytes, p);
     decode(max_files, p);
@@ -392,12 +392,12 @@ struct client_writeable_range_t {
   client_writeable_range_t() {}
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(std::list<client_writeable_range_t*>& ls);
 };
 
-inline void decode(client_writeable_range_t::byte_range_t& range, bufferlist::iterator& bl) {
+inline void decode(client_writeable_range_t::byte_range_t& range, bufferlist::const_iterator& bl) {
   decode(range.first, bl);
   decode(range.last, bl);
 }
@@ -450,7 +450,7 @@ public:
     return !(*this == o);
   }
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
 };
 WRITE_CLASS_ENCODER(inline_data_t)
 
@@ -611,7 +611,7 @@ struct inode_t {
   }
 
   void encode(bufferlist &bl, uint64_t features) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(std::list<inode_t*>& ls);
   /**
@@ -691,7 +691,7 @@ void inode_t<Allocator>::encode(bufferlist &bl, uint64_t features) const
 }
 
 template<template<typename> class Allocator>
-void inode_t<Allocator>::decode(bufferlist::iterator &p)
+void inode_t<Allocator>::decode(bufferlist::const_iterator &p)
 {
   DECODE_START_LEGACY_COMPAT_LEN(15, 6, 6, p);
 
@@ -931,7 +931,7 @@ inline void encode(const inode_t<Allocator> &c, ::ceph::bufferlist &bl, uint64_t
   ENCODE_DUMP_POST(cl);
 }
 template<template<typename> class Allocator>
-inline void decode(inode_t<Allocator> &c, ::ceph::bufferlist::iterator &p)
+inline void decode(inode_t<Allocator> &c, ::ceph::bufferlist::const_iterator &p)
 {
   c.decode(p);
 }
@@ -952,7 +952,7 @@ struct old_inode_t {
   xattr_map<Allocator> xattrs;
 
   void encode(bufferlist &bl, uint64_t features) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(std::list<old_inode_t*>& ls);
 };
@@ -969,7 +969,7 @@ void old_inode_t<Allocator>::encode(bufferlist& bl, uint64_t features) const
 }
 
 template<template<typename> class Allocator>
-void old_inode_t<Allocator>::decode(bufferlist::iterator& bl)
+void old_inode_t<Allocator>::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(first, bl);
@@ -1012,7 +1012,7 @@ inline void encode(const old_inode_t<Allocator> &c, ::ceph::bufferlist &bl, uint
   ENCODE_DUMP_POST(cl);
 }
 template<template<typename> class Allocator>
-inline void decode(old_inode_t<Allocator> &c, ::ceph::bufferlist::iterator &p)
+inline void decode(old_inode_t<Allocator> &c, ::ceph::bufferlist::const_iterator &p)
 {
   c.decode(p);
 }
@@ -1036,7 +1036,7 @@ struct fnode_t {
   utime_t localized_scrub_stamp;
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<fnode_t*>& ls);
   fnode_t() {}
@@ -1049,7 +1049,7 @@ struct old_rstat_t {
   nest_info_t rstat, accounted_rstat;
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& p);
+  void decode(bufferlist::const_iterator& p);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<old_rstat_t*>& ls);
 };
@@ -1084,7 +1084,7 @@ struct session_info_t {
   }
 
   void encode(bufferlist& bl, uint64_t features) const;
-  void decode(bufferlist::iterator& p);
+  void decode(bufferlist::const_iterator& p);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<session_info_t*>& ls);
 };
@@ -1124,7 +1124,7 @@ struct dentry_key_t {
     oss << name << "_" << b;
     key = oss.str();
   }
-  static void decode_helper(bufferlist::iterator& bl, string& nm, snapid_t& sn) {
+  static void decode_helper(bufferlist::const_iterator& bl, string& nm, snapid_t& sn) {
     string key;
     decode(key, bl);
     decode_helper(key, nm, sn);
@@ -1177,7 +1177,7 @@ struct string_snap_t {
   string_snap_t(const char *n, snapid_t s) : name(n), snapid(s) {}
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& p);
+  void decode(bufferlist::const_iterator& p);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<string_snap_t*>& ls);
 };
@@ -1205,7 +1205,7 @@ struct mds_table_pending_t {
   version_t tid = 0;
   mds_table_pending_t() {}
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<mds_table_pending_t*>& ls);
 };
@@ -1225,7 +1225,7 @@ struct metareqid_t {
     encode(name, bl);
     encode(tid, bl);
   }
-  void decode(bufferlist::iterator &p) {
+  void decode(bufferlist::const_iterator &p) {
     using ceph::decode;
     decode(name, p);
     decode(tid, p);
@@ -1288,9 +1288,9 @@ struct cap_reconnect_t {
     flockbl.claim(lb);
   }
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void encode_old(bufferlist& bl) const;
-  void decode_old(bufferlist::iterator& bl);
+  void decode_old(bufferlist::const_iterator& bl);
 
   void dump(Formatter *f) const;
   static void generate_test_instances(list<cap_reconnect_t*>& ls);
@@ -1339,7 +1339,7 @@ struct old_cap_reconnect_t {
     encode(path, bl);
     encode(capinfo, bl);
   }
-  void decode(bufferlist::iterator& bl) {
+  void decode(bufferlist::const_iterator& bl) {
     using ceph::decode;
     decode(path, bl);
     decode(capinfo, bl);
@@ -1363,7 +1363,7 @@ struct dirfrag_t {
     encode(ino, bl);
     encode(frag, bl);
   }
-  void decode(bufferlist::iterator& bl) {
+  void decode(bufferlist::const_iterator& bl) {
     using ceph::decode;
     decode(ino, bl);
     decode(frag, bl);
@@ -1425,18 +1425,18 @@ public:
       vec[i].reset(now);
   }
   void encode(bufferlist &bl) const;
-  void decode(const utime_t &t, bufferlist::iterator &p);
+  void decode(const utime_t &t, bufferlist::const_iterator &p);
   // for dencoder
-  void decode(bufferlist::iterator& p) { utime_t sample; decode(sample, p); }
+  void decode(bufferlist::const_iterator& p) { utime_t sample; decode(sample, p); }
   void dump(Formatter *f);
   static void generate_test_instances(list<inode_load_vec_t*>& ls);
 };
 inline void encode(const inode_load_vec_t &c, bufferlist &bl) { c.encode(bl); }
-inline void decode(inode_load_vec_t & c, const utime_t &t, bufferlist::iterator &p) {
+inline void decode(inode_load_vec_t & c, const utime_t &t, bufferlist::const_iterator &p) {
   c.decode(t, p);
 }
 // for dencoder
-inline void decode(inode_load_vec_t & c, bufferlist::iterator &p) {
+inline void decode(inode_load_vec_t & c, bufferlist::const_iterator &p) {
   utime_t sample;
   c.decode(sample, p);
 }
@@ -1463,7 +1463,7 @@ public:
     }
     ENCODE_FINISH(bl);
   }
-  void decode(const utime_t &t, bufferlist::iterator &p) {
+  void decode(const utime_t &t, bufferlist::const_iterator &p) {
     DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, p);
     for (auto &i : vec) {
       decode(i, t, p);
@@ -1471,7 +1471,7 @@ public:
     DECODE_FINISH(p);
   }
   // for dencoder infrastructure
-  void decode(bufferlist::iterator& p) {
+  void decode(bufferlist::const_iterator& p) {
     utime_t sample;
     decode(sample, p);
   }
@@ -1525,11 +1525,11 @@ public:
 };
 
 inline void encode(const dirfrag_load_vec_t &c, bufferlist &bl) { c.encode(bl); }
-inline void decode(dirfrag_load_vec_t& c, const utime_t &t, bufferlist::iterator &p) {
+inline void decode(dirfrag_load_vec_t& c, const utime_t &t, bufferlist::const_iterator &p) {
   c.decode(t, p);
 }
 // this for dencoder
-inline void decode(dirfrag_load_vec_t& c, bufferlist::iterator &p) {
+inline void decode(dirfrag_load_vec_t& c, bufferlist::const_iterator &p) {
   utime_t sample;
   c.decode(sample, p);
 }
@@ -1565,18 +1565,18 @@ struct mds_load_t {
   
   double mds_load();  // defiend in MDBalancer.cc
   void encode(bufferlist& bl) const;
-  void decode(const utime_t& now, bufferlist::iterator& bl);
+  void decode(const utime_t& now, bufferlist::const_iterator& bl);
   //this one is for dencoder infrastructure
-  void decode(bufferlist::iterator& bl) { utime_t sample; decode(sample, bl); }
+  void decode(bufferlist::const_iterator& bl) { utime_t sample; decode(sample, bl); }
   void dump(Formatter *f) const;
   static void generate_test_instances(list<mds_load_t*>& ls);
 };
 inline void encode(const mds_load_t &c, bufferlist &bl) { c.encode(bl); }
-inline void decode(mds_load_t &c, const utime_t &t, bufferlist::iterator &p) {
+inline void decode(mds_load_t &c, const utime_t &t, bufferlist::const_iterator &p) {
   c.decode(t, p);
 }
 // this one is for dencoder
-inline void decode(mds_load_t &c, bufferlist::iterator &p) {
+inline void decode(mds_load_t &c, bufferlist::const_iterator &p) {
   utime_t sample;
   c.decode(sample, p);
 }
@@ -1648,7 +1648,7 @@ public:
   MDSCacheObjectInfo() {}
 
   void encode(bufferlist& bl) const;
-  void decode(bufferlist::iterator& bl);
+  void decode(bufferlist::const_iterator& bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<MDSCacheObjectInfo*>& ls);
 };
diff --git a/src/mds/snap.cc b/src/mds/snap.cc
index dc913f3ed16..e53daef2c50 100644
--- a/src/mds/snap.cc
+++ b/src/mds/snap.cc
@@ -32,7 +32,7 @@ void SnapInfo::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void SnapInfo::decode(bufferlist::iterator& bl)
+void SnapInfo::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(snapid, bl);
@@ -92,7 +92,7 @@ void snaplink_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void snaplink_t::decode(bufferlist::iterator& bl)
+void snaplink_t::decode(bufferlist::const_iterator& bl)
 {
   DECODE_START_LEGACY_COMPAT_LEN(2, 2, 2, bl);
   decode(ino, bl);
@@ -138,7 +138,7 @@ void sr_t::encode(bufferlist& bl) const
   ENCODE_FINISH(bl);
 }
 
-void sr_t::decode(bufferlist::iterator& p)
+void sr_t::decode(bufferlist::const_iterator& p)
 {
   DECODE_START_LEGACY_COMPAT_LEN(6, 4, 4, p);
   if (struct_v == 2) {
diff --git a/src/mds/snap.h b/src/mds/snap.h
index f96b2870efc..d3a76b0ee9b 100644
--- a/src/mds/snap.h
+++ b/src/mds/snap.h
@@ -32,7 +32,7 @@ struct SnapInfo {
   mutable string long_name; ///< cached _$ino_$name
   
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<SnapInfo*>& ls);
 
@@ -59,7 +59,7 @@ struct snaplink_t {
   snapid_t first;
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<snaplink_t*>& ls);
 };
@@ -95,7 +95,7 @@ struct sr_t {
   {}
 
   void encode(bufferlist &bl) const;
-  void decode(bufferlist::iterator &bl);
+  void decode(bufferlist::const_iterator &bl);
   void dump(Formatter *f) const;
   static void generate_test_instances(list<sr_t*>& ls);
 };