osd: direct reads on SNAPDIR to either head or snapdir

The list_snaps operation needs to look at the SnapSet, and is logically
querying all revisions of the object.  Make requests to SNAPDIR be
read-only, and grab the head or snapdir obc transparently (whichever one
exists).  This allows us to list snaps when, say, the head does not
exist, but there are in fact snaps.

Signed-off-by: Sage Weil <sage@inktank.com>

diff --git a/src/osd/ReplicatedPG.cc b/src/osd/ReplicatedPG.cc
index 57be884b28d1a2edb7b4c70c294c607201bfb046..add7c4b45863f89e07324de462b5edf34a0ba2ae 100644 (file)
--- a/src/osd/ReplicatedPG.cc
+++ b/src/osd/ReplicatedPG.cc
@@ -673,6 +673,12 @@ void ReplicatedPG::do_op(OpRequestRef op)
     return;
   }
  
+  // asking for SNAPDIR is only ok for reads
+  if (m->get_snapid() == CEPH_SNAPDIR && op->may_write()) {
+    osd->reply_op_error(op, -EINVAL);
+    return;
+  }
+
   entity_inst_t client = m->get_source_inst();
 
   ObjectContext *obc;
@@ -4296,9 +4302,30 @@ int ReplicatedPG::find_object_context(const hobject_t& oid,
                                      bool can_create,
                                      snapid_t *psnapid)
 {
-  // want the head?
   hobject_t head(oid.oid, oid.get_key(), CEPH_NOSNAP, oid.hash,
                 info.pgid.pool());
+  hobject_t snapdir(oid.oid, oid.get_key(), CEPH_SNAPDIR, oid.hash,
+                   info.pgid.pool());
+
+  // want the snapdir?
+  if (oid.snap == CEPH_SNAPDIR) {
+    // return head or snapdir, whichever exists.
+    ObjectContext *obc = get_object_context(head, oloc, can_create);
+    if (!obc) {
+      obc = get_object_context(snapdir, oloc, can_create);
+    }
+    if (!obc)
+      return -ENOENT;
+    dout(10) << "find_object_context " << oid << " @" << oid.snap << dendl;
+    *pobc = obc;
+
+    // always populate ssc for SNAPDIR...
+    if (!obc->ssc)
+      obc->ssc = get_snapset_context(oid.oid, oid.get_key(), oid.hash, true);
+    return 0;
+  }
+
+  // want the head?
   if (oid.snap == CEPH_NOSNAP) {
     ObjectContext *obc = get_object_context(head, oloc, can_create);
     if (!obc)