crimson/os/alienstore: fix nullptr deref in OnCommit::finish().

`seastar::engine()` is available only for Seastar's threads;
it shouldn't be called outside of a reactor thread.
Unfortunately, this assumption is violated in `AlienStore`
where `OnCommit::finish()`, executed from a finisher thread
of `BlueStore`, calls `alien()` on `seastar::engine()`.
The net effect are crashes like the following one:

```
INFO  2021-09-22 14:26:33,214 [shard 0] osd - operator() writing superblock cluster_fsid 1d8f7908-2ebf-4a91-ae70-f445668c126b osd_fsid 4da9fe9a-1da5-4ea9-aa79-a1178165ede5         [381/1839]
Segmentation fault.
Backtrace:
 0# print_backtrace(std::basic_string_view<char, std::char_traits<char> >) at /home/rzarzynski/ceph1/build/../src/crimson/common/fatal_signal.cc:80
 1# FatalSignal::signaled(int, siginfo_t const&) at /opt/rh/gcc-toolset-9/root/usr/include/c++/9/ostream:570
 2# FatalSignal::install_oneshot_signal_handler<11>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) at /home/rzarzynski/ceph1/build/../src/crimson/common/fatal_signal.cc:
62
 3# 0x00007F16BBA13B30 in /lib64/libpthread.so.0
 4# (anonymous namespace)::OnCommit::finish(int) at /home/rzarzynski/ceph1/build/../src/crimson/os/alienstore/alien_store.cc:53
 5# Context::complete(int) at /home/rzarzynski/ceph1/build/../src/include/Context.h:100
 6# Finisher::finisher_thread_entry() at /home/rzarzynski/ceph1/build/../src/common/Finisher.cc:65
 7# 0x00007F16BBA0915A in /lib64/libpthread.so.0
 8# clone in /lib64/libc.so.6
Dump of siginfo:
  ...
  si_addr: 0x10
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>

diff --git a/src/crimson/os/alienstore/alien_store.cc b/src/crimson/os/alienstore/alien_store.cc
index c9dd14501d8a44dd26bc26cb62e72eb060c85666..719a3d1b63fd509e6a239986eaced3aa79d64529 100644 (file)
--- a/src/crimson/os/alienstore/alien_store.cc
+++ b/src/crimson/os/alienstore/alien_store.cc
@@ -39,18 +39,23 @@ class OnCommit final: public Context
 {
   const int cpuid;
   Context *oncommit;
+  seastar::alien::instance &alien;
   seastar::promise<> &alien_done;
 public:
   OnCommit(
     int id,
     seastar::promise<> &done,
     Context *oncommit,
+    seastar::alien::instance &alien,
     ceph::os::Transaction& txn)
-    : cpuid(id), oncommit(oncommit),
-      alien_done(done) {}
+    : cpuid(id),
+      oncommit(oncommit),
+      alien(alien),
+      alien_done(done) {
+  }
 
   void finish(int) final {
-    return seastar::alien::submit_to(seastar::engine().alien(), cpuid, [this] {
+    return seastar::alien::submit_to(alien, cpuid, [this] {
       if (oncommit) {
         oncommit->complete(0);
       }
@@ -425,9 +430,9 @@ seastar::future<> AlienStore::do_transaction(CollectionRef ch,
            ceph::os::Transaction::collect_all_contexts(txn);
          assert(tp);
          return tp->submit(ch->get_cid().hash_to_shard(tp->size()),
-           [this, ch, id, crimson_wrapper, &txn, &done] {
+           [this, ch, id, crimson_wrapper, &txn, &done, &alien=seastar::engine().alien()] {
            txn.register_on_commit(new OnCommit(id, done, crimson_wrapper,
-                                               txn));
+                                               alien, txn));
            auto c = static_cast<AlienCollection*>(ch.get());
            return store->queue_transaction(c->collection, std::move(txn));
          });