os/bluestore/BlueFS: prevent _compact_log_async reentry

_should_compact_log uses new_log != nullptr to tell whether compaction is
already in progress, but we don't set it until we are midway through the
process.  Set it at the top of the method to prevent reentry.

See 455cc6c, which failed to implement
this properly.

Fixes: http://tracker.ceph.com/issues/21250
Signed-off-by: Sage Weil <[email protected]>

src/os/bluestore/BlueFS.cc

@@ -1187,6 +1187,11 @@ void BlueFS::_compact_log_async(std::unique_lock<std::mutex>& l)
   assert(!new_log);
   assert(!new_log_writer);
 
+  // create a new log [writer] so that we know compaction is in progress
+  // (see _should_compact_log)
+  new_log = new File;
+  new_log->fnode.ino = 0;   // so that _flush_range won't try to log the fnode
+
   // 1. allocate new log space and jump to it.
   old_log_jump_to = log_file->fnode.get_allocated();
   uint64_t need = old_log_jump_to + cct->_conf->bluefs_max_log_runway;
@@ -1228,9 +1233,7 @@ void BlueFS::_compact_log_async(std::unique_lock<std::mutex>& l)
   dout(10) << __func__ << " new_log_jump_to 0x" << std::hex << new_log_jump_to
 	   << std::dec << dendl;
 
-  // create a new log [writer]
-  new_log = new File;
-  new_log->fnode.ino = 0;   // so that _flush_range won't try to log the fnode
+  // allocate
   int r = _allocate(BlueFS::BDEV_DB, new_log_jump_to,
                     &new_log->fnode.extents);
   assert(r == 0);