Lock database while running drop database in Hot Standby to protect

against concurrent reconnection. Failure during testing showed issue
was possible, even though earlier analysis seemed to indicate it
would not be required. Use LockSharedObjectForSession() before
ResolveRecoveryConflictWithDatabase() and hold lock until end of
processing for that WAL record. Simple approach to avoid introducing
further bugs at this stage of development on an improbable issue.

src/backend/commands/dbcommands.c

@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.232 2010/01/14 11:08:00 sriggs Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.233 2010/01/16 14:16:31 sriggs Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1944,7 +1944,15 @@ dbase_redo(XLogRecPtr lsn, XLogRecord *record)
 		dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_id);
 
 		if (InHotStandby)
+		{
+			/*
+			 * Lock database while we resolve conflicts to ensure that InitPostgres()
+			 * cannot fully re-execute concurrently. This avoids backends re-connecting
+			 * automatically to same database, which can happen in some cases.
+			 */
+			LockSharedObjectForSession(DatabaseRelationId, xlrec->db_id, 0, AccessExclusiveLock);
 			ResolveRecoveryConflictWithDatabase(xlrec->db_id);
+		}
 
 		/* Drop pages for this database that are in the shared buffer cache */
 		DropDatabaseBuffers(xlrec->db_id);
@@ -1960,6 +1968,17 @@ dbase_redo(XLogRecPtr lsn, XLogRecord *record)
 			ereport(WARNING,
 					(errmsg("some useless files may be left behind in old database directory \"%s\"",
 							dst_path)));
+
+		if (InHotStandby)
+		{
+			/*
+			 * Release locks prior to commit. XXX There is a race condition here that may allow
+			 * backends to reconnect, but the window for this is small because the gap between
+			 * here and commit is mostly fairly small and it is unlikely that people will be
+			 * dropping databases that we are trying to connect to anyway.
+			 */
+			UnlockSharedObjectForSession(DatabaseRelationId, xlrec->db_id, 0, AccessExclusiveLock);
+		}
 	}
 	else
 		elog(PANIC, "dbase_redo: unknown op code %u", info);