Re: v12.0: segfault in reindex CONCURRENTLY

[Alvaro Herrera]

On 2019-Oct-13, Justin Pryzby wrote:

> Looks like it's a race condition and dereferencing *holder=NULL.  The first
> crash was probably the same bug, due to report query running during "reindex
> CONCURRENTLY", and probably finished at nearly the same time as another 
> locker.

Ooh, right, makes sense.  There's another spot with the same mistake ...
this patch should fix it.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 70f9b6729a..5f4ee86f70 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -388,8 +388,9 @@ WaitForOlderSnapshots(TransactionId limitXmin, bool progress)
 			{
 				PGPROC	   *holder = BackendIdGetProc(old_snapshots[i].backendId);
 
-				pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
-											 holder->pid);
+				if (holder)
+					pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
+												 holder->pid);
 			}
 			VirtualXactLock(old_snapshots[i], true);
 		}
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 4682438114..63c4188efc 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -908,8 +908,9 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 			{
 				PGPROC	   *holder = BackendIdGetProc(lockholders->backendId);
 
-				pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
-											 holder->pid);
+				if (holder)
+					pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
+												 holder->pid);
 			}
 			VirtualXactLock(*lockholders, true);
 			lockholders++;