Fix crash when reporting CREATE INDEX progress

alvherre · alvherre · commit 0d21f919eb86 · 2019-10-16T14:51:34.000+02:00
A race condition can make us try to dereference a NULL pointer to the PGPROC struct of a process that's already finished. That results in crashes during REINDEX CONCURRENTLY and CREATE INDEX CONCURRENTLY. This was introduced in ab0dfc9, so backpatch to pg12. Reported by: Justin Pryzby Reviewed-by: Michaël Paquier Discussion: https://postgr.es/m/20191012004446.GT10470@telsasoft.com
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
@@ -384,12 +384,14 @@ WaitForOlderSnapshots(TransactionId limitXmin, bool progress)
 
 		if (VirtualTransactionIdIsValid(old_snapshots[i]))
 		{
+			/* If requested, publish who we're going to wait for. */
 			if (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
@@ -900,16 +900,14 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 
 		while (VirtualTransactionIdIsValid(*lockholders))
 		{
-			/*
-			 * If requested, publish who we're going to wait for.  This is not
-			 * 100% accurate if they're already gone, but we don't care.
-			 */
+			/* If requested, publish who we're going to wait for. */
 			if (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++;

Original file line number	Diff line number	Diff line change
`@@ -384,12 +384,14 @@ WaitForOlderSnapshots(TransactionId limitXmin, bool progress)`
`384`	`384`
`385`	`385`	`if (VirtualTransactionIdIsValid(old_snapshots[i]))`
`386`	`386`	`{`
	`387`	`+ /* If requested, publish who we're going to wait for. */`
`387`	`388`	`if (progress)`
`388`	`389`	`{`
`389`	`390`	`PGPROC *holder = BackendIdGetProc(old_snapshots[i].backendId);`
`390`	`391`
`391`		`- pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,`
`392`		`- holder->pid);`
	`392`	`+ if (holder)`
	`393`	`+ pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,`
	`394`	`+ holder->pid);`
`393`	`395`	`}`
`394`	`396`	`VirtualXactLock(old_snapshots[i], true);`
`395`	`397`	`}`
Original file line number	Diff line number	Diff line change
`@@ -900,16 +900,14 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)`
`900`	`900`
`901`	`901`	`while (VirtualTransactionIdIsValid(*lockholders))`
`902`	`902`	`{`
`903`		`- /*`
`904`		`- * If requested, publish who we're going to wait for. This is not`
`905`		`- * 100% accurate if they're already gone, but we don't care.`
`906`		`- */`
	`903`	`+ /* If requested, publish who we're going to wait for. */`
`907`	`904`	`if (progress)`
`908`	`905`	`{`
`909`	`906`	`PGPROC *holder = BackendIdGetProc(lockholders->backendId);`
`910`	`907`
`911`		`- pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,`
`912`		`- holder->pid);`
	`908`	`+ if (holder)`
	`909`	`+ pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,`
	`910`	`+ holder->pid);`
`913`	`911`	`}`
`914`	`912`	`VirtualXactLock(*lockholders, true);`
`915`	`913`	`lockholders++;`