Switch memory contexts in ReinitializeParallelDSM.

robertmhaas · jeevanchalke · robertmhaas · commit 12c2f843cdda · 2025-12-16T10:59:02.000-05:00
We already do this in CreateParallelContext, InitializeParallelDSM, and LaunchParallelWorkers. I suspect the reason why the matching logic was omitted from ReinitializeParallelDSM is that I failed to realize that any memory allocation was happening here -- but shm_mq_attach does allocate, which could result in a shm_mq_handle being allocated in a shorter-lived context than the ParallelContext which points to it. That could result in a crash if the shorter-lived context is freed before the parallel context is destroyed. As far as I am currently aware, there is no way to reach a crash using only code that is present in core PostgreSQL, but extensions could potentially trip over this. Fixing this in the back-branches appears low-risk, so back-patch to all supported versions. Author: Jakub Wartak <jakub.wartak@enterprisedb.com> Co-authored-by: Jeevan Chalke <jeevan.chalke@enterprisedb.com> Backpatch-through: 14 Discussion: http://postgr.es/m/CAKZiRmwfVripa3FGo06=5D1EddpsLu9JY2iJOTgbsxUQ339ogQ@mail.gmail.com
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
@@ -503,8 +503,12 @@ InitializeParallelDSM(ParallelContext *pcxt)
 void
 ReinitializeParallelDSM(ParallelContext *pcxt)
 {
+	MemoryContext oldcontext;
 	FixedParallelState *fps;
 
+	/* We might be running in a very short-lived memory context. */
+	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+
 	/* Wait for any old workers to exit. */
 	if (pcxt->nworkers_launched > 0)
 	{
@@ -542,6 +546,9 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
 		}
 	}
+
+	/* Restore previous memory context. */
+	MemoryContextSwitchTo(oldcontext);
 }
 
 /*

Original file line number	Diff line number	Diff line change
`@@ -503,8 +503,12 @@ InitializeParallelDSM(ParallelContext *pcxt)`
`503`	`503`	`void`
`504`	`504`	`ReinitializeParallelDSM(ParallelContext *pcxt)`
`505`	`505`	`{`
	`506`	`+ MemoryContext oldcontext;`
`506`	`507`	`FixedParallelState *fps;`
`507`	`508`
	`509`	`+ /* We might be running in a very short-lived memory context. */`
	`510`	`+ oldcontext = MemoryContextSwitchTo(TopTransactionContext);`
	`511`	`+`
`508`	`512`	`/* Wait for any old workers to exit. */`
`509`	`513`	`if (pcxt->nworkers_launched > 0)`
`510`	`514`	`{`
`@@ -542,6 +546,9 @@ ReinitializeParallelDSM(ParallelContext *pcxt)`
`542`	`546`	`pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);`
`543`	`547`	`}`
`544`	`548`	`}`
	`549`	`+`
	`550`	`+ /* Restore previous memory context. */`
	`551`	`+ MemoryContextSwitchTo(oldcontext);`
`545`	`552`	`}`
`546`	`553`
`547`	`554`	`/*`