Switch memory contexts in ReinitializeParallelDSM.

robertmhaas · jeevanchalke · robertmhaas · commit f1a6e622bd94 · 2025-12-16T12:24:55.000-05:00
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
@@ -508,8 +508,12 @@ 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)
 	{
@@ -547,6 +551,9 @@ 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
`@@ -508,8 +508,12 @@ InitializeParallelDSM(ParallelContext *pcxt)`
`508`	`508`	`void`
`509`	`509`	`ReinitializeParallelDSM(ParallelContext *pcxt)`
`510`	`510`	`{`
	`511`	`+ MemoryContext oldcontext;`
`511`	`512`	`FixedParallelState *fps;`
`512`	`513`
	`514`	`+ /* We might be running in a very short-lived memory context. */`
	`515`	`+ oldcontext = MemoryContextSwitchTo(TopTransactionContext);`
	`516`	`+`
`513`	`517`	`/* Wait for any old workers to exit. */`
`514`	`518`	`if (pcxt->nworkers_launched > 0)`
`515`	`519`	`{`
`@@ -547,6 +551,9 @@ ReinitializeParallelDSM(ParallelContext *pcxt)`
`547`	`551`	`pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);`
`548`	`552`	`}`
`549`	`553`	`}`
	`554`	`+`
	`555`	`+ /* Restore previous memory context. */`
	`556`	`+ MemoryContextSwitchTo(oldcontext);`
`550`	`557`	`}`
`551`	`558`
`552`	`559`	`/*`