Switch memory contexts in ReinitializeParallelDSM.

robertmhaas · jeevanchalke · robertmhaas · commit bd2726d3e15f · 2025-12-16T10:59:01.000-05:00
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
@@ -492,8 +492,12 @@ 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)
 	{
@@ -531,6 +535,9 @@ 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
`@@ -492,8 +492,12 @@ InitializeParallelDSM(ParallelContext *pcxt)`
`492`	`492`	`void`
`493`	`493`	`ReinitializeParallelDSM(ParallelContext *pcxt)`
`494`	`494`	`{`
	`495`	`+ MemoryContext oldcontext;`
`495`	`496`	`FixedParallelState *fps;`
`496`	`497`
	`498`	`+ /* We might be running in a very short-lived memory context. */`
	`499`	`+ oldcontext = MemoryContextSwitchTo(TopTransactionContext);`
	`500`	`+`
`497`	`501`	`/* Wait for any old workers to exit. */`
`498`	`502`	`if (pcxt->nworkers_launched > 0)`
`499`	`503`	`{`
`@@ -531,6 +535,9 @@ ReinitializeParallelDSM(ParallelContext *pcxt)`
`531`	`535`	`pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);`
`532`	`536`	`}`
`533`	`537`	`}`
	`538`	`+`
	`539`	`+ /* Restore previous memory context. */`
	`540`	`+ MemoryContextSwitchTo(oldcontext);`
`534`	`541`	`}`
`535`	`542`
`536`	`543`	`/*`