Clear MyProc and MyProcSignalState before they become invalid.

robertmhaas · robertmhaas · commit 458b20f2da84 · 2014-01-31T21:41:09.000-05:00
Evidence from buildfarm member crake suggests that the new test_shm_mq
module is routinely crashing the server due to the arrival of a SIGUSR1
after the shared memory segment has been unmapped.  Although processes
using the new dynamic background worker facilities are more likely to
receive a SIGUSR1 around this time, the problem is also possible on older
branches, so I'm back-patching the parts of this change that apply to
older branches as far as they apply.

It's already generally the case that code checks whether these pointers
are NULL before deferencing them, so the important thing is mostly to
make sure that they do get set to NULL before they become invalid.  But
in master, there's one case in procsignal_sigusr1_handler that lacks a
NULL guard, so add that.

Patch by me; review by Tom Lane.
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
@@ -591,6 +591,7 @@ ProcKill(int code, Datum arg)
 {
 	/* use volatile pointer to prevent code rearrangement */
 	volatile PROC_HDR *procglobal = ProcGlobal;
+	PGPROC	   *proc;
 
 	Assert(MyProc != NULL);
 
@@ -601,23 +602,28 @@ ProcKill(int code, Datum arg)
 	 */
 	LWLockReleaseAll();
 
+	/*
+	 * Clear MyProc first before after putting it back on the global list,
+	 * so that signal handlers won't try to access it after it's no longer
+	 * ours.
+	 */
+	proc = MyProc;
+	MyProc = NULL;
+
 	SpinLockAcquire(ProcStructLock);
 
 	/* Return PGPROC structure (and semaphore) to freelist */
 	if (IsAutoVacuumWorkerProcess())
 	{
-		MyProc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
-		procglobal->autovacFreeProcs = MyProc;
+		proc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
+		procglobal->autovacFreeProcs = proc;
 	}
 	else
 	{
-		MyProc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
-		procglobal->freeProcs = MyProc;
+		proc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
+		procglobal->freeProcs = proc;
 	}
 
-	/* PGPROC struct isn't mine anymore */
-	MyProc = NULL;
-
 	/* Update shared estimate of spins_per_delay */
 	procglobal->spins_per_delay = update_spins_per_delay(procglobal->spins_per_delay);
 
@@ -645,6 +651,7 @@ AuxiliaryProcKill(int code, Datum arg)
 {
 	int			proctype = DatumGetInt32(arg);
 	PGPROC	   *auxproc;
+	PGPROC	   *proc;
 
 	Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);
 
@@ -655,13 +662,18 @@ AuxiliaryProcKill(int code, Datum arg)
 	/* Release any LW locks I am holding (see notes above) */
 	LWLockReleaseAll();
 
+	/*
+	 * Clear MyProc first before after putting it back on the global list,
+	 * so that signal handlers won't try to access it after it's no longer
+	 * ours.
+	 */
+	proc = MyProc;
+	MyProc = NULL;
+
 	SpinLockAcquire(ProcStructLock);
 
 	/* Mark auxiliary proc no longer in use */
-	MyProc->pid = 0;
-
-	/* PGPROC struct isn't mine anymore */
-	MyProc = NULL;
+	proc->pid = 0;
 
 	/* Update shared estimate of spins_per_delay */
 	ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);
