Centralize decision-making about where to get a backend's PGPROC.

This code was originally written as part of parallel query effort, but
it seems to have independent value, because if we make one decision
about where to get a PGPROC when we allocate and then put it back on a
different list at backend-exit time, bad things happen.  This isn't
just a theoretical risk; we fixed an actual problem of this type in
commit e280c630a87e1b8325770c6073097d109d79a00f.

src/backend/storage/lmgr/proc.c

@@ -242,18 +242,21 @@ InitProcGlobal(void)
 			/* PGPROC for normal backend, add to freeProcs list */
 			procs[i].links.next = (SHM_QUEUE *) ProcGlobal->freeProcs;
 			ProcGlobal->freeProcs = &procs[i];
+			procs[i].procgloballist = &ProcGlobal->freeProcs;
 		}
 		else if (i < MaxConnections + autovacuum_max_workers + 1)
 		{
 			/* PGPROC for AV launcher/worker, add to autovacFreeProcs list */
 			procs[i].links.next = (SHM_QUEUE *) ProcGlobal->autovacFreeProcs;
 			ProcGlobal->autovacFreeProcs = &procs[i];
+			procs[i].procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
 		else if (i < MaxBackends)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			procs[i].links.next = (SHM_QUEUE *) ProcGlobal->bgworkerFreeProcs;
 			ProcGlobal->bgworkerFreeProcs = &procs[i];
+			procs[i].procgloballist = &ProcGlobal->bgworkerFreeProcs;
 		}
 
 		/* Initialize myProcLocks[] shared memory queues. */
@@ -281,6 +284,7 @@ InitProcess(void)
 {
 	/* use volatile pointer to prevent code rearrangement */
 	volatile PROC_HDR *procglobal = ProcGlobal;
+	PGPROC * volatile * procgloballist;
 
 	/*
 	 * ProcGlobal should be set up already (if we are a backend, we inherit
@@ -292,9 +296,17 @@ InitProcess(void)
 	if (MyProc != NULL)
 		elog(ERROR, "you already exist");
 
+	/* Decide which list should supply our PGPROC. */
+	if (IsAnyAutoVacuumProcess())
+		procgloballist = &procglobal->autovacFreeProcs;
+	else if (IsBackgroundWorker)
+		procgloballist = &procglobal->bgworkerFreeProcs;
+	else
+		procgloballist = &procglobal->freeProcs;
+
 	/*
-	 * Try to get a proc struct from the free list.  If this fails, we must be
+	 * Try to get a proc struct from the appropriate free list.  If this
-	 * out of PGPROC structures (not to mention semaphores).
+	 * fails, we must be out of PGPROC structures (not to mention semaphores).
 	 *
 	 * While we are holding the ProcStructLock, also copy the current shared
 	 * estimate of spins_per_delay to local storage.
@@ -303,21 +315,11 @@ InitProcess(void)
 
 	set_spins_per_delay(procglobal->spins_per_delay);
 
-	if (IsAnyAutoVacuumProcess())
+	MyProc = *procgloballist;
-		MyProc = procglobal->autovacFreeProcs;
-	else if (IsBackgroundWorker)
-		MyProc = procglobal->bgworkerFreeProcs;
-	else
-		MyProc = procglobal->freeProcs;
 
 	if (MyProc != NULL)
 	{
-		if (IsAnyAutoVacuumProcess())
+		*procgloballist = (PGPROC *) MyProc->links.next;
-			procglobal->autovacFreeProcs = (PGPROC *) MyProc->links.next;
-		else if (IsBackgroundWorker)
-			procglobal->bgworkerFreeProcs = (PGPROC *) MyProc->links.next;
-		else
-			procglobal->freeProcs = (PGPROC *) MyProc->links.next;
 		SpinLockRelease(ProcStructLock);
 	}
 	else
@@ -335,6 +337,12 @@ InitProcess(void)
 	}
 	MyPgXact = &ProcGlobal->allPgXact[MyProc->pgprocno];
 
+	/*
+	 * Cross-check that the PGPROC is of the type we expect; if this were
+	 * not the case, it would get returned to the wrong list.
+	 */
+	Assert(MyProc->procgloballist == procgloballist);
+
 	/*
 	 * Now that we have a PGPROC, mark ourselves as an active postmaster
 	 * child; this is so that the postmaster can detect it if we exit without
@@ -761,6 +769,7 @@ ProcKill(int code, Datum arg)
 	/* use volatile pointer to prevent code rearrangement */
 	volatile PROC_HDR *procglobal = ProcGlobal;
 	PGPROC	   *proc;
+	PGPROC * volatile * procgloballist;
 
 	Assert(MyProc != NULL);
 
@@ -799,24 +808,12 @@ ProcKill(int code, Datum arg)
 	MyProc = NULL;
 	DisownLatch(&proc->procLatch);
 
+	procgloballist = proc->procgloballist;
 	SpinLockAcquire(ProcStructLock);
 
 	/* Return PGPROC structure (and semaphore) to appropriate freelist */
-	if (IsAnyAutoVacuumProcess())
+	proc->links.next = (SHM_QUEUE *) *procgloballist;
-	{
+	*procgloballist = proc;
-		proc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
-		procglobal->autovacFreeProcs = proc;
-	}
-	else if (IsBackgroundWorker)
-	{
-		proc->links.next = (SHM_QUEUE *) procglobal->bgworkerFreeProcs;
-		procglobal->bgworkerFreeProcs = proc;
-	}
-	else
-	{
-		proc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
-		procglobal->freeProcs = proc;
-	}
 
 	/* Update shared estimate of spins_per_delay */
 	procglobal->spins_per_delay = update_spins_per_delay(procglobal->spins_per_delay);

src/include/storage/proc.h

@@ -78,6 +78,7 @@ struct PGPROC
 {
 	/* proc->links MUST BE FIRST IN STRUCT (see ProcSleep,ProcWakeup,etc) */
 	SHM_QUEUE	links;			/* list link if process is in a list */
+	PGPROC	  **procgloballist;	/* procglobal list that owns this PGPROC */
 
 	PGSemaphoreData sem;		/* ONE semaphore to sleep on */
 	int			waitStatus;		/* STATUS_WAITING, STATUS_OK or STATUS_ERROR */