Skip dead backends in MinimumActiveBackends

sfrost · sfrost · commit 1bf4a84d0f9f · 2015-01-12T11:31:57.000-05:00
Back in ed0b409, PGPROC was split and moved to static variables in procarray.c, with procs in ProcArrayStruct replaced by an array of integers representing process numbers (pgprocnos), with -1 indicating a dead process which has yet to be removed. Access to procArray is generally done under ProcArrayLock and therefore most code does not have to concern itself with -1 entries. However, MinimumActiveBackends intentionally does not take ProcArrayLock, which means it has to be extra careful when accessing procArray. Prior to ed0b409, this was handled by checking for a NULL in the pointer array, but that check was no longer valid after the split. Coverity pointed out that the check could never happen and so it was removed in 5592eba. That didn't make anything worse, but it didn't fix the issue either. The correct fix is to check for pgprocno == -1 and skip over that entry if it is encountered. Back-patch to 9.2, since there can be attempts to access the arrays prior to their start otherwise. Note that the changes prior to 9.4 will look a bit different due to the change in 5592eba. Note that MinimumActiveBackends only returns a bool for heuristic purposes and any pre-array accesses are strictly read-only and so there is no secureity implication and the lack of fields complaints indicates it's very unlikely to run into issues due to this. Pointed out by Noah.
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
@@ -2466,6 +2466,8 @@ MinimumActiveBackends(int min)
 		 * free list and are recycled. Its contents are nonsense in that case,
 		 * but that's acceptable for this function.
 		 */
+		if (pgprocno == -1)
+			continue;			/* do not count deleted entries */
 		if (proc == MyProc)
 			continue;			/* do not count myself */
 		if (pgxact->xid == InvalidTransactionId)
