Clean up shm_mq cleanup.

The logic around shm_mq_detach was a few bricks shy of a load, because
(contrary to the comments for shm_mq_attach) all it did was update the
shared shm_mq state.  That left us leaking a bit of process-local
memory, but much worse, the on_dsm_detach callback for shm_mq_detach
was still armed.  That means that whenever we ultimately detach from
the DSM segment, we'd run shm_mq_detach again for already-detached,
possibly long-dead queues.  This accidentally fails to fail today,
because we only ever re-use a shm_mq's memory for another shm_mq, and
multiple detach attempts on the last such shm_mq are fairly harmless.
But it's gonna bite us someday, so let's clean it up.

To do that, change shm_mq_detach's API so it takes a shm_mq_handle
not the underlying shm_mq.  This makes the callers simpler in most
cases anyway.  Also fix a few places in parallel.c that were just
pfree'ing the handle structs rather than doing proper cleanup.

Back-patch to v10 because of the risk that the revenant shm_mq_detach
callbacks would cause a live bug sometime.  Since this is an API
change, it's too late to do it in 9.6.  (We could make a variant
patch that preserves API, but I'm not excited enough to do that.)

Discussion: https://postgr.es/m/8670.1504192177@sss.pgh.pa.us

src/backend/access/transam/parallel.c

@@ -480,7 +480,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 			 */
 			any_registrations_failed = true;
 			pcxt->worker[i].bgwhandle = NULL;
-			pfree(pcxt->worker[i].error_mqh);
+			shm_mq_detach(pcxt->worker[i].error_mqh);
 			pcxt->worker[i].error_mqh = NULL;
 		}
 	}
@@ -612,7 +612,7 @@ DestroyParallelContext(ParallelContext *pcxt)
 			{
 				TerminateBackgroundWorker(pcxt->worker[i].bgwhandle);
 
-				pfree(pcxt->worker[i].error_mqh);
+				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
 			}
 		}
@@ -861,7 +861,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 		case 'X':				/* Terminate, indicating clean exit */
 			{
-				pfree(pcxt->worker[i].error_mqh);
+				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
 				break;
 			}

src/backend/executor/tqueue.c

@@ -578,7 +578,9 @@ tqueueShutdownReceiver(DestReceiver *self)
 {
 	TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
 
-	shm_mq_detach(shm_mq_get_queue(tqueue->queue));
+	if (tqueue->queue != NULL)
+		shm_mq_detach(tqueue->queue);
+	tqueue->queue = NULL;
 }
 
 /*
@@ -589,6 +591,9 @@ tqueueDestroyReceiver(DestReceiver *self)
 {
 	TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
 
+	/* We probably already detached from queue, but let's be sure */
+	if (tqueue->queue != NULL)
+		shm_mq_detach(tqueue->queue);
 	if (tqueue->tmpcontext != NULL)
 		MemoryContextDelete(tqueue->tmpcontext);
 	if (tqueue->recordhtab != NULL)
@@ -650,7 +655,7 @@ CreateTupleQueueReader(shm_mq_handle *handle, TupleDesc tupledesc)
 void
 DestroyTupleQueueReader(TupleQueueReader *reader)
 {
-	shm_mq_detach(shm_mq_get_queue(reader->queue));
+	shm_mq_detach(reader->queue);
 	if (reader->typmodmap != NULL)
 		hash_destroy(reader->typmodmap);
 	/* Is it worth trying to free substructure of the remap tree? */

src/backend/libpq/pqmq.c

@@ -21,7 +21,6 @@
 #include "tcop/tcopprot.h"
 #include "utils/builtins.h"
 
-static shm_mq *pq_mq;
 static shm_mq_handle *pq_mq_handle;
 static bool pq_mq_busy = false;
 static pid_t pq_mq_parallel_master_pid = 0;
@@ -56,7 +55,6 @@ void
 pq_redirect_to_shm_mq(dsm_segment *seg, shm_mq_handle *mqh)
 {
 	PqCommMethods = &PqCommMqMethods;
-	pq_mq = shm_mq_get_queue(mqh);
 	pq_mq_handle = mqh;
 	whereToSendOutput = DestRemote;
 	FrontendProtocol = PG_PROTOCOL_LATEST;
@@ -70,7 +68,6 @@ pq_redirect_to_shm_mq(dsm_segment *seg, shm_mq_handle *mqh)
 static void
 pq_cleanup_redirect_to_shm_mq(dsm_segment *seg, Datum arg)
 {
-	pq_mq = NULL;
 	pq_mq_handle = NULL;
 	whereToSendOutput = DestNone;
 }
@@ -135,9 +132,8 @@ mq_putmessage(char msgtype, const char *s, size_t len)
 	 */
 	if (pq_mq_busy)
 	{
-		if (pq_mq != NULL)
-			shm_mq_detach(pq_mq);
-		pq_mq = NULL;
+		if (pq_mq_handle != NULL)
+			shm_mq_detach(pq_mq_handle);
 		pq_mq_handle = NULL;
 		return EOF;
 	}
@@ -148,7 +144,7 @@ mq_putmessage(char msgtype, const char *s, size_t len)
 	 * be generated late in the shutdown sequence, after all DSMs have already
 	 * been detached.
 	 */
-	if (pq_mq == NULL)
+	if (pq_mq_handle == NULL)
 		return 0;
 
 	pq_mq_busy = true;

src/backend/storage/ipc/shm_mq.c

@@ -83,7 +83,9 @@ struct shm_mq
  * This structure is a backend-private handle for access to a queue.
  *
  * mqh_queue is a pointer to the queue we've attached, and mqh_segment is
- * a pointer to the dynamic shared memory segment that contains it.
+ * an optional pointer to the dynamic shared memory segment that contains it.
+ * (If mqh_segment is provided, we register an on_dsm_detach callback to
+ * make sure we detach from the queue before detaching from DSM.)
  *
  * If this queue is intended to connect the current process with a background
  * worker that started it, the user can pass a pointer to the worker handle
@@ -139,6 +141,7 @@ struct shm_mq_handle
 	MemoryContext mqh_context;
 };
 
+static void shm_mq_detach_internal(shm_mq *mq);
 static shm_mq_result shm_mq_send_bytes(shm_mq_handle *mq, Size nbytes,
 				  const void *data, bool nowait, Size *bytes_written);
 static shm_mq_result shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed,
@@ -288,14 +291,15 @@ shm_mq_attach(shm_mq *mq, dsm_segment *seg, BackgroundWorkerHandle *handle)
 	Assert(mq->mq_receiver == MyProc || mq->mq_sender == MyProc);
 	mqh->mqh_queue = mq;
 	mqh->mqh_segment = seg;
-	mqh->mqh_buffer = NULL;
 	mqh->mqh_handle = handle;
+	mqh->mqh_buffer = NULL;
 	mqh->mqh_buflen = 0;
 	mqh->mqh_consume_pending = 0;
-	mqh->mqh_context = CurrentMemoryContext;
 	mqh->mqh_partial_bytes = 0;
+	mqh->mqh_expected_bytes = 0;
 	mqh->mqh_length_word_complete = false;
 	mqh->mqh_counterparty_attached = false;
+	mqh->mqh_context = CurrentMemoryContext;
 
 	if (seg != NULL)
 		on_dsm_detach(seg, shm_mq_detach_callback, PointerGetDatum(mq));
@@ -765,7 +769,28 @@ shm_mq_wait_for_attach(shm_mq_handle *mqh)
 }
 
 /*
- * Detach a shared message queue.
+ * Detach from a shared message queue, and destroy the shm_mq_handle.
+ */
+void
+shm_mq_detach(shm_mq_handle *mqh)
+{
+	/* Notify counterparty that we're outta here. */
+	shm_mq_detach_internal(mqh->mqh_queue);
+
+	/* Cancel on_dsm_detach callback, if any. */
+	if (mqh->mqh_segment)
+		cancel_on_dsm_detach(mqh->mqh_segment,
+							 shm_mq_detach_callback,
+							 PointerGetDatum(mqh->mqh_queue));
+
+	/* Release local memory associated with handle. */
+	if (mqh->mqh_buffer != NULL)
+		pfree(mqh->mqh_buffer);
+	pfree(mqh);
+}
+
+/*
+ * Notify counterparty that we're detaching from shared message queue.
  *
  * The purpose of this function is to make sure that the process
  * with which we're communicating doesn't block forever waiting for us to
@@ -773,9 +798,13 @@ shm_mq_wait_for_attach(shm_mq_handle *mqh)
  * detaches, the receiver can read any messages remaining in the queue;
  * further reads will return SHM_MQ_DETACHED.  If the receiver detaches,
  * further attempts to send messages will likewise return SHM_MQ_DETACHED.
+ *
+ * This is separated out from shm_mq_detach() because if the on_dsm_detach
+ * callback fires, we only want to do this much.  We do not try to touch
+ * the local shm_mq_handle, as it may have been pfree'd already.
  */
-void
-shm_mq_detach(shm_mq *mq)
+static void
+shm_mq_detach_internal(shm_mq *mq)
 {
 	volatile shm_mq *vmq = mq;
 	PGPROC	   *victim;
@@ -1193,5 +1222,5 @@ shm_mq_detach_callback(dsm_segment *seg, Datum arg)
 {
 	shm_mq	   *mq = (shm_mq *) DatumGetPointer(arg);
 
-	shm_mq_detach(mq);
+	shm_mq_detach_internal(mq);
 }

src/include/storage/shm_mq.h

@@ -62,8 +62,8 @@ extern shm_mq_handle *shm_mq_attach(shm_mq *mq, dsm_segment *seg,
 /* Associate worker handle with shm_mq. */
 extern void shm_mq_set_handle(shm_mq_handle *, BackgroundWorkerHandle *);
 
-/* Break connection. */
-extern void shm_mq_detach(shm_mq *);
+/* Break connection, release handle resources. */
+extern void shm_mq_detach(shm_mq_handle *mqh);
 
 /* Get the shm_mq from handle. */
 extern shm_mq *shm_mq_get_queue(shm_mq_handle *mqh);