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

1 files changed, 36 insertions, 7 deletions

diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index f45a67cc278..770559a03e3 100644
--- a/src/backend/storage/ipc/shm_mq.c
+++ b/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);
 }