5 files changed, 30 insertions, 14 deletions
diff --git a/src/backend/libpq/pqmq.c b/src/backend/libpq/pqmq.c
index 0a3c2b70cbe..549e76dfa06 100644
--- a/src/backend/libpq/pqmq.c
+++ b/src/backend/libpq/pqmq.c
@@ -172,8 +172,8 @@ mq_putmessage(char msgtype, const char *s, size_t len)
 			break;
 
 		WaitLatch(&MyProc->procLatch, WL_LATCH_SET, 0);
-		CHECK_FOR_INTERRUPTS();
 		ResetLatch(&MyProc->procLatch);
+		CHECK_FOR_INTERRUPTS();
 	}
 
 	pq_mq_busy = false;
diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index f88e624a413..65348dca97b 100644
--- a/src/backend/storage/ipc/shm_mq.c
+++ b/src/backend/storage/ipc/shm_mq.c
@@ -868,11 +868,11 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
 			 */
 			WaitLatch(MyLatch, WL_LATCH_SET, 0);
 
-			/* An interrupt may have occurred while we were waiting. */
-			CHECK_FOR_INTERRUPTS();
-
 			/* Reset the latch so we don't spin. */
 			ResetLatch(MyLatch);
+
+			/* An interrupt may have occurred while we were waiting. */
+			CHECK_FOR_INTERRUPTS();
 		}
 		else
 		{
@@ -965,11 +965,11 @@ shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed, bool nowait,
 		 */
 		WaitLatch(MyLatch, WL_LATCH_SET, 0);
 
-		/* An interrupt may have occurred while we were waiting. */
-		CHECK_FOR_INTERRUPTS();
-
 		/* Reset the latch so we don't spin. */
 		ResetLatch(MyLatch);
+
+		/* An interrupt may have occurred while we were waiting. */
+		CHECK_FOR_INTERRUPTS();
 	}
 }
 
@@ -1071,11 +1071,11 @@ shm_mq_wait_internal(volatile shm_mq *mq, PGPROC *volatile * ptr,
 			/* Wait to be signalled. */
 			WaitLatch(MyLatch, WL_LATCH_SET, 0);
 
-			/* An interrupt may have occurred while we were waiting. */
-			CHECK_FOR_INTERRUPTS();
-
 			/* Reset the latch so we don't spin. */
 			ResetLatch(MyLatch);
+
+			/* An interrupt may have occurred while we were waiting. */
+			CHECK_FOR_INTERRUPTS();
 		}
 	}
 	PG_CATCH();
diff --git a/src/include/storage/latch.h b/src/include/storage/latch.h
index 28fc684d240..273836d5530 100644
--- a/src/include/storage/latch.h
+++ b/src/include/storage/latch.h
@@ -52,6 +52,22 @@
  * do. Otherwise, if someone sets the latch between the check and the
  * ResetLatch call, you will miss it and Wait will incorrectly block.
  *
+ * Another valid coding pattern looks like:
+ *
+ * for (;;)
+ * {
+ *	   if (work to do)
+ *		   Do Stuff(); // in particular, exit loop if some condition satisfied
+ *	   WaitLatch();
+ *	   ResetLatch();
+ * }
+ *
+ * This is useful to reduce latch traffic if it's expected that the loop's
+ * termination condition will often be satisfied in the first iteration;
+ * the cost is an extra loop iteration before blocking when it is not.
+ * What must be avoided is placing any checks for asynchronous events after
+ * WaitLatch and before ResetLatch, as that creates a race condition.
+ *
  * To wake up the waiter, you must first set a global flag or something
  * else that the wait loop tests in the "if (work to do)" part, and call
  * SetLatch *after* that. SetLatch is designed to return quickly if the
diff --git a/src/test/modules/test_shm_mq/setup.c b/src/test/modules/test_shm_mq/setup.c
index 7f2f5fd3ff7..061af8b4676 100644
--- a/src/test/modules/test_shm_mq/setup.c
+++ b/src/test/modules/test_shm_mq/setup.c
@@ -287,11 +287,11 @@ wait_for_workers_to_become_ready(worker_state *wstate,
 			/* Wait to be signalled. */
 			WaitLatch(MyLatch, WL_LATCH_SET, 0);
 
-			/* An interrupt may have occurred while we were waiting. */
-			CHECK_FOR_INTERRUPTS();
-
 			/* Reset the latch so we don't spin. */
 			ResetLatch(MyLatch);
+
+			/* An interrupt may have occurred while we were waiting. */
+			CHECK_FOR_INTERRUPTS();
 		}
 	}
 	PG_CATCH();
diff --git a/src/test/modules/test_shm_mq/test.c b/src/test/modules/test_shm_mq/test.c
index 732376daff2..b36b4bf1293 100644
--- a/src/test/modules/test_shm_mq/test.c
+++ b/src/test/modules/test_shm_mq/test.c
@@ -231,8 +231,8 @@ test_shm_mq_pipelined(PG_FUNCTION_ARGS)
 			 * for us to do.
 			 */
 			WaitLatch(MyLatch, WL_LATCH_SET, 0);
-			CHECK_FOR_INTERRUPTS();
 			ResetLatch(MyLatch);
+			CHECK_FOR_INTERRUPTS();
 		}
 	}