Client-side fixes for delayed NOTIFY receipt.

PQnotifies() is defined to just process already-read data, not try to read
any more from the socket.  (This is a debatable decision, perhaps, but I'm
hesitant to change longstanding library behavior.)  The documentation has
long recommended calling PQconsumeInput() before PQnotifies() to ensure
that any already-arrived message would get absorbed and processed.
However, psql did not get that memo, which explains why it's not very
reliable about reporting notifications promptly.

Also, most (not quite all) callers called PQconsumeInput() just once before
a PQnotifies() loop.  Taking this recommendation seriously implies that we
should do PQconsumeInput() before each call.  This is more important now
that we have "payload" strings in notification messages than it was before;
that increases the probability of having more than one packet's worth
of notify messages.  Hence, adjust code as well as documentation examples
to do it like that.

Back-patch to 9.5 to match related server fixes.  In principle we could
probably go back further with these changes, but given lack of field
complaints I doubt it's worthwhile.

Discussion: https://postgr.es/m/CAOYf6ec-TmRYjKBXLLaGaB-jrd=mjG1Hzn1a1wufUAR39PQYhw@mail.gmail.com

5 files changed, 12 insertions, 4 deletions

diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 5e7931ba901..f7b6f918b16 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -5284,7 +5284,7 @@ typedef struct pgNotify
   <para>
    <function>PQnotifies</function> does not actually read data from the
    server; it just returns messages previously absorbed by another
-   <application>libpq</application> function.  In prior releases of
+   <application>libpq</application> function.  In ancient releases of
    <application>libpq</application>, the only way to ensure timely receipt
    of <command>NOTIFY</command> messages was to constantly submit commands, even
    empty ones, and then check <function>PQnotifies</function> after each
@@ -8677,6 +8677,7 @@ main(int argc, char **argv)
                     notify->relname, notify->be_pid);
             PQfreemem(notify);
             nnotifies++;
+            PQconsumeInput(conn);
         }
     }
 
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index b56995925bd..62c2928e6bb 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -836,7 +836,8 @@ PrintNotifications(void)
 {
 	PGnotify   *notify;
 
-	while ((notify = PQnotifies(pset.db)))
+	PQconsumeInput(pset.db);
+	while ((notify = PQnotifies(pset.db)) != NULL)
 	{
 		/* for backward compatibility, only show payload if nonempty */
 		if (notify->extra[0])
@@ -847,6 +848,7 @@ PrintNotifications(void)
 					notify->relname, notify->be_pid);
 		fflush(pset.queryFout);
 		PQfreemem(notify);
+		PQconsumeInput(pset.db);
 	}
 }
 
diff --git a/src/interfaces/ecpg/ecpglib/execute.c b/src/interfaces/ecpg/ecpglib/execute.c
index f5d0359cc24..6ef6df0ea0c 100644
--- a/src/interfaces/ecpg/ecpglib/execute.c
+++ b/src/interfaces/ecpg/ecpglib/execute.c
@@ -1729,12 +1729,13 @@ ecpg_process_output(struct statement *stmt, bool clear_result)
 	}
 
 	/* check for asynchronous returns */
-	notify = PQnotifies(stmt->connection->connection);
-	if (notify)
+	PQconsumeInput(stmt->connection->connection);
+	while ((notify = PQnotifies(stmt->connection->connection)) != NULL)
 	{
 		ecpg_log("ecpg_process_output on line %d: asynchronous notification of \"%s\" from backend PID %d received\n",
 				 stmt->lineno, notify->relname, notify->be_pid);
 		PQfreemem(notify);
+		PQconsumeInput(stmt->connection->connection);
 	}
 
 	return status;
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index 4c0114c514d..93c334b380e 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -2239,6 +2239,9 @@ sendFailed:
  * no unhandled async notification from the backend
  *
  * the CALLER is responsible for FREE'ing the structure returned
+ *
+ * Note that this function does not read any new data from the socket;
+ * so usually, caller should call PQconsumeInput() first.
  */
 PGnotify *
 PQnotifies(PGconn *conn)
diff --git a/src/test/examples/testlibpq2.c b/src/test/examples/testlibpq2.c
index 62ecd68b55e..6cdf8c8631b 100644
--- a/src/test/examples/testlibpq2.c
+++ b/src/test/examples/testlibpq2.c
@@ -140,6 +140,7 @@ main(int argc, char **argv)
 					notify->relname, notify->be_pid);
 			PQfreemem(notify);
 			nnotifies++;
+			PQconsumeInput(conn);
 		}
 	}