libpq: Improve idle state handling in pipeline mode

We were going into IDLE state too soon when executing queries via
PQsendQuery in pipeline mode, causing several scenarios to misbehave in
different ways -- most notably, as reported by Daniele Varrazzo, that a
warning message is produced by libpq:
  message type 0x33 arrived from server while idle
But it is also possible, if queries are sent and results consumed not in
lockstep, for the expected mediating NULL result values from PQgetResult
to be lost (a problem which has not been reported, but which is more
serious).

Fix this by introducing two new concepts: one is a command queue element
PGQUERY_CLOSE to tell libpq to wait for the CloseComplete server
response to the Close message that is sent by PQsendQuery.  Because the
application is not expecting any PGresult from this, the mechanism to
consume it is a bit hackish.

The other concept, authored by Horiguchi-san, is a PGASYNC_PIPELINE_IDLE
state for libpq's state machine to differentiate "really idle" from
merely "the idle state that occurs in between reading results from the
server for elements in the pipeline".  This makes libpq not go fully
IDLE when the libpq command queue contains entries; in normal cases, we
only go IDLE once at the end of the pipeline, when the server response
to the final SYNC message is received.  (However, there are corner cases
it doesn't fix, such as terminating the query sequence by
PQsendFlushRequest instead of PQpipelineSync; this sort of scenario is
what requires PGQUERY_CLOSE bit above.)

This last bit helps make the libpq state machine clearer; in particular
we can get rid of an ugly hack in pqParseInput3 to avoid considering
IDLE as such when the command queue contains entries.

A new test mode is added to libpq_pipeline.c to tickle some related
problematic cases.

Reported-by: Daniele Varrazzo <daniele.varrazzo@gmail.com>
Co-authored-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA+mi_8bvD0_CW3sumgwPvWdNzXY32itoG_16tDYRu_1S2gV2iw@mail.gmail.com

1 files changed, 17 insertions, 13 deletions

diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 10c76daf6ed..f267dfd33c5 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -159,18 +159,6 @@ pqParseInput3(PGconn *conn)
 				return;
 
 			/*
-			 * We're also notionally not-IDLE when in pipeline mode the state
-			 * says "idle" (so we have completed receiving the results of one
-			 * query from the server and dispatched them to the application)
-			 * but another query is queued; yield back control to caller so
-			 * that they can initiate processing of the next query in the
-			 * queue.
-			 */
-			if (conn->pipelineStatus != PQ_PIPELINE_OFF &&
-				conn->cmd_queue_head != NULL)
-				return;
-
-			/*
 			 * Unexpected message in IDLE state; need to recover somehow.
 			 * ERROR messages are handled using the notice processor;
 			 * ParameterStatus is handled normally; anything else is just
@@ -296,8 +284,24 @@ pqParseInput3(PGconn *conn)
 					}
 					break;
 				case '2':		/* Bind Complete */
+					/* Nothing to do for this message type */
+					break;
 				case '3':		/* Close Complete */
-					/* Nothing to do for these message types */
+					/*
+					 * If we get CloseComplete when waiting for it, consume
+					 * the queue element and keep going.  A result is not
+					 * expected from this message; it is just there so that
+					 * we know to wait for it when PQsendQuery is used in
+					 * pipeline mode, before going in IDLE state.  Failing to
+					 * do this makes us receive CloseComplete when IDLE, which
+					 * creates problems.
+					 */
+					if (conn->cmd_queue_head &&
+						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
+					{
+						pqCommandQueueAdvance(conn);
+					}
+
 					break;
 				case 'S':		/* parameter status */
 					if (getParameterStatus(conn))