[Denis Laxalde <denis.laxalde@dalibo.com>]

Hello,

I'm trying to make single-row mode and pipeline mode work together in 
Psycopg using libpq. I think there is something wrong with respect to 
the single-row mode flag, not being correctly reset, in some situations.

The minimal case I'm considering is (in a pipeline):
* send query 1,
* get its results in single-row mode,
* send query 2,
* get its results *not* in single-row mode.

It seems that, as the command queue in the pipeline is empty after 
getting the results of query 1, the single-row mode flag is not reset 
and is still active for query 2, thus leading to an unexpected 
PGRES_SINGLE_TUPLE status.

The attached patch demonstrates this in the test suite. It also suggests 
to move the statement resetting single-row mode up in 
pqPipelineProcessQueue(), before exiting the function when the command 
queue is empty in particular.

Thanks for considering,
Denis

Attachments:

0001-Reset-single-row-processing-mode-at-end-of-pipeline-.patch (text/x-patch, 5.3K) download
From de28d9ac1e541c3b3a0279dc58fb5a7f23f775f8 Mon Sep 17 00:00:00 2001
From: Denis Laxalde <denis.laxalde@dalibo.com>
Date: Fri, 7 Oct 2022 14:00:24 +0200
Subject: [PATCH] Reset single-row processing mode at end of pipeline commands
 queue

Previously, we'd reset the single-row mode only when processing results
of the next command in a pipeline and not if there's no more commands in
the queue. But if the client emits a query, retrieves its results in
single-row mode (thus setting single-row mode flag), then emits another
query and wants to retrieve its result in "normal" mode, the single-row
mode was not reset in pqPipelineProcessQueue(); this lead to an
erroneous PGRES_SINGLE_TUPLE instead of, e.g., PGRES_TUPLES_OK.

We fix this by resetting the single-row mode *even* if the command queue
is empty in pqPipelineProcessQueue().
---
 src/interfaces/libpq/fe-exec.c                | 12 +++---
 .../modules/libpq_pipeline/libpq_pipeline.c   | 43 ++++++++++++++++++-
 .../libpq_pipeline/traces/singlerow.trace     | 20 +++++++++
 3 files changed, 68 insertions(+), 7 deletions(-)

diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index 0274c1b156..64701b562b 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -3096,6 +3096,12 @@ pqPipelineProcessQueue(PGconn *conn)
 			break;
 	}
 
+	/*
+	 * Reset single-row processing mode.  (Client has to set it up for each
+	 * query, if desired.)
+	 */
+	conn->singleRowMode = false;
+
 	/*
 	 * If there are no further commands to process in the queue, get us in
 	 * "real idle" mode now.
@@ -3115,12 +3121,6 @@ pqPipelineProcessQueue(PGconn *conn)
 	/* Initialize async result-accumulation state */
 	pqClearAsyncResult(conn);
 
-	/*
-	 * Reset single-row processing mode.  (Client has to set it up for each
-	 * query, if desired.)
-	 */
-	conn->singleRowMode = false;
-
 	if (conn->pipelineStatus == PQ_PIPELINE_ABORTED &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SYNC)
 	{
diff --git a/src/test/modules/libpq_pipeline/libpq_pipeline.c b/src/test/modules/libpq_pipeline/libpq_pipeline.c
index c609f42258..4b9ccbfb4c 100644
--- a/src/test/modules/libpq_pipeline/libpq_pipeline.c
+++ b/src/test/modules/libpq_pipeline/libpq_pipeline.c
@@ -1153,11 +1153,11 @@ test_singlerowmode(PGconn *conn)
 	int			i;
 	bool		pipeline_ended = false;
 
-	/* 1 pipeline, 3 queries in it */
 	if (PQenterPipelineMode(conn) != 1)
 		pg_fatal("failed to enter pipeline mode: %s",
 				 PQerrorMessage(conn));
 
+	/* One series of three commands, using single-row mode for the first two. */
 	for (i = 0; i < 3; i++)
 	{
 		char	   *param[1];
@@ -1249,6 +1249,47 @@ test_singlerowmode(PGconn *conn)
 			pg_fatal("didn't get expected terminating TUPLES_OK");
 	}
 
+	/*
+	 * Now issue one command, get its results in with single-row mode, then
+	 * issue another command, and get its results in normal mode; make sure
+	 * the single-row mode flag is reset as expected.
+	 */
+	if (PQsendQueryParams(conn, "SELECT generate_series(0, 0)", 0, NULL, NULL, NULL, NULL, 0) != 1)
+			pg_fatal("failed to send query: %s",
+					 PQerrorMessage(conn));
+	if (PQsendFlushRequest(conn) != 1)
+		pg_fatal("failed to send flush request");
+	if (PQsetSingleRowMode(conn) != 1)
+		pg_fatal("PQsetSingleRowMode() failed");
+	res = PQgetResult(conn);
+	if (res == NULL)
+		pg_fatal("unexpected NULL");
+	if (PQresultStatus(res) != PGRES_SINGLE_TUPLE)
+		pg_fatal("Expected PGRES_SINGLE_TUPLE, got %s",
+				 PQresStatus(PQresultStatus(res)));
+	res = PQgetResult(conn);
+	if (res == NULL)
+		pg_fatal("unexpected NULL");
+	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		pg_fatal("Expected PGRES_TUPLES_OK, got %s",
+				 PQresStatus(PQresultStatus(res)));
+	if (PQgetResult(conn) != NULL)
+		pg_fatal("expected NULL result");
+
+	if (PQsendQueryParams(conn, "SELECT 1", 0, NULL, NULL, NULL, NULL, 0) != 1)
+			pg_fatal("failed to send query: %s",
+					 PQerrorMessage(conn));
+	if (PQsendFlushRequest(conn) != 1)
+		pg_fatal("failed to send flush request");
+	res = PQgetResult(conn);
+	if (res == NULL)
+		pg_fatal("unexpected NULL");
+	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		pg_fatal("Expected PGRES_TUPLES_OK, got %s",
+				 PQresStatus(PQresultStatus(res)));
+	if (PQgetResult(conn) != NULL)
+		pg_fatal("expected NULL result");
+
 	if (PQexitPipelineMode(conn) != 1)
 		pg_fatal("failed to end pipeline mode: %s", PQerrorMessage(conn));
 
diff --git a/src/test/modules/libpq_pipeline/traces/singlerow.trace b/src/test/modules/libpq_pipeline/traces/singlerow.trace
index 9de99befcc..83043e1407 100644
--- a/src/test/modules/libpq_pipeline/traces/singlerow.trace
+++ b/src/test/modules/libpq_pipeline/traces/singlerow.trace
@@ -36,4 +36,24 @@ B	12	DataRow	 1 2 '45'
 B	12	DataRow	 1 2 '46'
 B	13	CommandComplete	 "SELECT 5"
 B	5	ReadyForQuery	 I
+F	36	Parse	 "" "SELECT generate_series(0, 0)" 0
+F	14	Bind	 "" "" 0 0 1 0
+F	6	Describe	 P ""
+F	9	Execute	 "" 0
+F	4	Flush
+B	4	ParseComplete
+B	4	BindComplete
+B	40	RowDescription	 1 "generate_series" NNNN 0 NNNN 4 -1 0
+B	11	DataRow	 1 1 '0'
+B	13	CommandComplete	 "SELECT 1"
+F	16	Parse	 "" "SELECT 1" 0
+F	14	Bind	 "" "" 0 0 1 0
+F	6	Describe	 P ""
+F	9	Execute	 "" 0
+F	4	Flush
+B	4	ParseComplete
+B	4	BindComplete
+B	33	RowDescription	 1 "?column?" NNNN 0 NNNN 4 -1 0
+B	11	DataRow	 1 1 '1'
+B	13	CommandComplete	 "SELECT 1"
 F	4	Terminate
-- 
2.30.2