Fix assertion failure in pgbench when handling multiple pipeline sync messages.

Previously, when running pgbench in pipeline mode with a custom script
that triggered retriable errors (e.g., serialization errors),
an assertion failure could occur:

    Assertion failed: (res == ((void*)0)), function discardUntilSync, file pgbench.c, line 3515.

The root cause was that pgbench incorrectly assumed only a single
pipeline sync message would be received at the end. In reality,
multiple pipeline sync messages can be sent and must be handled properly.

This commit fixes the issue by updating pgbench to correctly process
multiple pipeline sync messages, preventing the assertion failure.

Back-patch to v15, where the bug was introduced.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: https://postgr.es/m/CAHGQGwFAX56Tfx+1ppo431OSWiLLuW72HaGzZ39NkLkop6bMzQ@mail.gmail.com
Backpatch-through: 15

diff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.c
index 4c3a6a44f43d526a3ccd4a26b2bb45945f661605..34ac280c1b82e48dec136dd88ec6030c6b8678d0 100644 (file)
--- a/src/bin/pgbench/pgbench.c
+++ b/src/bin/pgbench/pgbench.c
@@ -3459,6 +3459,8 @@ doRetry(CState *st, pg_time_usec_t *now)
 static int
 discardUntilSync(CState *st)
 {
+   bool        received_sync = false;
+
    /* send a sync */
    if (!PQpipelineSync(st->con))
    {
@@ -3473,10 +3475,16 @@ discardUntilSync(CState *st)
        PGresult   *res = PQgetResult(st->con);
 
        if (PQresultStatus(res) == PGRES_PIPELINE_SYNC)
+           received_sync = true;
+       else if (received_sync)
        {
-           PQclear(res);
-           res = PQgetResult(st->con);
+           /*
+            * PGRES_PIPELINE_SYNC must be followed by another
+            * PGRES_PIPELINE_SYNC or NULL; otherwise, assert failure.
+            */
            Assert(res == NULL);
+
+           PQclear(res);
            break;
        }
        PQclear(res);