Hopefully make libpq_pipeline's new cancel test more reliable

The newly introduced cancel test in libpq_pipeline was flaky. It's not
completely clear why, but one option is that the check for "active" was
actually seeing the active state for the previous query. This change
should address any such race condition by first waiting until the
connection is reported as idle.

Author: Jelte Fennema-Nio <me@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQRvmUK5-d68A+cm+fgmfht9Dv2uZ28-qq3QiaF6EAZqPQ@mail.gmail.com

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -114,50 +114,38 @@ confirm_query_canceled_impl(int line, PGconn *conn)
 		PQconsumeInput(conn);
 }
 
-#define send_cancellable_query(conn, monitorConn) \
-	send_cancellable_query_impl(__LINE__, conn, monitorConn)
+/*
+ * Using monitorConn, query pg_stat_activity to see that the connection with
+ * the given PID is in the given state.  We never stop until it does.
+ */
 static void
-send_cancellable_query_impl(int line, PGconn *conn, PGconn *monitorConn)
+wait_for_connection_state(int line, PGconn *monitorConn, int procpid, char *state)
 {
-	const char *env_wait;
-	const Oid	paramTypes[1] = {INT4OID};
-	int			procpid = PQbackendPID(conn);
-
-	env_wait = getenv("PG_TEST_TIMEOUT_DEFAULT");
-	if (env_wait == NULL)
-		env_wait = "180";
+	const Oid	paramTypes[] = {INT4OID, TEXTOID};
+	const char *paramValues[2];
+	char	   *pidstr = psprintf("%d", procpid);
 
-	if (PQsendQueryParams(conn, "SELECT pg_sleep($1)", 1, paramTypes,
-						  &env_wait, NULL, NULL, 0) != 1)
-		pg_fatal_impl(line, "failed to send query: %s", PQerrorMessage(conn));
+	paramValues[0] = pidstr;
+	paramValues[1] = state;
 
-	/*
-	 * Wait until the query is actually running. Otherwise sending a
-	 * cancellation request might not cancel the query due to race conditions.
-	 */
 	while (true)
 	{
-		char	   *value;
 		PGresult   *res;
-		const char *paramValues[1];
-		char		pidval[16];
-
-		snprintf(pidval, 16, "%d", procpid);
-		paramValues[0] = pidval;
+		char	   *value;
 
 		res = PQexecParams(monitorConn,
 						   "SELECT count(*) FROM pg_stat_activity WHERE "
-						   "pid = $1 AND state = 'active'",
-						   1, NULL, paramValues, NULL, NULL, 1);
+						   "pid = $1 AND state = $2",
+						   2, paramTypes, paramValues, NULL, NULL, 1);
 
 		if (PQresultStatus(res) != PGRES_TUPLES_OK)
-			pg_fatal("could not query pg_stat_activity: %s", PQerrorMessage(monitorConn));
+			pg_fatal_impl(line, "could not query pg_stat_activity: %s", PQerrorMessage(monitorConn));
 		if (PQntuples(res) != 1)
-			pg_fatal("unexpected number of rows received: %d", PQntuples(res));
+			pg_fatal_impl(line, "unexpected number of rows received: %d", PQntuples(res));
 		if (PQnfields(res) != 1)
-			pg_fatal("unexpected number of columns received: %d", PQnfields(res));
+			pg_fatal_impl(line, "unexpected number of columns received: %d", PQnfields(res));
 		value = PQgetvalue(res, 0, 0);
-		if (*value != '0')
+		if (value[0] != '0')
 		{
 			PQclear(res);
 			break;
@@ -167,6 +155,38 @@ send_cancellable_query_impl(int line, PGconn *conn, PGconn *monitorConn)
 		/* wait 10ms before polling again */
 		pg_usleep(10000);
 	}
+
+	pfree(pidstr);
+}
+
+#define send_cancellable_query(conn, monitorConn) \
+	send_cancellable_query_impl(__LINE__, conn, monitorConn)
+static void
+send_cancellable_query_impl(int line, PGconn *conn, PGconn *monitorConn)
+{
+	const char *env_wait;
+	const Oid	paramTypes[1] = {INT4OID};
+
+	/*
+	 * Wait for the connection to be idle, so that our check for an active
+	 * connection below is reliable, instead of possibly seeing an outdated
+	 * state.
+	 */
+	wait_for_connection_state(line, monitorConn, PQbackendPID(conn), "idle");
+
+	env_wait = getenv("PG_TEST_TIMEOUT_DEFAULT");
+	if (env_wait == NULL)
+		env_wait = "180";
+
+	if (PQsendQueryParams(conn, "SELECT pg_sleep($1)", 1, paramTypes,
+						  &env_wait, NULL, NULL, 0) != 1)
+		pg_fatal_impl(line, "failed to send query: %s", PQerrorMessage(conn));
+
+	/*
+	 * Wait for the query to start, because if the query is not running yet
+	 * the cancel request that we send won't have any effect.
+	 */
+	wait_for_connection_state(line, monitorConn, PQbackendPID(conn), "active");
 }
 
 /*