postgres_fdw: Fix bug in checking of return value of PQsendQuery().

[Fujii Masao]

Hi,

I found that fetch_more_data_begin() in postgres_fdw reports an error when 
PQsendQuery() returns the value less than 0 as follows though PQsendQuery() can 
return only 1 or 0. I think this is  a bug. Attached is the patch that fixes 
this bug. This needs to be back-ported to v14 where async execution was 
supported in postgres_fdw.

        if (PQsendQuery(fsstate->conn, sql) < 0)
                pgfdw_report_error(ERROR, NULL, fsstate->conn, false, 
fsstate->query);

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
From 538f33a17f3623cec54768a7325d64f8e97abe06 Mon Sep 17 00:00:00 2001
From: Fujii Masao <fu...@postgresql.org>
Date: Thu, 21 Jul 2022 22:52:50 +0900
Subject: [PATCH] postgres_fdw: Fix bug in checking of return value of
 PQsendQuery().

When postgres_fdw begins an asynchronous data fetch, it submits FETCH query
by using PQsendQuery(). If PQsendQuery() fails and returns 0, postgres_fdw
should report an error. But, previously, postgres_fdw reported an error
only when the return value is less than 0, though PQsendQuery() never return
the values other than 0 and 1. Therefore postgres_fdw could not handle
the failure to send FETCH query in an asynchronous data fetch.

This commit fixes postgres_fdw so that it reports an error
when PQsendQuery() returns 0.

Back-patch to v14 where asynchronous execution was supported in postgres_fdw.

Author: Fujii Masao
Reviewed-by:
Discussion: https://postgr.es/m/
---
 contrib/postgres_fdw/postgres_fdw.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/contrib/postgres_fdw/postgres_fdw.c 
b/contrib/postgres_fdw/postgres_fdw.c
index f3b93954ee..048db542d3 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -7070,7 +7070,7 @@ fetch_more_data_begin(AsyncRequest *areq)
        snprintf(sql, sizeof(sql), "FETCH %d FROM c%u",
                         fsstate->fetch_size, fsstate->cursor_number);
 
-       if (PQsendQuery(fsstate->conn, sql) < 0)
+       if (!PQsendQuery(fsstate->conn, sql))
                pgfdw_report_error(ERROR, NULL, fsstate->conn, false, 
fsstate->query);
 
        /* Remember that the request is in process */
-- 
2.36.0