[PATCH] pg_upgrade: report the reason for failing to open the cluster version file

Hi Hackers,

The other day I was helping someone with pg_upgrade on IRC, and they got
a rather unhelpful error message:

  ERROR: could not open version file /path/to/new/cluster/PG_VERSION

It would have saved some minutes of debugging time if that had included
the reason why the open failed, so here's a patch to do so.

- ilmari
-- 
- Twitter seems more influential [than blogs] in the 'gets reported in
  the mainstream press' sense at least.               - Matt McLeod
- That'd be because the content of a tweet is easier to condense down
  to a mainstream media article.                      - Calle Dybedahl

From 84b99582a3447213f666c2f6f52c22ef518d82d5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dagfinn=20Ilmari=20Manns=C3=A5ker?= <ilmari@ilmari.org>
Date: Tue, 25 Feb 2020 18:07:14 +0000
Subject: [PATCH] pg_upgrade: add %m to version file open failure message

---
 src/bin/pg_upgrade/server.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/bin/pg_upgrade/server.c b/src/bin/pg_upgrade/server.c
index e244256501..c9b56ae175 100644
--- a/src/bin/pg_upgrade/server.c
+++ b/src/bin/pg_upgrade/server.c
@@ -164,7 +164,7 @@ get_major_server_version(ClusterInfo *cluster)
     snprintf(ver_filename, sizeof(ver_filename), "%s/PG_VERSION",
              cluster->pgdata);
     if ((version_fd = fopen(ver_filename, "r")) == NULL)
-        pg_fatal("could not open version file: %s\n", ver_filename);
+        pg_fatal("could not open version file \"%s\": %m\n", ver_filename);
 
     if (fscanf(version_fd, "%63s", cluster->major_version_str) == 0 ||
         sscanf(cluster->major_version_str, "%d.%d", &v1, &v2) < 1)
-- 
2.22.0