Improve dblink error message when remote does not provide it

When dblink or postgres_fdw detects an error on the remote side of the
connection, it will try to construct a local error message as best it
can using libpq's PQresultErrorField(). When no primary message is
available, it was bailing out with an unhelpful "unknown error". Make
that message better and more style guide compliant. Per discussion
on hackers.

Backpatch to 9.2 except postgres_fdw which didn't exist before 9.3.

Discussion: https://postgr.es/m/19872.1482338965%40sss.pgh.pa.us

diff --git a/contrib/dblink/dblink.c b/contrib/dblink/dblink.c
index 6f972be0d7f22cec463b12050510c4918dc3f5ff..e6eb7075d0b0c606c0338562d1d1c41bcd5254e5 100644 (file)
--- a/contrib/dblink/dblink.c
+++ b/contrib/dblink/dblink.c
@@ -2633,7 +2633,7 @@ dblink_res_error(const char *conname, PGresult *res, const char *dblink_context_
        ereport(level,
                        (errcode(sqlstate),
                         message_primary ? errmsg_internal("%s", message_primary) :
-                        errmsg("unknown error"),
+                        errmsg("could not obtain message string for remote error"),
                         message_detail ? errdetail_internal("%s", message_detail) : 0,
                         message_hint ? errhint("%s", message_hint) : 0,
                         message_context ? errcontext("%s", message_context) : 0,