Improve connection-failure error handling in contrib/postgres_fdw.

postgres_fdw tended to say "unknown error" if it tried to execute a command
on an already-dead connection, because some paths in libpq just return a
null PGresult for such cases.  Out-of-memory might result in that, too.
To fix, pass the PGconn to pgfdw_report_error, and look at its
PQerrorMessage() string if we can't get anything out of the PGresult.

Also, fix the transaction-exit logic to reliably drop a dead connection.
It was attempting to do that already, but it assumed that only connection
cache entries with xact_depth > 0 needed to be examined.  The folly in that
is that if we fail while issuing START TRANSACTION, we'll not have bumped
xact_depth.  (At least for the case I was testing, this fix masks the
other problem; but it still seems like a good idea to have the PGconn
fallback logic.)

Per investigation of bug #9087 from Craig Lucas.  Backpatch to 9.3 where
this code was introduced.

1 files changed, 2 insertions, 2 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.h b/contrib/postgres_fdw/postgres_fdw.h
index c782d4fea12..cd0fab17417 100644
--- a/contrib/postgres_fdw/postgres_fdw.h
+++ b/contrib/postgres_fdw/postgres_fdw.h
@@ -30,8 +30,8 @@ extern PGconn *GetConnection(ForeignServer *server, UserMapping *user,
 extern void ReleaseConnection(PGconn *conn);
 extern unsigned int GetCursorNumber(PGconn *conn);
 extern unsigned int GetPrepStmtNumber(PGconn *conn);
-extern void pgfdw_report_error(int elevel, PGresult *res, bool clear,
-				   const char *sql);
+extern void pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
+				   bool clear, const char *sql);
 
 /* in option.c */
 extern int ExtractConnectionOptions(List *defelems,