Make dblink try harder to form useful error messages

When libpq encounters a connection-level error, e.g. runs out of memory
while forming a result, there will be no error associated with PGresult,
but a message will be placed into PGconn's error buffer. postgres_fdw
takes care to use the PGconn error message when PGresult does not have
one, but dblink has been negligent in that regard. Modify dblink to mirror
what postgres_fdw has been doing.

Back-patch to all supported branches.

Author: Joe Conway
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/02fa2d90-2efd-00bc-fefc-c23c00eb671e%40joeconway.com

contrib/dblink/dblink.c

@@ -113,7 +113,8 @@ static Relation get_rel_from_relname(text *relname_text, LOCKMODE lockmode, AclM
 static char *generate_relation_name(Relation rel);
 static void dblink_connstr_check(const char *connstr);
 static void dblink_security_check(PGconn *conn, remoteConn *rconn);
-static void dblink_res_error(const char *conname, PGresult *res, const char *dblink_context_msg, bool fail);
+static void dblink_res_error(PGconn *conn, const char *conname, PGresult *res,
+							 const char *dblink_context_msg, bool fail);
 static char *get_connect_string(const char *servername);
 static char *escape_param_str(const char *from);
 static void validate_pkattnums(Relation rel,
@@ -428,7 +429,7 @@ dblink_open(PG_FUNCTION_ARGS)
 	res = PQexec(conn, buf.data);
 	if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
-		dblink_res_error(conname, res, "could not open cursor", fail);
+		dblink_res_error(conn, conname, res, "could not open cursor", fail);
 		PG_RETURN_TEXT_P(cstring_to_text("ERROR"));
 	}
 
@@ -497,7 +498,7 @@ dblink_close(PG_FUNCTION_ARGS)
 	res = PQexec(conn, buf.data);
 	if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
-		dblink_res_error(conname, res, "could not close cursor", fail);
+		dblink_res_error(conn, conname, res, "could not close cursor", fail);
 		PG_RETURN_TEXT_P(cstring_to_text("ERROR"));
 	}
 
@@ -600,7 +601,8 @@ dblink_fetch(PG_FUNCTION_ARGS)
 		(PQresultStatus(res) != PGRES_COMMAND_OK &&
 		 PQresultStatus(res) != PGRES_TUPLES_OK))
 	{
-		dblink_res_error(conname, res, "could not fetch from cursor", fail);
+		dblink_res_error(conn, conname, res,
+						 "could not fetch from cursor", fail);
 		return (Datum) 0;
 	}
 	else if (PQresultStatus(res) == PGRES_COMMAND_OK)
@@ -751,8 +753,8 @@ dblink_record_internal(FunctionCallInfo fcinfo, bool is_async)
 				if (PQresultStatus(res) != PGRES_COMMAND_OK &&
 					PQresultStatus(res) != PGRES_TUPLES_OK)
 				{
-					dblink_res_error(conname, res, "could not execute query",
-									 fail);
+					dblink_res_error(conn, conname, res,
+									 "could not execute query", fail);
 					/* if fail isn't set, we'll return an empty query result */
 				}
 				else
@@ -997,7 +999,8 @@ materializeQueryResult(FunctionCallInfo fcinfo,
 			PGresult   *res1 = res;
 
 			res = NULL;
-			dblink_res_error(conname, res1, "could not execute query", fail);
+			dblink_res_error(conn, conname, res1,
+							 "could not execute query", fail);
 			/* if fail isn't set, we'll return an empty query result */
 		}
 		else if (PQresultStatus(res) == PGRES_COMMAND_OK)
@@ -1432,7 +1435,8 @@ dblink_exec(PG_FUNCTION_ARGS)
 			(PQresultStatus(res) != PGRES_COMMAND_OK &&
 			 PQresultStatus(res) != PGRES_TUPLES_OK))
 		{
-			dblink_res_error(conname, res, "could not execute command", fail);
+			dblink_res_error(conn, conname, res,
+							 "could not execute command", fail);
 
 			/*
 			 * and save a copy of the command status string to return as our
@@ -2663,7 +2667,8 @@ dblink_connstr_check(const char *connstr)
 }
 
 static void
-dblink_res_error(const char *conname, PGresult *res, const char *dblink_context_msg, bool fail)
+dblink_res_error(PGconn *conn, const char *conname, PGresult *res,
+				 const char *dblink_context_msg, bool fail)
 {
 	int			level;
 	char	   *pg_diag_sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
@@ -2697,6 +2702,14 @@ dblink_res_error(const char *conname, PGresult *res, const char *dblink_context_
 	xpstrdup(message_hint, pg_diag_message_hint);
 	xpstrdup(message_context, pg_diag_context);
 
+	/*
+	 * If we don't get a message from the PGresult, try the PGconn.  This
+	 * is needed because for connection-level failures, PQexec may just
+	 * return NULL, not a PGresult at all.
+	 */
+	if (message_primary == NULL)
+		message_primary = PQerrorMessage(conn);
+
 	if (res)
 		PQclear(res);