postgres_fdw: Save foreign server OID in connection cache entry.

The foreign server OID stored in the connection cache entry is used as
a lookup key to directly get the server name.

Previously since the connection cache entry did not have the server OID,
postgres_fdw had to get the server OID at first from user mapping before
getting the server name. So if the corresponding user mapping was dropped,
postgres_fdw could raise the error "cache lookup failed for user mapping"
while looking up user mapping and fail to get the server name even though
the server had not been dropped yet.

Author: Bharath Rupireddy
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CALj2ACVRZPUB7ZwqLn-6DY8C_UmPs6084gSpHA92YBv++1AJXA@mail.gmail.com

contrib/postgres_fdw/connection.c

@@ -57,6 +57,7 @@ typedef struct ConnCacheEntry
 	bool		have_error;		/* have any subxacts aborted in this xact? */
 	bool		changing_xact_state;	/* xact state change in process */
 	bool		invalidated;	/* true if reconnect is pending */
+	Oid			serverid;		/* foreign server OID used to get server name */
 	uint32		server_hashvalue;	/* hash value of foreign server OID */
 	uint32		mapping_hashvalue;	/* hash value of user mapping OID */
 } ConnCacheEntry;
@@ -273,6 +274,7 @@ make_new_connection(ConnCacheEntry *entry, UserMapping *user)
 	entry->have_error = false;
 	entry->changing_xact_state = false;
 	entry->invalidated = false;
+	entry->serverid = server->serverid;
 	entry->server_hashvalue =
 		GetSysCacheHashValue1(FOREIGNSERVEROID,
 							  ObjectIdGetDatum(server->serverid));
@@ -1138,8 +1140,6 @@ pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue)
 static void
 pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry)
 {
-	HeapTuple	tup;
-	Form_pg_user_mapping umform;
 	ForeignServer *server;
 
 	/* nothing to do for inactive entries and entries of sane state */
@@ -1150,13 +1150,7 @@ pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry)
 	disconnect_pg_server(entry);
 
 	/* find server name to be shown in the message below */
-	tup = SearchSysCache1(USERMAPPINGOID,
-						  ObjectIdGetDatum(entry->key));
-	if (!HeapTupleIsValid(tup))
-		elog(ERROR, "cache lookup failed for user mapping %u", entry->key);
-	umform = (Form_pg_user_mapping) GETSTRUCT(tup);
-	server = GetForeignServer(umform->umserver);
-	ReleaseSysCache(tup);
+	server = GetForeignServer(entry->serverid);
 
 	ereport(ERROR,
 			(errcode(ERRCODE_CONNECTION_EXCEPTION),