Don't require a user mapping for FDWs to work.

Commit fbe5a3fb73 accidentally changed
this behavior; put things back the way they were, and add some
regression tests.

Report by Andres Freund; patch by Ashutosh Bapat, with a bit of
kibitzing by me.

src/backend/foreign/foreign.c

@@ -31,7 +31,7 @@
 extern Datum pg_options_to_table(PG_FUNCTION_ARGS);
 extern Datum postgresql_fdw_validator(PG_FUNCTION_ARGS);
 
-static HeapTuple find_user_mapping(Oid userid, Oid serverid);
+static HeapTuple find_user_mapping(Oid userid, Oid serverid, bool missing_ok);
 
 /*
  * GetForeignDataWrapper -	look up the foreign-data wrapper by OID.
@@ -223,7 +223,7 @@ GetUserMapping(Oid userid, Oid serverid)
 	bool		isnull;
 	UserMapping *um;
 
-	tp = find_user_mapping(userid, serverid);
+	tp = find_user_mapping(userid, serverid, false);
 
 	um = (UserMapping *) palloc(sizeof(UserMapping));
 	um->umid = HeapTupleGetOid(tp);
@@ -250,14 +250,23 @@ GetUserMapping(Oid userid, Oid serverid)
  *
  * If no mapping is found for the supplied user, we also look for
  * PUBLIC mappings (userid == InvalidOid).
+ *
+ * If missing_ok is true, the function returns InvalidOid when it does not find
+ * required user mapping. Otherwise, find_user_mapping() throws error if it
+ * does not find required user mapping. 
  */
 Oid
-GetUserMappingId(Oid userid, Oid serverid)
+GetUserMappingId(Oid userid, Oid serverid, bool missing_ok)
 {
 	HeapTuple	tp;
 	Oid			umid;
 
-	tp = find_user_mapping(userid, serverid);
+	tp = find_user_mapping(userid, serverid, missing_ok);
+
+	Assert(missing_ok || tp);
+
+	if (!tp && missing_ok)
+		return InvalidOid;
 
 	/* Extract the Oid */
 	umid = HeapTupleGetOid(tp);
@@ -273,9 +282,13 @@ GetUserMappingId(Oid userid, Oid serverid)
  *
  * If no mapping is found for the supplied user, we also look for
  * PUBLIC mappings (userid == InvalidOid).
+ *
+ * If missing_ok is true, the function returns NULL, if it does not find
+ * the required user mapping. Otherwise, it throws error if it does not
+ * find the required user mapping.
  */
 static HeapTuple
-find_user_mapping(Oid userid, Oid serverid)
+find_user_mapping(Oid userid, Oid serverid, bool missing_ok)
 {
 	HeapTuple	tp;
 
@@ -292,10 +305,15 @@ find_user_mapping(Oid userid, Oid serverid)
 						 ObjectIdGetDatum(serverid));
 
 	if (!HeapTupleIsValid(tp))
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("user mapping not found for \"%s\"",
-						MappingUserName(userid))));
+	{
+		if (missing_ok)
+			return NULL;
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("user mapping not found for \"%s\"",
+							MappingUserName(userid))));
+	}
 
 	return tp;
 }

src/backend/optimizer/util/relnode.c

@@ -180,11 +180,15 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 		 * ensure that it gets invalidated in the case of a user OID change.
 		 * See RevalidateCachedQuery and more generally the hasForeignJoin
 		 * flags in PlannerGlobal and PlannedStmt.
+		 *
+		 * It's possible, and not necessarily an error, for rel->umid to be
+		 * InvalidOid even though rel->serverid is set.  That just means there
+		 * is a server with no user mapping.
 		 */
 		Oid		userid;
 
 		userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
-		rel->umid = GetUserMappingId(userid, rel->serverid);
+		rel->umid = GetUserMappingId(userid, rel->serverid, true);
 	}
 	else
 		rel->umid = InvalidOid;
@@ -435,12 +439,16 @@ build_join_rel(PlannerInfo *root,
 	 *
 	 * Otherwise those fields are left invalid, so FDW API will not be called
 	 * for the join relation.
+	 *
+	 * For FDWs like file_fdw, which ignore user mapping, the user mapping id
+	 * associated with the joining relation may be invalid. A valid serverid
+	 * distinguishes between a pushed down join with no user mapping and
+	 * a join which can not be pushed down because of user mapping mismatch.
 	 */
 	if (OidIsValid(outer_rel->serverid) &&
 		inner_rel->serverid == outer_rel->serverid &&
 		inner_rel->umid == outer_rel->umid)
 	{
-		Assert(OidIsValid(outer_rel->umid));
 		joinrel->serverid = outer_rel->serverid;
 		joinrel->umid = outer_rel->umid;
 		joinrel->fdwroutine = outer_rel->fdwroutine;

src/include/foreign/foreign.h

@@ -72,7 +72,7 @@ typedef struct ForeignTable
 extern ForeignServer *GetForeignServer(Oid serverid);
 extern ForeignServer *GetForeignServerByName(const char *name, bool missing_ok);
 extern UserMapping *GetUserMapping(Oid userid, Oid serverid);
-extern Oid GetUserMappingId(Oid userid, Oid serverid);
+extern Oid GetUserMappingId(Oid userid, Oid serverid, bool missing_ok);
 extern UserMapping *GetUserMappingById(Oid umid);
 extern ForeignDataWrapper *GetForeignDataWrapper(Oid fdwid);
 extern ForeignDataWrapper *GetForeignDataWrapperByName(const char *name,