Stop accessing checkAsUser via RTE in some cases

A future commit will move the checkAsUser field from RangeTblEntry
to a new node that, unlike RTEs, will only be created for tables
mentioned in the query but not for the inheritance child relations
added to the query by the planner.  So, checkAsUser value for a
given child relation will have to be obtained by referring to that
for its ancestor mentioned in the query.

In preparation, it seems better to expand the use of RelOptInfo.userid
during planning in place of rte->checkAsUser so that there will be
fewer places to adjust for the above change.

Given that the child-to-ancestor mapping is not available during the
execution of a given "child" ForeignScan node, add a checkAsUser
field to ForeignScan to carry the child relation's RelOptInfo.userid.

Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqGFCs2uq7VRKi7g+FFKbP6Ea_2_HkgZb2HPhUfaAKT3ng@mail.gmail.com

contrib/postgres_fdw/postgres_fdw.c

@@ -624,7 +624,6 @@ postgresGetForeignRelSize(PlannerInfo *root,
 {
 	PgFdwRelationInfo *fpinfo;
 	ListCell   *lc;
-	RangeTblEntry *rte = planner_rt_fetch(baserel->relid, root);
 
 	/*
 	 * We use PgFdwRelationInfo to pass various information to subsequent
@@ -663,8 +662,9 @@ postgresGetForeignRelSize(PlannerInfo *root,
 	 */
 	if (fpinfo->use_remote_estimate)
 	{
-		Oid			userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+		Oid			userid;
 
+		userid = OidIsValid(baserel->userid) ? baserel->userid : GetUserId();
 		fpinfo->user = GetUserMapping(userid, fpinfo->server->serverid);
 	}
 	else
@@ -1510,16 +1510,14 @@ postgresBeginForeignScan(ForeignScanState *node, int eflags)
 
 	/*
 	 * Identify which user to do the remote access as.  This should match what
-	 * ExecCheckRTEPerms() does.  In case of a join or aggregate, use the
-	 * lowest-numbered member RTE as a representative; we would get the same
-	 * result from any.
+	 * ExecCheckRTEPerms() does.
 	 */
+	userid = OidIsValid(fsplan->checkAsUser) ? fsplan->checkAsUser : GetUserId();
 	if (fsplan->scan.scanrelid > 0)
 		rtindex = fsplan->scan.scanrelid;
 	else
 		rtindex = bms_next_member(fsplan->fs_relids, -1);
 	rte = exec_rt_fetch(rtindex, estate);
-	userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
 
 	/* Get info about foreign table. */
 	table = GetForeignTable(rte->relid);
@@ -2633,7 +2631,6 @@ postgresBeginDirectModify(ForeignScanState *node, int eflags)
 	EState	   *estate = node->ss.ps.state;
 	PgFdwDirectModifyState *dmstate;
 	Index		rtindex;
-	RangeTblEntry *rte;
 	Oid			userid;
 	ForeignTable *table;
 	UserMapping *user;
@@ -2655,11 +2652,10 @@ postgresBeginDirectModify(ForeignScanState *node, int eflags)
 	 * Identify which user to do the remote access as.  This should match what
 	 * ExecCheckRTEPerms() does.
 	 */
-	rtindex = node->resultRelInfo->ri_RangeTableIndex;
-	rte = exec_rt_fetch(rtindex, estate);
-	userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+	userid = OidIsValid(fsplan->checkAsUser) ? fsplan->checkAsUser : GetUserId();
 
 	/* Get info about foreign table. */
+	rtindex = node->resultRelInfo->ri_RangeTableIndex;
 	if (fsplan->scan.scanrelid == 0)
 		dmstate->rel = ExecOpenScanRelation(estate, rtindex, eflags);
 	else
@@ -3983,7 +3979,7 @@ create_foreign_modify(EState *estate,
 	 * Identify which user to do the remote access as.  This should match what
 	 * ExecCheckRTEPerms() does.
 	 */
-	userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+	userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
 
 	/* Get info about foreign table. */
 	table = GetForeignTable(RelationGetRelid(rel));

src/backend/executor/execMain.c

@@ -631,7 +631,7 @@ ExecCheckRTEPerms(RangeTblEntry *rte)
 	 * call it once in ExecCheckRTPerms and pass the userid down from there.
 	 * But for now, no need for the extra clutter.
 	 */
-	userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+	userid = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
 
 	/*
 	 * We must have *all* the requiredPerms bits, but some of the bits can be

src/backend/optimizer/plan/createplan.c

@@ -4148,6 +4148,9 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 	/* Copy cost data from Path to Plan; no need to make FDW do this */
 	copy_generic_path_info(&scan_plan->scan.plan, &best_path->path);
 
+	/* Copy user OID to access as; likewise no need to make FDW do this */
+	scan_plan->checkAsUser = rel->userid;
+
 	/* Copy foreign server OID; likewise, no need to make FDW do this */
 	scan_plan->fs_server = rel->serverid;
 
@@ -5794,7 +5797,8 @@ make_foreignscan(List *qptlist,
 	node->operation = CMD_SELECT;
 	node->resultRelation = 0;
 
-	/* fs_server will be filled in by create_foreignscan_plan */
+	/* checkAsUser, fs_server will be filled in by create_foreignscan_plan */
+	node->checkAsUser = InvalidOid;
 	node->fs_server = InvalidOid;
 	node->fdw_exprs = fdw_exprs;
 	node->fdw_private = fdw_private;

src/backend/rewrite/rowsecurity.c

@@ -128,7 +128,7 @@ get_row_security_policies(Query *root, RangeTblEntry *rte, int rt_index,
 		return;
 
 	/* Switch to checkAsUser if it's set */
-	user_id = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+	user_id = OidIsValid(rte->checkAsUser) ? rte->checkAsUser : GetUserId();
 
 	/* Determine the state of RLS for this, pass checkAsUser explicitly */
 	rls_status = check_enable_rls(rte->relid, rte->checkAsUser, false);

src/backend/statistics/extended_stats.c

@@ -1598,6 +1598,7 @@ statext_is_compatible_clause(PlannerInfo *root, Node *clause, Index relid,
 							 Bitmapset **attnums, List **exprs)
 {
 	RangeTblEntry *rte = root->simple_rte_array[relid];
+	RelOptInfo *rel = root->simple_rel_array[relid];
 	RestrictInfo *rinfo;
 	int			clause_relid;
 	Oid			userid;
@@ -1646,10 +1647,9 @@ statext_is_compatible_clause(PlannerInfo *root, Node *clause, Index relid,
 		return false;
 
 	/*
-	 * Check that the user has permission to read all required attributes. Use
-	 * checkAsUser if it's set, in case we're accessing the table via a view.
+	 * Check that the user has permission to read all required attributes.
 	 */
-	userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+	userid = OidIsValid(rel->userid) ? rel->userid : GetUserId();
 
 	/* Table-level SELECT privilege is sufficient for all columns */
 	if (pg_class_aclcheck(rte->relid, userid, ACL_SELECT) != ACLCHECK_OK)

src/backend/utils/adt/selfuncs.c

@@ -5155,10 +5155,11 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 								Assert(rte->rtekind == RTE_RELATION);
 
 								/*
-								 * Use checkAsUser if it's set, in case we're
-								 * accessing the table via a view.
+								 * Use onerel->userid if it's set, in case
+								 * we're accessing the table via a view.
 								 */
-								userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+								userid = OidIsValid(onerel->userid) ?
+									onerel->userid : GetUserId();
 
 								/*
 								 * For simplicity, we insist on the whole
@@ -5210,7 +5211,8 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 										rte = planner_rt_fetch(varno, root);
 										Assert(rte->rtekind == RTE_RELATION);
 
-										userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+										userid = OidIsValid(onerel->userid) ?
+											onerel->userid : GetUserId();
 
 										vardata->acl_ok =
 											rte->securityQuals == NIL &&
@@ -5290,10 +5292,11 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 					vardata->freefunc = ReleaseDummy;
 
 					/*
-					 * Use checkAsUser if it's set, in case we're accessing
+					 * Use onerel->userid if it's set, in case we're accessing
 					 * the table via a view.
 					 */
-					userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+					userid = OidIsValid(onerel->userid) ?
+						onerel->userid : GetUserId();
 
 					/*
 					 * For simplicity, we insist on the whole table being
@@ -5341,7 +5344,8 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 							rte = planner_rt_fetch(varno, root);
 							Assert(rte->rtekind == RTE_RELATION);
 
-							userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+							userid = OidIsValid(onerel->userid) ?
+								onerel->userid : GetUserId();
 
 							vardata->acl_ok =
 								rte->securityQuals == NIL &&
@@ -5402,15 +5406,17 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 
 		if (HeapTupleIsValid(vardata->statsTuple))
 		{
+			RelOptInfo *onerel = find_base_rel(root, var->varno);
 			Oid			userid;
 
 			/*
 			 * Check if user has permission to read this column.  We require
 			 * all rows to be accessible, so there must be no securityQuals
-			 * from security barrier views or RLS policies.  Use checkAsUser
-			 * if it's set, in case we're accessing the table via a view.
+			 * from security barrier views or RLS policies.  Use
+			 * onerel->userid if it's set, in case we're accessing the table
+			 * via a view.
 			 */
-			userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+			userid = OidIsValid(onerel->userid) ?  onerel->userid : GetUserId();
 
 			vardata->acl_ok =
 				rte->securityQuals == NIL &&
@@ -5479,7 +5485,8 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 				rte = planner_rt_fetch(varno, root);
 				Assert(rte->rtekind == RTE_RELATION);
 
-				userid = rte->checkAsUser ? rte->checkAsUser : GetUserId();
+				userid = OidIsValid(onerel->userid) ?
+					onerel->userid : GetUserId();
 
 				vardata->acl_ok =
 					rte->securityQuals == NIL &&

src/backend/utils/misc/rls.c

@@ -51,7 +51,7 @@
 int
 check_enable_rls(Oid relid, Oid checkAsUser, bool noError)
 {
-	Oid			user_id = checkAsUser ? checkAsUser : GetUserId();
+	Oid			user_id = OidIsValid(checkAsUser) ? checkAsUser : GetUserId();
 	HeapTuple	tuple;
 	Form_pg_class classform;
 	bool		relrowsecurity;

src/include/nodes/pathnodes.h

@@ -901,7 +901,7 @@ typedef struct RelOptInfo
 	 */
 	/* identifies server for the table or join */
 	Oid			serverid;
-	/* identifies user to check access as */
+	/* identifies user to check access as; 0 means to check as current user */
 	Oid			userid;
 	/* join is only valid for current user */
 	bool		useridiscurrent;

src/include/nodes/plannodes.h

@@ -703,6 +703,8 @@ typedef struct ForeignScan
 	Scan		scan;
 	CmdType		operation;		/* SELECT/INSERT/UPDATE/DELETE */
 	Index		resultRelation; /* direct modification target's RT index */
+	Oid			checkAsUser;	/* user to perform the scan as; 0 means to
+								 * check as current user */
 	Oid			fs_server;		/* OID of foreign server */
 	List	   *fdw_exprs;		/* expressions that FDW may evaluate */
 	List	   *fdw_private;	/* private data for FDW */