Avoid doing catalog lookups in postgres_fdw's conversion_error_callback.

As in 50371df26, this is a bad idea since the callback can't really
know what error is being thrown and thus whether or not it is safe
to attempt catalog accesses.  Rather than pushing said accesses into
the mainline code where they'd usually be a waste of cycles, we can
look at the query's rangetable instead.

This change does mean that we'll be printing query aliases (if any
were used) rather than the table or column's true name.  But that
doesn't seem like a bad thing: it's certainly a more useful definition
in self-join cases, for instance.  In any case, it seems unlikely that
any applications would be depending on this detail, so it seems safe
to change.

Patch by me.  Original complaint by Andres Freund; Bharath Rupireddy
noted the connection to conversion_error_callback.

Discussion: https://postgr.es/m/20210106020229.ne5xnuu6wlondjpe@alap3.anarazel.de

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -4096,15 +4096,17 @@ DROP TABLE reind_fdw_parent;
 -- conversion error
 -- ===================================================================
 ALTER FOREIGN TABLE ft1 ALTER COLUMN c8 TYPE int;
-SELECT * FROM ft1 WHERE c1 = 1;  -- ERROR
+SELECT * FROM ft1 ftx(x1,x2,x3,x4,x5,x6,x7,x8) WHERE x1 = 1;  -- ERROR
 ERROR:  invalid input syntax for type integer: "foo"
-CONTEXT:  column "c8" of foreign table "ft1"
-SELECT  ft1.c1,  ft2.c2, ft1.c8 FROM ft1, ft2 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = 1; -- ERROR
+CONTEXT:  column "x8" of foreign table "ftx"
+SELECT ftx.x1, ft2.c2, ftx.x8 FROM ft1 ftx(x1,x2,x3,x4,x5,x6,x7,x8), ft2
+  WHERE ftx.x1 = ft2.c1 AND ftx.x1 = 1; -- ERROR
 ERROR:  invalid input syntax for type integer: "foo"
-CONTEXT:  column "c8" of foreign table "ft1"
-SELECT  ft1.c1,  ft2.c2, ft1 FROM ft1, ft2 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = 1; -- ERROR
+CONTEXT:  column "x8" of foreign table "ftx"
+SELECT ftx.x1, ft2.c2, ftx FROM ft1 ftx(x1,x2,x3,x4,x5,x6,x7,x8), ft2
+  WHERE ftx.x1 = ft2.c1 AND ftx.x1 = 1; -- ERROR
 ERROR:  invalid input syntax for type integer: "foo"
-CONTEXT:  whole-row reference to foreign table "ft1"
+CONTEXT:  whole-row reference to foreign table "ftx"
 SELECT sum(c2), array_agg(c8) FROM ft1 GROUP BY c8; -- ERROR
 ERROR:  invalid input syntax for type integer: "foo"
 CONTEXT:  processing expression at position 2 in select list

contrib/postgres_fdw/postgres_fdw.c

@@ -302,16 +302,8 @@ typedef struct
  */
 typedef struct ConversionLocation
 {
-	Relation	rel;			/* foreign table's relcache entry. */
 	AttrNumber	cur_attno;		/* attribute number being processed, or 0 */
-
-	/*
-	 * In case of foreign join push down, fdw_scan_tlist is used to identify
-	 * the Var node corresponding to the error location and
-	 * fsstate->ss.ps.state gives access to the RTEs of corresponding relation
-	 * to get the relation name and attribute name.
-	 */
-	ForeignScanState *fsstate;
+	ForeignScanState *fsstate;	/* plan node being processed */
 } ConversionLocation;
 
 /* Callback argument for ec_member_matches_foreign */
@@ -7082,7 +7074,6 @@ make_tuple_from_result_row(PGresult *res,
 	/*
 	 * Set up and install callback to report where conversion error occurs.
 	 */
-	errpos.rel = rel;
 	errpos.cur_attno = 0;
 	errpos.fsstate = fsstate;
 	errcallback.callback = conversion_error_callback;
@@ -7185,34 +7176,32 @@ make_tuple_from_result_row(PGresult *res,
 /*
  * Callback function which is called when error occurs during column value
  * conversion.  Print names of column and relation.
+ *
+ * Note that this function mustn't do any catalog lookups, since we are in
+ * an already-failed transaction.  Fortunately, we can get the needed info
+ * from the query's rangetable instead.
  */
 static void
 conversion_error_callback(void *arg)
 {
+	ConversionLocation *errpos = (ConversionLocation *) arg;
+	ForeignScanState *fsstate = errpos->fsstate;
+	ForeignScan *fsplan = castNode(ForeignScan, fsstate->ss.ps.plan);
+	int			varno = 0;
+	AttrNumber	colno = 0;
 	const char *attname = NULL;
 	const char *relname = NULL;
 	bool		is_wholerow = false;
-	ConversionLocation *errpos = (ConversionLocation *) arg;
 
-	if (errpos->rel)
+	if (fsplan->scan.scanrelid > 0)
 	{
 		/* error occurred in a scan against a foreign table */
-		TupleDesc	tupdesc = RelationGetDescr(errpos->rel);
-		Form_pg_attribute attr = TupleDescAttr(tupdesc, errpos->cur_attno - 1);
-
-		if (errpos->cur_attno > 0 && errpos->cur_attno <= tupdesc->natts)
-			attname = NameStr(attr->attname);
-		else if (errpos->cur_attno == SelfItemPointerAttributeNumber)
-			attname = "ctid";
-
-		relname = RelationGetRelationName(errpos->rel);
+		varno = fsplan->scan.scanrelid;
+		colno = errpos->cur_attno;
 	}
 	else
 	{
 		/* error occurred in a scan against a foreign join */
-		ForeignScanState *fsstate = errpos->fsstate;
-		ForeignScan *fsplan = castNode(ForeignScan, fsstate->ss.ps.plan);
-		EState	   *estate = fsstate->ss.ps.state;
 		TargetEntry *tle;
 
 		tle = list_nth_node(TargetEntry, fsplan->fdw_scan_tlist,
@@ -7220,35 +7209,40 @@ conversion_error_callback(void *arg)
 
 		/*
 		 * Target list can have Vars and expressions.  For Vars, we can get
-		 * its relation, however for expressions we can't.  Thus for
+		 * some information, however for expressions we can't.  Thus for
 		 * expressions, just show generic context message.
 		 */
 		if (IsA(tle->expr, Var))
 		{
-			RangeTblEntry *rte;
 			Var		   *var = (Var *) tle->expr;
 
-			rte = exec_rt_fetch(var->varno, estate);
-
-			if (var->varattno == 0)
-				is_wholerow = true;
-			else
-				attname = get_attname(rte->relid, var->varattno, false);
-
-			relname = get_rel_name(rte->relid);
+			varno = var->varno;
+			colno = var->varattno;
 		}
-		else
-			errcontext("processing expression at position %d in select list",
-					   errpos->cur_attno);
 	}
 
-	if (relname)
+	if (varno > 0)
 	{
-		if (is_wholerow)
+		EState	   *estate = fsstate->ss.ps.state;
+		RangeTblEntry *rte = exec_rt_fetch(varno, estate);
+
+		relname = rte->eref->aliasname;
+
+		if (colno == 0)
+			is_wholerow = true;
+		else if (colno > 0 && colno <= list_length(rte->eref->colnames))
+			attname = strVal(list_nth(rte->eref->colnames, colno - 1));
+		else if (colno == SelfItemPointerAttributeNumber)
+			attname = "ctid";
+	}
+
+	if (relname && is_wholerow)
 		errcontext("whole-row reference to foreign table \"%s\"", relname);
-		else if (attname)
+	else if (relname && attname)
 		errcontext("column \"%s\" of foreign table \"%s\"", attname, relname);
-	}
+	else
+		errcontext("processing expression at position %d in select list",
+				   errpos->cur_attno);
 }
 
 /*

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1129,9 +1129,11 @@ DROP TABLE reind_fdw_parent;
 -- conversion error
 -- ===================================================================
 ALTER FOREIGN TABLE ft1 ALTER COLUMN c8 TYPE int;
-SELECT * FROM ft1 WHERE c1 = 1;  -- ERROR
-SELECT  ft1.c1,  ft2.c2, ft1.c8 FROM ft1, ft2 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = 1; -- ERROR
-SELECT  ft1.c1,  ft2.c2, ft1 FROM ft1, ft2 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = 1; -- ERROR
+SELECT * FROM ft1 ftx(x1,x2,x3,x4,x5,x6,x7,x8) WHERE x1 = 1;  -- ERROR
+SELECT ftx.x1, ft2.c2, ftx.x8 FROM ft1 ftx(x1,x2,x3,x4,x5,x6,x7,x8), ft2
+  WHERE ftx.x1 = ft2.c1 AND ftx.x1 = 1; -- ERROR
+SELECT ftx.x1, ft2.c2, ftx FROM ft1 ftx(x1,x2,x3,x4,x5,x6,x7,x8), ft2
+  WHERE ftx.x1 = ft2.c1 AND ftx.x1 = 1; -- ERROR
 SELECT sum(c2), array_agg(c8) FROM ft1 GROUP BY c8; -- ERROR
 ALTER FOREIGN TABLE ft1 ALTER COLUMN c8 TYPE user_enum;