Prevent dangling-pointer access when update trigger returns old tuple.

A before-update row trigger may choose to return the "new" or "old" tuple
unmodified.  ExecBRUpdateTriggers failed to consider the second
possibility, and would proceed to free the "old" tuple even if it was the
one returned, leading to subsequent access to already-deallocated memory.
In debug builds this reliably leads to an "invalid memory alloc request
size" failure; in production builds it might accidentally work, but data
corruption is also possible.

This is a very old bug.  There are probably a couple of reasons it hasn't
been noticed up to now.  It would be more usual to return NULL if one
wanted to suppress the update action; returning "old" is significantly less
efficient since the update will occur anyway.  Also, none of the standard
PLs would ever cause this because they all returned freshly-manufactured
tuples even if they were just copying "old".  But commit 4b93f5799 changed
that for plpgsql, making it possible to see the bug with a plpgsql trigger.
Still, this is certainly legal behavior for a trigger function, so it's
ExecBRUpdateTriggers's fault not plpgsql's.

It seems worth creating a test case that exercises returning "old" directly
with a C-language trigger; testing this through plpgsql seems unreliable
because its behavior might change again.

Report and fix by Rushabh Lathia; regression test case by me.
Back-patch to all supported branches.

Discussion: https://postgr.es/m/CAGPqQf1P4pjiNPrMof=P_16E-DFjt457j+nH2ex3=nBTew7tXw@mail.gmail.com

src/backend/commands/trigger.c

@@ -2497,7 +2497,7 @@ ExecBRUpdateTriggers(EState *estate, EPQState *epqstate,
 			return NULL;		/* "do nothing" */
 		}
 	}
-	if (trigtuple != fdw_trigtuple)
+	if (trigtuple != fdw_trigtuple && trigtuple != newtuple)
 		heap_freetuple(trigtuple);
 
 	if (newtuple != slottuple)

src/test/regress/expected/triggers.out

@@ -133,6 +133,32 @@ DROP TABLE fkeys2;
 -- select count(*) from dup17 where x = 13;
 --
 -- DROP TABLE dup17;
+-- Check behavior when trigger returns unmodified trigtuple
+create table trigtest (f1 int, f2 text);
+create trigger trigger_return_old
+	before insert or delete or update on trigtest
+	for each row execute procedure trigger_return_old();
+insert into trigtest values(1, 'foo');
+select * from trigtest;
+ f1 | f2  
+----+-----
+  1 | foo
+(1 row)
+
+update trigtest set f2 = f2 || 'bar';
+select * from trigtest;
+ f1 | f2  
+----+-----
+  1 | foo
+(1 row)
+
+delete from trigtest;
+select * from trigtest;
+ f1 | f2 
+----+----
+(0 rows)
+
+drop table trigtest;
 create sequence ttdummy_seq increment 10 start 0 minvalue 0;
 create table tttest (
 	price_id	int4,

src/test/regress/input/create_function_1.source

@@ -42,6 +42,11 @@ CREATE FUNCTION funny_dup17 ()
         AS '@libdir@/regress@DLSUFFIX@'
         LANGUAGE C;
 
+CREATE FUNCTION trigger_return_old ()
+        RETURNS trigger
+        AS '@libdir@/regress@DLSUFFIX@'
+        LANGUAGE C;
+
 CREATE FUNCTION ttdummy ()
         RETURNS trigger
         AS '@libdir@/regress@DLSUFFIX@'

src/test/regress/output/create_function_1.source

@@ -39,6 +39,10 @@ CREATE FUNCTION funny_dup17 ()
         RETURNS trigger
         AS '@libdir@/regress@DLSUFFIX@'
         LANGUAGE C;
+CREATE FUNCTION trigger_return_old ()
+        RETURNS trigger
+        AS '@libdir@/regress@DLSUFFIX@'
+        LANGUAGE C;
 CREATE FUNCTION ttdummy ()
         RETURNS trigger
         AS '@libdir@/regress@DLSUFFIX@'

src/test/regress/regress.c

@@ -456,6 +456,22 @@ funny_dup17(PG_FUNCTION_ARGS)
 	return PointerGetDatum(tuple);
 }
 
+PG_FUNCTION_INFO_V1(trigger_return_old);
+
+Datum
+trigger_return_old(PG_FUNCTION_ARGS)
+{
+	TriggerData *trigdata = (TriggerData *) fcinfo->context;
+	HeapTuple	tuple;
+
+	if (!CALLED_AS_TRIGGER(fcinfo))
+		elog(ERROR, "trigger_return_old: not fired by trigger manager");
+
+	tuple = trigdata->tg_trigtuple;
+
+	return PointerGetDatum(tuple);
+}
+
 #define TTDUMMY_INFINITY	999999
 
 static SPIPlanPtr splan = NULL;

src/test/regress/sql/triggers.sql

@@ -131,6 +131,22 @@ DROP TABLE fkeys2;
 --
 -- DROP TABLE dup17;
 
+-- Check behavior when trigger returns unmodified trigtuple
+create table trigtest (f1 int, f2 text);
+
+create trigger trigger_return_old
+	before insert or delete or update on trigtest
+	for each row execute procedure trigger_return_old();
+
+insert into trigtest values(1, 'foo');
+select * from trigtest;
+update trigtest set f2 = f2 || 'bar';
+select * from trigtest;
+delete from trigtest;
+select * from trigtest;
+
+drop table trigtest;
+
 create sequence ttdummy_seq increment 10 start 0 minvalue 0;
 
 create table tttest (