Fix ON CONFLICT UPDATE bug breaking AFTER UPDATE triggers.

ExecOnConflictUpdate() passed t_ctid of the to-be-updated tuple to
ExecUpdate(). That's problematic primarily because of two reason: First
and foremost t_ctid could point to a different tuple. Secondly, and
that's what triggered the complaint by Stanislav, t_ctid is changed by
heap_update() to point to the new tuple version.  The behavior of AFTER
UPDATE triggers was therefore broken, with NEW.* and OLD.* tuples
spuriously identical within AFTER UPDATE triggers.

To fix both issues, pass a pointer to t_self of a on-stack HeapTuple
instead.

Fixing this bug lead to one change in regression tests, which previously
failed due to the first issue mentioned above. There's a reasonable
expectation that test fails, as it updates one row repeatedly within one
INSERT ... ON CONFLICT statement. That is only possible if the second
update is triggered via ON CONFLICT ... SET, ON CONFLICT ... WHERE, or
by a WITH CHECK expression, as those are executed after
ExecOnConflictUpdate() does a visibility check. That could easily be
prohibited, but given it's allowed for plain UPDATEs and a rare corner
case, it doesn't seem worthwhile.

Reported-By: Stanislav Grozev
Author: Andres Freund and Peter Geoghegan
Discussion: CAA78GVqy1+LisN-8DygekD_Ldfy=BJLarSpjGhytOsgkpMavfQ@mail.gmail.com
Backpatch: 9.5, where ON CONFLICT was introduced

src/backend/executor/nodeModifyTable.c

@@ -1181,8 +1181,17 @@ ExecOnConflictUpdate(ModifyTableState *mtstate,
 	/* Project the new tuple version */
 	ExecProject(resultRelInfo->ri_onConflictSetProj, NULL);
 
+	/*
+	 * Note that it is possible that the target tuple has been modified in
+	 * this session, after the above heap_lock_tuple. We choose to not error
+	 * out in that case, in line with ExecUpdate's treatment of similar
+	 * cases. This can happen if an UPDATE is triggered from within
+	 * ExecQual(), ExecWithCheckOptions() or ExecProject() above, e.g. by
+	 * selecting from a wCTE in the ON CONFLICT's SET.
+	 */
+
 	/* Execute UPDATE with projection */
-	*returning = ExecUpdate(&tuple.t_data->t_ctid, NULL,
+	*returning = ExecUpdate(&tuple.t_self, NULL,
 							mtstate->mt_conflproj, planSlot,
 							&mtstate->mt_epqstate, mtstate->ps.state,
 							canSetTag);

src/test/regress/expected/triggers.out

@@ -1703,7 +1703,7 @@ create function upsert_after_func()
 $$
 begin
   if (TG_OP = 'UPDATE') then
-    raise warning 'after update (old): %', new.*::text;
+    raise warning 'after update (old): %', old.*::text;
     raise warning 'after update (new): %', new.*::text;
   elsif (TG_OP = 'INSERT') then
     raise warning 'after insert (new): %', new.*::text;
@@ -1724,7 +1724,7 @@ insert into upsert values(3, 'orange') on conflict (key) do update set color = '
 WARNING:  before insert (new): (3,orange)
 WARNING:  before update (old): (3,"red trig modified")
 WARNING:  before update (new): (3,"updated red trig modified")
-WARNING:  after update (old): (3,"updated red trig modified")
+WARNING:  after update (old): (3,"red trig modified")
 WARNING:  after update (new): (3,"updated red trig modified")
 insert into upsert values(4, 'green') on conflict (key) do update set color = 'updated ' || upsert.color;
 WARNING:  before insert (new): (4,green)
@@ -1734,7 +1734,7 @@ insert into upsert values(5, 'purple') on conflict (key) do update set color = '
 WARNING:  before insert (new): (5,purple)
 WARNING:  before update (old): (5,"green trig modified")
 WARNING:  before update (new): (5,"updated green trig modified")
-WARNING:  after update (old): (5,"updated green trig modified")
+WARNING:  after update (old): (5,"green trig modified")
 WARNING:  after update (new): (5,"updated green trig modified")
 insert into upsert values(6, 'white') on conflict (key) do update set color = 'updated ' || upsert.color;
 WARNING:  before insert (new): (6,white)
@@ -1744,7 +1744,7 @@ insert into upsert values(7, 'pink') on conflict (key) do update set color = 'up
 WARNING:  before insert (new): (7,pink)
 WARNING:  before update (old): (7,"white trig modified")
 WARNING:  before update (new): (7,"updated white trig modified")
-WARNING:  after update (old): (7,"updated white trig modified")
+WARNING:  after update (old): (7,"white trig modified")
 WARNING:  after update (new): (7,"updated white trig modified")
 insert into upsert values(8, 'yellow') on conflict (key) do update set color = 'updated ' || upsert.color;
 WARNING:  before insert (new): (8,yellow)

src/test/regress/expected/with.out

@@ -1875,8 +1875,9 @@ ON CONFLICT (k) DO UPDATE SET v = (SELECT b || ' update' FROM aa WHERE a = 'a' L
 WITH aa AS (SELECT 1 a, 2 b)
 INSERT INTO z VALUES(1, (SELECT b || ' insert' FROM aa WHERE a = 1 ))
 ON CONFLICT (k) DO UPDATE SET v = (SELECT b || ' update' FROM aa WHERE a = 1 LIMIT 1);
--- This shows an attempt to update an invisible row, which should really be
--- reported as a cardinality violation, but it doesn't seem worth fixing:
+-- Update a row more than once, in different parts of a wCTE. That is
+-- an allowed, presumably very rare, edge case, but since it was
+-- broken in the past, having a test seems worthwhile.
 WITH simpletup AS (
   SELECT 2 k, 'Green' v),
 upsert_cte AS (
@@ -1886,7 +1887,10 @@ upsert_cte AS (
 INSERT INTO z VALUES(2, 'Red') ON CONFLICT (k) DO
 UPDATE SET (k, v) = (SELECT k, v FROM upsert_cte WHERE upsert_cte.k = z.k)
 RETURNING k, v;
-ERROR:  attempted to update invisible tuple
+ k | v 
+---+---
+(0 rows)
+
 DROP TABLE z;
 -- check that run to completion happens in proper ordering
 TRUNCATE TABLE y;

src/test/regress/sql/triggers.sql

@@ -1215,7 +1215,7 @@ create function upsert_after_func()
 $$
 begin
   if (TG_OP = 'UPDATE') then
-    raise warning 'after update (old): %', new.*::text;
+    raise warning 'after update (old): %', old.*::text;
     raise warning 'after update (new): %', new.*::text;
   elsif (TG_OP = 'INSERT') then
     raise warning 'after insert (new): %', new.*::text;

src/test/regress/sql/with.sql

@@ -838,8 +838,9 @@ WITH aa AS (SELECT 1 a, 2 b)
 INSERT INTO z VALUES(1, (SELECT b || ' insert' FROM aa WHERE a = 1 ))
 ON CONFLICT (k) DO UPDATE SET v = (SELECT b || ' update' FROM aa WHERE a = 1 LIMIT 1);
 
--- This shows an attempt to update an invisible row, which should really be
--- reported as a cardinality violation, but it doesn't seem worth fixing:
+-- Update a row more than once, in different parts of a wCTE. That is
+-- an allowed, presumably very rare, edge case, but since it was
+-- broken in the past, having a test seems worthwhile.
 WITH simpletup AS (
   SELECT 2 k, 'Green' v),
 upsert_cte AS (