invalid memory alloc request size error with commit 4b93f579

Hi,

With commit 4b93f57999a2ca9b9c9e573ea32ab1aeaa8bf496, which plpgsql
use its DTYPE_REC code paths for composite-type variables - below
test started failing with "invalid memory alloc request size 2139062167"
error.

Testcase:

create table foo ( name varchar(20), type varchar(20));

insert into foo values ( 'Ford', 'Car');

CREATE OR REPLACE FUNCTION Trigger_Function() returns trigger as
$$
BEGIN
  RAISE NOTICE 'OLD: %, NEW: %', OLD, NEW;
  IF NEW.name = 'Ford' THEN
    return OLD; -- return old tuple
  END IF;
  return NEW; -- return original tuple
END;
$$ language plpgsql;

CREATE TRIGGER Before_Update_Trigger BEFORE UPDATE ON foo FOR EACH ROW EXECUTE PROCEDURE Trigger_Function();

UPDATE foo SET type = 'New Car' where name = 'Ford';

Error coming while trying to copy the invalid tuple from
(heap_copytuple() <- ExecCopySlotTuple() <- ExecMaterializeSlot() <-
ExecUpdate() <- ExecModifyTable())

Here ExecBRUpdateTriggers() returns the invalid tuple when trigger
return old tuple.  Looking further I found that tuple is getting free
at ExecBRUpdateTriggers(), below code:

    if (trigtuple != fdw_trigtuple)
        heap_freetuple(trigtuple);


It seems like before commit 4b93f57999a2ca9b9c9e573ea32ab1aeaa8bf496,
plpgsql_exec_trigger() always used to copy the old and new tuple but
after that commit it doen't copy the "old" and "new" tuple if
if user just did "return new" or "return old" without changing anything.

With commit 4b93f57999a2ca9b9c9e573ea32ab1aeaa8bf496, which plpgsql
use its DTYPE_REC code paths for composite-type variables - below
test started failing with "invalid memory alloc request size 2139062167"
error.

Testcase:

create table foo ( name varchar(20), type varchar(20));

insert into foo values ( 'Ford', 'Car');

CREATE OR REPLACE FUNCTION Trigger_Function() returns trigger as
$$
BEGIN
  RAISE NOTICE 'OLD: %, NEW: %', OLD, NEW;
  IF NEW.name = 'Ford' THEN
    return OLD; -- return old tuple
  END IF;
  return NEW; -- return original tuple
END;
$$ language plpgsql;

CREATE TRIGGER Before_Update_Trigger BEFORE UPDATE ON foo FOR EACH ROW EXECUTE PROCEDURE Trigger_Function();

UPDATE foo SET type = 'New Car' where name = 'Ford';

Error coming while trying to copy the invalid tuple from
(heap_copytuple() <- ExecCopySlotTuple() <- ExecMaterializeSlot() <-
ExecUpdate() <- ExecModifyTable())

Here ExecBRUpdateTriggers() returns the invalid tuple when trigger
return old tuple.  Looking further I found that tuple is getting free
at ExecBRUpdateTriggers(), below code:

    if (trigtuple != fdw_trigtuple)
        heap_freetuple(trigtuple);


It seems like before commit 4b93f57999a2ca9b9c9e573ea32ab1aeaa8bf496,
plpgsql_exec_trigger() always used to copy the old and new tuple but
after that commit it doen't copy the "old" and "new" tuple if
if user just did "return new" or "return old" without changing anything.

+           /*
+            * Copy tuple to upper executor memory.  But if user just did
+            * "return new" or "return old" without changing anything, there's
+            * no need to copy; we can return the original tuple (which will
+            * save a few cycles in trigger.c as well as here).
+            */
+           if (rettup != trigdata->tg_newtuple &&
+               rettup != trigdata->tg_trigtuple)
+               rettup = SPI_copytuple(rettup);

In ExecBRUpdateTriggers(), we need to add a check that if trigtuple is same
as newtuple, then we don't require to free the trigtuple.

ExecBRDeleteTriggers() also does the similar things, but their we don't
need a check because it doesn't care about the return tuple.

PFA patch which add a check to not free the trigtuple if newtuple is same

as trigtuple and also added the related testcase.