subtransactions -- storage manager

Hackers,

This patch adds subtransaction support into the storage manager. Files
created or dropped inside a subtransaction are correctly dealt with at
subtransaction commit or abort.

This works:
create table foo (a int);
create table foo2 (a int);
begin;
begin;
create table bar (a int);
select relfilenode, relname from pg_class where relname in ('foo', 'bar');
drop table foo2;
rollback;
drop table foo;
create table baz (a int);
select relfilenode, relname from pg_class where relname='baz';
commit;

At this point, the files for "bar" and "foo" have disappeared, while
"foo2" and "baz" remain. (Note however that the catalog entries are not
correct -- this is because we don't have correctly recorded results in
pg_clog.)

While making this I realized I had made a mistake regarding portal
memory, so I also correct it with this patch. As a side effect, the
following works;

begin;
begin;
declare foo cursor for select 1;
commit;
begin;
declare bar cursor for select 1;
rollback;
fetch all from foo; -- returns 1 row
fetch all from bar; -- no such cursor
rollback;

(This patch will only apply cleanly with the previous patch applied.)

Still missing:

- support for prepared statements, async notifies. Easy.
- support for on commit actions. Not sure.
- support for deferred triggers. Not so easy, maybe hard.
- correct LWLock handling. Should be easy (release them all on abort)
- correct regular lock handling. Not so easy.
- pg_clog/pg_subtrans. Need a solution.

PS: somehow I managed to get tired of the phrase "nested transactions"
and I'm using the term "subtransactions" instead. In my head they are
the same thing ...

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
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