RE: Copy data to DSA area

From: Thomas Munro [mailto:thomas.munro@enterprisedb.com]
>
>On Tue, Nov 13, 2018 at 9:45 AM Robert Haas <robertmhaas@gmail.com> wrote:
>> On Thu, Nov 8, 2018 at 9:05 PM Thomas Munro
>> <thomas.munro@enterprisedb.com> wrote:
>> > * I had some ideas about some kind of "allocation rollback" interface:
>> > you begin an "allocation transaction", allocate a bunch of stuff
>> > (perhaps indirectly, by calling some API that makes query plans or
>> > whatever and is totally unaware of this stuff).  Then if there is an
>> > error, whatever was allocated so far is freed in the usual cleanup
>> > paths by a rollback that happens via the resource manager machinery.
>> > If you commit, then the allocation becomes permanent.  Then you only
>> > commit stuff that you promise not to leak (perhaps stuff that has
>> > been added to a very carefully managed cluster-wide plan cache).  I
>> > am not sure of the details, and this might be crazy...
>>
>> Hmm, my first thought was that you were just reinventing memory
>> contexts, but it's really not quite the same thing, because you want
>> the allocations to end up owned by a long-lived context when you
>> succeed but a transient context when you fail.  Still, if it weren't
>> for the fact that the memory context interface is hostile to dynamic
>> shared memory's map-this-anywhere semantics, I suspect we'd try to
>> find a way to make memory contexts fit the bill, maybe by reparenting
>> contexts or even individual allocations.  You could imagine using the
>> sorts of union algorithms that are described in
>> https://en.wikipedia.org/wiki/Disjoint-set_data_structure to get very
>> low asymptotic complexity here.
>
>Yeah, I was imagining something that still works with MemoryContexts.
>Interesting idea re: unions.  I haven't got as far as thinking about how you'd actually
>make that work.  But I think we're both describing the same general kind of
>semantics; you need to be able to build stuff with automatic clean-up on non-local exit,
>but also keep that stuff for longer once you decide you're ready.
>
>Anyway, avoiding all the hard questions about new kinds of foot gun for now, here is a
>stupid hack that shows a DSA area inside the traditional fixed-address shared memory
>region, wrapped in a custom (and somewhat defective for now) MemoryContext.  It
>shows a "List"
>being manipulated by standard PostgreSQL list code that is entirely unaware that it is
>working in shared memory:
>
>postgres=# call hoge_list(3);
>NOTICE:  Contents of list:
>NOTICE:   1
>NOTICE:   2
>NOTICE:   3
>CALL
>
>You can call that procedure from various different backends to add and remove
>integers from a List.  The point is that it could just as easily be a query plan or any
>other palloc-based stuff in our tree, and the pointers work in any backend.

Thanks for the patch. I know it's a rough sketch but basically that's what I want to do. 
I tried it and it works well.

Regards,
Takeshi Ideriha