Re: [HACKERS] Error: dsa_area could not attach to a segment thathas been freed

On Wed, Sep 20, 2017 at 6:14 PM, Gaddam Sai Ram

<gaddamsairam.n@zohocorp.com> wrote:

> Thank you very much! That fixed my issue! :)

> I was in an assumption that pinning the area will increase its lifetime but

> yeah after taking memory context into consideration its working fine!

So far the success rate in confusing people who first try to make

long-lived DSA areas and DSM segments is 100%. Basically, this is all

designed to ensure automatic cleanup of resources in short-lived

scopes.

Good luck for your graph project. I think you're going to have to

expend a lot of energy trying to avoid memory leaks if your DSA lives

as long as the database cluster, since error paths won't automatically

free any memory you allocated in it. Right now I don't have any

particularly good ideas for mechanisms to deal with that. PostgreSQL

C has exception-like error handling, but doesn't (and probably can't)

have a language feature like scoped destructors from C++. IMHO

exceptions need either destructors or garbage collection to keep you

sane. There is a kind of garbage collection for palloc'd memory and

also for other resources like file handles, but if you're using a big

long lived DSA area you have nothing like that. You can use

PG_TRY/PG_CATCH very carefully to clean up, or (probably better) you

can try to make sure that all your interaction with shared memory is

no-throw (note that that means using dsa_allocate_extended(x,

DSA_ALLOC_NO_OOM), because dsa_allocate itself can raise errors). The

first thing I'd try would probably be to keep all shmem-allocating

code in as few routines as possible, and use only no-throw operations

in the 'critical' regions of them, and maybe look into some kind of

undo log of things to free or undo in case of error to manage

multi-allocation operations if that turned out to be necessary.

--

Thomas Munro

http://www.enterprisedb.com