On 28/06/16 19:24, Robert Haas wrote:
On Tue, Jun 28, 2016 at 10:11 AM, Max Fomichev <max.fomitc...@gmail.com> wrote:
sorry for my repost from psql-novice, probably it was not a right place for
my question.

I'm trying to understand how to work with dynamic shared memory, message
queues and workers.
The problem is I can not initialize any dsm segment -

     void _PG_init() {
         dsm_segment *seg = dsm_create(32768, 0); // Segmentation fault here
         BackgroundWorker worker;
         sprintf(worker.bgw_name, "mystem wrapper process");
         worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
         worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
         worker.bgw_restart_time = BGW_NEVER_RESTART;
         worker.bgw_main = mainProc;
         worker.bgw_notify_pid = 0;

Also I was trying to move dsm_create call to a worker, but with the same
result -

     static void mainProc(Datum) {
         dsm_segment *seg = dsm_create(32768, 0); // Segmentation fault here
         pqsignal(SIGTERM, mystemSigterm);

What could be a reason and what am I doing wrong?
I think there are two problems.

1. You need to set up a ResourceOwner before you can attach to a DSM
segment.  Something like: CurrentResourceOwner =
ResourceOwnerCreate(NULL, "name of my extension").

2. You can't do this from inside a _PG_init() block.  That will run in
every process that loads this, which is probably not what you want,
and it will run in the postmaster also, which will not work: the
postmaster cannot use DSM.

Actually, I'd like to change #1 at some point, so that if
CurrentResourceOwner = NULL and you create or attach to a DSM, you
just get a backend-lifespan mapping.  The current setup is annoying
rather than helpful.  But currently that's how it is.

It works now with CurrentResourceOwner = ResourceOwnerCreate(NULL, "name of my extension")

I am a little bit confused about test/modules/test_shm_mq, where CurrentResourceOwner is set up before dsm_attach, not dsm_create -

     * Connect to the dynamic shared memory segment.
     * The backend that registered this worker passed us the ID of a shared
     * memory segment to which we must attach for further instructions.  In
     * order to attach to dynamic shared memory, we need a resource owner.
* Once we've mapped the segment in our address space, attach to the table * of contents so we can locate the various data structures we'll need to
     * find within the segment.
    CurrentResourceOwner = ResourceOwnerCreate(NULL, "test_shm_mq worker");
    seg = dsm_attach(DatumGetInt32(main_arg));

Best regards,
Max Fomichev

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