seg fault on dsm_create call

On Tue, Jun 28, 2016 at 10:11 AM, Max Fomichev <max.fomitchev@gmail.com> wrote:

Hello,
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;
RegisterBackgroundWorker(&worker);
}

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);
BackgroundWorkerUnblockSignals();
...

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.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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On 28/06/16 19:24, Robert Haas wrote:

On Tue, Jun 28, 2016 at 10:11 AM, Max Fomichev <max.fomitchev@gmail.com> wrote:

Hello,
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;
RegisterBackgroundWorker(&worker);
}

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);
BackgroundWorkerUnblockSignals();
...

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.

Thanks.
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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On Tue, Jun 28, 2016 at 12:45 PM, Max Fomichev <max.fomitchev@gmail.com> wrote:

On 28/06/16 19:24, Robert Haas wrote:
Thanks.
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 -

You need a resource owner for either dsm_attach or dsm_create.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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