Thread: Shmem queue is not flushed if receiver is not yet attached

Shmem queue is not flushed if receiver is not yet attached

From
Pavan Deolasee
Date:
Hello,

While testing on the current PG master, I noticed a problem between backends communicating over a shared memory queue. I think `shm_mq_sendv()` fails to flush the queue, even if  `force_flush` is set to true, if the receiver is not yet attached to the queue. This simple fix solves the problem for me.

On another note, `shm_mq.h` declares `shm_mq_flush()`, but I don't see it being implemented. Maybe just a leftover from the previous work? Though it seems useful to implement that API.

Thanks,
Pavan

--
Pavan Deolasee
EnterpriseDB: https://www.enterprisedb..com
Attachment

Re: Shmem queue is not flushed if receiver is not yet attached

From
Robert Haas
Date:
On Thu, Mar 17, 2022 at 3:13 AM Pavan Deolasee <pavan.deolasee@gmail.com> wrote:
> While testing on the current PG master, I noticed a problem between backends communicating over a shared memory
queue.I think `shm_mq_sendv()` fails to flush the queue, even if  `force_flush` is set to true, if the receiver is not
yetattached to the queue. This simple fix solves the problem for me. 
>
> On another note, `shm_mq.h` declares `shm_mq_flush()`, but I don't see it being implemented. Maybe just a leftover
fromthe previous work? Though it seems useful to implement that API. 

I think that this patch is basically correct, except that it's not
correct to set mqh_counterparty_attached when receiver is still NULL.
Here's a v2 where I've attempted to correct that while preserving the
essence of your proposed fix.

I'm not sure that we need a shm_mq_flush(), but we definitely don't
have one currently, so I've also adjusted your patch to remove the
dead prototype.

Please let me know your thoughts on the attached.

Thanks,

--
Robert Haas
EDB: http://www.enterprisedb.com

Attachment

Re: Shmem queue is not flushed if receiver is not yet attached

From
Japin Li
Date:
On Tue, 24 May 2022 at 23:05, Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Mar 17, 2022 at 3:13 AM Pavan Deolasee <pavan.deolasee@gmail.com> wrote:
>> While testing on the current PG master, I noticed a problem between backends communicating over a shared memory
queue.I think `shm_mq_sendv()` fails to flush the queue, even if  `force_flush` is set to true, if the receiver is not
yetattached to the queue. This simple fix solves the problem for me.
 
>>
>> On another note, `shm_mq.h` declares `shm_mq_flush()`, but I don't see it being implemented. Maybe just a leftover
fromthe previous work? Though it seems useful to implement that API.
 
>
> I think that this patch is basically correct, except that it's not
> correct to set mqh_counterparty_attached when receiver is still NULL.
> Here's a v2 where I've attempted to correct that while preserving the
> essence of your proposed fix.
>
> I'm not sure that we need a shm_mq_flush(), but we definitely don't
> have one currently, so I've also adjusted your patch to remove the
> dead prototype.
>
> Please let me know your thoughts on the attached.
>
> Thanks,

Hi,

I have a problem that is also related to shmem queue [1], however, I cannot
reproduce it.  How did you reproduce this problem?

[1]
https://www.postgresql.org/message-id/MEYP282MB1669C8D88F0997354C2313C1B6CA9%40MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.



Re: Shmem queue is not flushed if receiver is not yet attached

From
Pavan Deolasee
Date:

Hi Robert,

On Tue, May 24, 2022 at 8:35 PM Robert Haas <robertmhaas@gmail.com> wrote:


I think that this patch is basically correct, except that it's not
correct to set mqh_counterparty_attached when receiver is still NULL.
Here's a v2 where I've attempted to correct that while preserving the
essence of your proposed fix.

This looks good to me,
 

I'm not sure that we need a shm_mq_flush(), but we definitely don't
have one currently, so I've also adjusted your patch to remove the
dead prototype.


Makes sense to me.

Thanks,
Pavan 

--
Pavan Deolasee
EnterpriseDB: https://www.enterprisedb..com

Re: Shmem queue is not flushed if receiver is not yet attached

From
Pavan Deolasee
Date:

On Wed, May 25, 2022 at 7:01 AM Japin Li <japinli@hotmail.com> wrote:

I have a problem that is also related to shmem queue [1], however, I cannot
reproduce it.  How did you reproduce this problem?


I discovered this bug while working on an extension that makes use of the shared memory queue facility. Not sure how useful that is for your purpose.

Thanks,
Pavan

--
Pavan Deolasee
EnterpriseDB: https://www.enterprisedb..com

Re: Shmem queue is not flushed if receiver is not yet attached

From
Robert Haas
Date:
On Mon, May 30, 2022 at 3:06 AM Pavan Deolasee <pavan.deolasee@gmail.com> wrote:
>> I think that this patch is basically correct, except that it's not
>> correct to set mqh_counterparty_attached when receiver is still NULL.
>> Here's a v2 where I've attempted to correct that while preserving the
>> essence of your proposed fix.
>
> This looks good to me,

Thanks for checking. Committed.

-- 
Robert Haas
EDB: http://www.enterprisedb.com