Re: Listen / Notify - what to do when the queue is full

On Wed, Feb 3, 2010 at 4:34 AM, Joachim Wieland <joe(at)mcknight(dot)de> wrote:
> On Wed, Feb 3, 2010 at 2:05 AM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
>>> Thanks, very well spotted... Actually the same is true for LISTEN... I
>>> have reworked the patch to do the changes to listenChannels only in
>>> the post-commit functions.
>>
>> I'm worried that this creates the opposite problem: that a LISTEN
>> transaction might commit before a NOTIFY transaction, and yet miss the
>> notification.
>
> See the following comment and let me know if you agree...
>
> ! /*
> !  * Exec_ListenBeforeCommit --- subroutine for AtCommit_NotifyBeforeCommit
> !  *
> !  * Note that we do only set our pointer here and do not yet add the channel to
> !  * listenChannels. Since our transaction could still roll back we do this only
> !  * after commit. We know that our tail pointer won't move between here and
> !  * directly after commit, so we won't miss a notification.
> !  */
>
> However this introduces a new problem when an initial LISTEN aborts:
> Then we are not listening to anything but for other backends it looks
> like we were. This is tracked by the boolean variable
> backendExecutesInitialListen and gets cleaned up in AtAbort_Notify().
>
>
>> It seems safest to me to add a backend (LISTEN) to the list before
>> commit, and remove a backend (UNLISTEN) after commit. That way we are
>> sure to only receive spurious notifications, and can't miss any.
>
> If a LISTEN aborted we would not only receive a few spurious
> notifications from it but would receive notifications on this channel
> forever even though we have never executed LISTEN on it successfully.

Jeff, do you think this patch is ready for committer? If so, please
mark it as such on commitfest.postgresql.org - otherwise, please
clarify what you think the action items are.

Thanks,

...Robert