On Wed, Feb 3, 2010 at 4:34 AM, Joachim Wieland <j...@mcknight.de> wrote: > On Wed, Feb 3, 2010 at 2:05 AM, Jeff Davis <pg...@j-davis.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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
