Re: LISTEN denial of service with aborted transaction

Matt Newell <newellm(at)blur(dot)com> writes:
> Patch attached works great. I added the dboid to the QueueBackendStatus
> struct but that might not be needed if there is an easy and fast way to get a
> db oid from a backend pid.

Not particularly; I agree with adding it to this data structure. One
reason is it makes the array stride a power of 2, so array accesses may be
a shade cheaper than before.

> I also skip the max pos calculation loop if QUEUE_HEAD is on the same page as
> QUEUE_TAIL, with the thought that it's not desirable to increase the time that
> AsyncQueueLock is held if the queue is small and
> asyncQueueReadAllNotifications will execute quickly.

Check.

There were a couple of minor bugs in this, but I fixed them and committed
it.

> I then noticed that we are taking the same lock twice in a row to read the
> same values, first in Exec_ListenPreCommit then in
> asyncQueueReadAllNotifications, and much of the time the latter will simply
> return because pos will already be at QUEUE_HEAD. I prepared a second patch
> that splits asyncQueueReadAllNotifications. Exec_ListPreCommit then only calls
> the worker version only when needed. This avoids the duplicate lock.

I was unimpressed with this; the refactoring of asyncQueueReadAllNotifications
seemed messy and definitely was undocumented. I think the main actual
value is to skip doing anything during Exec_ListenPreCommit when
max == head, so I extracted that part and included it.

regards, tom lane