On Thu, Dec 15, 2016 at 4:17 AM, Amit Kapila <amit.kapil...@gmail.com> wrote:
> On Thu, Dec 15, 2016 at 10:04 AM, Andres Freund <and...@anarazel.de> wrote:
>> Hi,
>> On 2016-12-12 16:46:38 +0900, Michael Paquier wrote:
>>> Ashutosh, could you try it and see if it improves things?
>> So what's the theory of why this triggers pldebugger hangs, but
>> apparently causes not many other problems?
> The theory is that with pldebugger latch event gets triggered before
> FD_READ due to which it seems like the second event gets lost and
> WaitForMultipleObjects() keeps on waiting indefinitely.  The probable
> reason is that we fail to reset the event due to which we are seeing
> this behavior. That seems to be required as per msdn as well, as
> pointed by Robert upthread.
> Find attached patch to implement the resetting of event only for the
> required case.  This fixes the reported problem.

After some study I don't think this is quite right yet.  The client
code is not obliged to call ModifyWaitEvent() before again calling
WaitEventSetWait().  I think it should be the responsibility of
WaitEventSetWaitBlock() to reset the event, if needed, before calling
WaitForMultipleObjects().  BTW, I suggest this rewritten comment:

             * FD_READ events are edge-triggered on Windows per
             * but users of the latch facility are entitled to expect
             * level-triggered behavior.  Indeed, this function itself
             * expects level-triggered behavior, since it doesn't guarantee
             * that a read event will be returned if the latch is set at the
             * same time.  Even if it did, the caller might drop that event
             * expecting it to reoccur on next call.  So, we must force
             * the event to be reset if this WaitEventSet is used again in
             * order to avoid an indefinite hang.

The dashes keep pgindent from inserting a line break into the link.

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

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