Re: Latches API on windows

On 9 October 2017 at 21:26, Abbas Butt <abbas(dot)butt(at)enterprisedb(dot)com> wrote:
> Hi,
> I am working on a contrib module that uses RegisterDynamicBackgroundWorker
> API
> to create a couple of worker processes. For synchronization between the
> background worker processes I am using InitSharedLatch, SetLatch, WaitLatch
> APIs.
> One of the processes is supposed to wait for the latch, the other is
> supposed to set it.
> The system works perfectly fine as long as its run on Linux, however when
> tried
> on Windows, it fails giving the error:
> ResetEvent failed: error code 6
> Error code 6 means invalid handle. Debugging reveals that the handle
> contains
> a valid value, however it seems that the handle is not accessible (was not
> created)
> in the process that is calling ResetEvent.
>
> Debugging the issue lead me to the following comment on top of
> InitSharedLatch:
>
> * InitSharedLatch needs to be called in postmaster before forking child
> * processes, usually right after allocating the shared memory block
> * containing the latch with ShmemInitStruct. (The Unix implementation
> * doesn't actually require that, but the Windows one does.)
>
> In my case this is not true, I am calling InitSharedLatch in _PG_init
> which gets called at CREATE EXTENSION time.
> My question : Is there a way to get the latches API work on windows
> the way it is working on Linux?

I suspect you'd need to do it by having your extension load via
shared_preload_libraries, registering its latch in shmem_startup_hook
.

But ... that's an off-the-cuff guess.

--
Craig Ringer http://www.2ndQuadrant.com/
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