Latches API on windows

On 9 October 2017 at 21:26, Abbas Butt <abbas.butt@enterprisedb.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.

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Craig Ringer <craig@2ndquadrant.com> writes:

On 9 October 2017 at 21:26, Abbas Butt <abbas.butt@enterprisedb.com> wrote:

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

Yeah. That would also let you request your shared memory area honestly,
instead of relying on there being some slop in the initial allocation.

regards, tom lane

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Thanks for the suggestion.

On Mon, Oct 9, 2017 at 6:56 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Craig Ringer <craig@2ndquadrant.com> writes:

On 9 October 2017 at 21:26, Abbas Butt <abbas.butt@enterprisedb.com>

wrote:

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

Yeah. That would also let you request your shared memory area honestly,
instead of relying on there being some slop in the initial allocation.

regards, tom lane

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On October 9, 2017 6:56:10 AM PDT, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Craig Ringer <craig@2ndquadrant.com> writes:

On 9 October 2017 at 21:26, Abbas Butt <abbas.butt@enterprisedb.com>

wrote:

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

Yeah. That would also let you request your shared memory area
honestly,
instead of relying on there being some slop in the initial allocation.

Might be dsm style memory.

I think the right approach here, regardless of the source of the memory, is to actually bit create a new latch, but instead to store a pointer the the owning processes preexisting latch. Besides solving this issue, it also avoids problems with various routines already waiting on the proclatch.

Andres

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