Re: Synchronization problem with posix_spawn

[Ken Brown <kbrown@cornell.edu>]

On 8/3/2020 7:22 AM, Corinna Vinschen wrote:
> On Aug  2 12:21, Ken Brown via Cygwin-developers wrote:
>> On 8/2/2020 10:50 AM, Corinna Vinschen wrote:
>>> On Aug  2 13:55, Corinna Vinschen wrote:
>>>> [Moving this discussion to cygwin-developers]
>>>> On Jul 31 10:10, Corinna Vinschen wrote:
>>>>> On Jul 30 19:04, Ken Brown via Cygwin wrote:
>>>>>> On 7/30/2020 1:17 PM, Corinna Vinschen wrote:
>>>>>>> On Jul 30 13:59, Corinna Vinschen wrote:
>>>>>>>> On Jul 29 19:12, Ken Brown via Cygwin wrote:
>>>>>>>>> On 7/29/2020 4:17 PM, Ken Brown via Cygwin wrote:
>>>>>>>>>> posix_spawn(p) returns before the spawned process is fully up and
>>>>>>>>>> running.  [...]
>>>>>>>>> I just took a look at the source, and I see that posix_spawn
>>>>>>>>> was taken from FreeBSD.
>>>>>>>>> [...]
>>>>>>>>
>>>>>>>> Actually, this is a Cygwin problem.
>>>>>>>> [...]
>>>>>>>> IOW, we need a Cygwin-specific do_posix_spawn() using fork(2)
>>>>>>>> in conjunction with some synchronization the BSD function
>>>>>>>> gets "for free" by using its specific vfork(2).
>>>>>>>
>>>>>>> Below is a POC implementation for a Cygwin-specific do_posix_spawn().
>>>>>>> [...]
>>>>>>> Can you give it a try?
>>>>>>
>>>>>> It looks like something further is needed: 'wait' doesn't seem to recognize
>>>>>> the spawned process.
>>>>>
>>>>> Oh well.
>>>>> [...]
>>>>
>>>> I attached another patch.  This one is designed from the ground up and
>>>> I *think* it works as desired.  I added lots of comments so the idea
>>>> behind this patch should be clear enough.
>>>>
>>>> Please give it a try.
>>>
>>> Version 2 of the patch attached.  It occured to me belatedly, that
>>> parent and child have to be synchronized prior to calling WFMO in
>>> the parent.  Otherwise OpenProcess in __posix_spawn_sem_wait_and_close
>>> may end up opening the exec'ed process rather than the forked child.
>>
>> LGTM, and passes all the tests I could throw at it.
>>
>> FYI, I noticed the posix_spawn problem because I've been regularly testing
>> my FIFO code by running the test suite from the "casual" project
>> (https://bitbucket.org/casualcore/).  That project uses FIFOs extensively,
>> and it also uses posix_spawn.  I only realized a few days ago that some of
>> the test failures I had been seeing had nothing to do with FIFOs but rather
>> stemmed from the posix_spawn issue that I reported.  Those tests now all
>> pass with your patch.
> 
> Great, I pushed version 3 of the patch (with an additional minor fix).
> Can you check this version again, too?

All posix_spawn tests still pass.

Ken