Synchronization problem with posix_spawn

[Ken Brown]

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.

For the sake of my education, could you explain what made you decide that a 
semaphore was the right kind of synchronization object for this problem?  (If 
that doesn't have an easy answer, don't worry about it.)

Thanks.

Ken