Synchronization problem with posix_spawn

[Corinna Vinschen]

On Jul 30 19:04, Ken Brown via Cygwin wrote:
> Hi Corinna,
> 
> On 7/30/2020 1:17 PM, Corinna Vinschen wrote:
> > Hi Ken,
> > 
> > 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.  Does FreeBSD have the same problem?  Should applications just be
> > > > aware of this issue and insert a sleep after posix_spawn before sending
> > > > signals?
> > > 
> > > Actually, this is a Cygwin problem.  I just had a look into the
> > > newlib implementation myself, and it turns out that the code,
> > > in particular the do_posix_spawn() function, is BSD specific.  It
> > > relies on the BSD implementation of vfork(2).  Cygwin's vfork(2)
> > > on the other hand, is NOT following the historic idea of the
> > > BSD vfork(2), rather it's equivalent to fork(2).  This is
> > > POSIX compliant, but certainly the reliance of the BSD vfork
> > > behaviour makes do_posix_spawn() as it's implemented right now,
> > > not overly functional for Cygwin.
> > > 
> > > 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().
> > If this does the trick (at least your testcase works in my testing),
> > then I'm planning to move the function over to the winsup/cygwin dir
> > so it can be streamlined further.
> > 
> > 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 did a quick test with your new testcase (thanks for that!)
and it seems to be a bit more complicated than I anticipated yesterday.
The parent-child relationship between the processes is broken.  I have
to think a while about this problem, stay tuned.


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer