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Date:	Fri, 18 Jul 2025 17:41:08 +0900
To:	cygwin AT cygwin DOT com
Subject:	Re: Calling system() in multi-threads.
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From:	Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Fri, 18 Jul 2025 09:51:11 +0200
Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com> wrote:

> On Jul 18 00:44, Takashi Yano via Cygwin wrote:
> > On Thu, 17 Jul 2025 17:19:49 +0200
> > Corinna Vinschen wrote:
> > > On Jul 17 23:14, Takashi Yano via Cygwin wrote:
> > > > Hi Corinna,
> > > > 
> > > > On Wed, 16 Jul 2025 17:36:42 +0200
> > > > Corinna Vinschen wrote:
> > > > > On Jul 16 23:52, Takashi Yano via Cygwin wrote:
> > > > > > Do you have any idea?
> > > > > 
> > > > > Locking would be super-simple.
> > > > > 
> > > > > But theoretically it should be possible to use a local child_info_spawn
> > > > > variable at this point.  The ch_spawn child_info_spawn instance is not
> > > > > copied to the child anyway, so that should be safe.  The same goes for
> > > > > posix_spawn() then, btw.
> > > > > 
> > > > > I checked the sources and I don't see any dependency to ch_spawn
> > > > > from a spawning process, in contrast to an exec'ing process.  That
> > > > > doesn't mean there is none, just that I didn't find any.
> > > > 
> > > > Thanks!
> > > > As a starting point, I tried tntroducing locking. It almost works
> > > > as expected, however, sometimes my STC in my first report is hangs
> > > > if N is large e.g. 100. The patch is as attached.
> > > > 
> > > > What am I missing?
> > > 
> > > I don't know.  You're perhaps not releasing the lock in all cases.
> > > But I would have to debug this just like you ¯\_(ツ)_/¯
> > > 
> > > Out of curiosity, did you try using a locale child_info_spawn instance
> > > instead?  That would be a rather nice solution, but I'm pretty sure
> > > there's some other problem lurking in the dark...
> > 
> > I'm not sure what to do with local child_info_spawn.
> > Some of other modules refer to ch_spawn, such as exception.cc and
> > pinfo.cc. Also, has_execed* uses ch_spawn. What should we do for that?
> 
> Yes, that's what I meant with my above paragraph, i.e.
> 
> > > > > I checked the sources and I don't see any dependency to ch_spawn
> > > > > from a spawning process, in contrast to an exec'ing process.  That
> > > > > doesn't mean there is none, just that I didn't find any.
> 
> These external dependencies *should* only be of interest when performing
> an execve, but not when performing a spawnve.

So, the previous patch might not correct.
The following makes more sense, I suppose.

diff --git a/winsup/cygwin/spawn.cc b/winsup/cygwin/spawn.cc
index cb58b6eed..56fca6e45 100644
--- a/winsup/cygwin/spawn.cc
+++ b/winsup/cygwin/spawn.cc
@@ -944,6 +944,7 @@ spawnve (int mode, const char *path, const char *const *argv,
   int ret;
 
   syscall_printf ("spawnve (%s, %s, %p)", path, argv[0], envp);
+  child_info_spawn ch_spawn_local;
 
   if (!envp)
     envp = empty_env;
@@ -961,7 +962,7 @@ spawnve (int mode, const char *path, const char *const *argv,
     case _P_WAIT:
     case _P_DETACH:
     case _P_SYSTEM:
-      ret = ch_spawn.worker (path, argv, envp, mode);
+      ret = ch_spawn_local.worker (path, argv, envp, mode);
       break;
     default:
       set_errno (EINVAL);

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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