Calling system() in multi-threads.

[Takashi Yano]

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?

I've just tried simply the following patch, however, this also hangs
with my STC.

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;
@@ -951,7 +952,7 @@ spawnve (int mode, const char *path, const char *const *argv,
   switch (_P_MODE (mode))
     {
     case _P_OVERLAY:
-      ch_spawn.worker (path, argv, envp, mode);
+      ch_spawn_local.worker (path, argv, envp, mode);
       /* Errno should be set by worker.  */
       ret = -1;
       break;
@@ -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.yano@nifty.ne.jp>