Re: [PATCH v7] Cygwin: pty: Fix race issue between starting and exiting non-cygwin apps

Thanks Mark. Pushed.

On Wed, 24 Jun 2026 00:09:26 -0700
Mark Geisert wrote:
> Hi Takashi,
> 
> This v7 LGTM.  OK to push.
> Thanks,
> 
> ..mark
> 
> On 6/23/2026 3:51 PM, Takashi Yano wrote:
> > Without this patch, when a non-cygwin program (A) is about to exit, and
> > another non-cygwin program (B) is started, input transferring between
> > cyg-pipe and nat-pipe may not work as expected. When the non-cygwin
> > program (A) exits, input transferring from nat-pipe to cyg-pipe will be
> > performed. However, the the non-cygwin program (B) will performs input
> > transferring from cyg-pipe to nat-pipe at the same time. The mechanism
> > of the problem is as follows.
> >   1) The the non-cygwin program (A) checks current input pipe state,
> >      then it is nat-pipe since the this program is a non-cygwin program.
> >      The program (A) also checks if any handover target exists, but
> >      it is not found since the program (B) is not started yet. So,
> >      the program (A) decided to transfer input form nat-pipe to cyg-
> >      pipe.
> >   2) Before the non-cygwin (A) program performs input transferring,
> >      if the non-cygwin program (B) is started and checks the input
> >      pipe state, it is nat-pipe state, so the non-cygwin program (B)
> >      does not perform input transferring.
> >   3) However, just after that, the non-cygwin program (A) performs
> >      input transferring from nat-pipe to cyg-pipe, so typeahead input
> >      will be stored in cyg-pipe.
> >   4) The non-cygwin program (B) cannot read the typeahead input
> >      because it is now in the cyg-pipe.
> > 
> > The following code demonstrates the issue.
> >    #include <stdio.h>
> >    #include <stdlib.h>
> >    #include <unistd.h>
> > 
> >    int main(int argc, char *argv[])
> >    {
> >      int n = 1;
> >      if (argc > 1)
> >        n = atoi(argv[1]);
> >      if (fork()) {
> >        execlp("cmd.exe", "cmd", NULL);
> >        perror("execlp(\"cmd\"): ");
> >      }
> >      for (int i=0; i<n; i++) {
> >        if (fork() == 0) {
> >          execlp("./winsleep.exe", "winsleep", "0", NULL);
> >          perror("execlp(\"winsleep\"): ");
> >        }
> >      }
> >      return 0;
> >    }
> > 
> > Transferring input itself is guarded by input_mutex, but the pre-
> > check is not. With this patch, the guard is enhanced so that the
> > state check and transferring input are done in atomic way.
> > 
> > Fixes: f20641789427 ("Cygwin: pty: Reduce unecessary input transfer.")
> > Signed-off-by: Takashi Yano <[email protected]>
> > Reviewed-by: Mark Geisert <[email protected]>
> > ---
> >   winsup/cygwin/fhandler/pty.cc           | 99 +++++++++++++++----------
> >   winsup/cygwin/local_includes/fhandler.h |  2 +
> >   2 files changed, 60 insertions(+), 41 deletions(-)
> [...]


-- 
Takashi Yano <[email protected]>