On Wed, 15 Jul 2026 11:20:29 +0000
kikairoya wrote:
> Hello,
>
> After upgrading to Cygwin 3.6.10, opening a fifo endpoint causes a
> deadlock.
> (Found in the LLVM testsuite,
> llvm/unittests/Analysis/MLModelRunnerTest.cpp:
> InteractiveModelRunner::Evaluation).
>
>
> reproducer:
>
> $ gcc fifotest.c && ./a
> trying to open /tmp/thetestfifo.pipe for write
> trying to open /tmp/thetestfifo.pipe for read
> (hangs here, and some of runs ignore Ctrl+C)
>
> Opening a fifo is blocked correctly until another endpoint is opened.
> However, Cygwin 3.6.10 never returns, even after both endpoints are
> requested to be opened.
>
> Downgrading to Cygwin 3.6.9 results in successful runs:
>
> $ gcc fifotest.c && ./a
> trying to open /tmp/thetestfifo.pipe for write
> trying to open /tmp/thetestfifo.pipe for read
> wr = 3
> rd = 4
>
>
> fifotest.c:
>
> #include <fcntl.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <pthread.h>
>
> const char *name = "/tmp/thetestfifo.pipe";
>
> void *bg() {
> printf("trying to open %s for read\n", name);
> int rd = open(name, O_RDONLY);
> printf("rd = %d\n", rd);
> close(rd);
> return 0;
> }
>
> int main() {
> mkfifo(name, 0666);
>
> pthread_t thr;
> pthread_create(&thr, 0, bg, 0);
>
> printf("trying to open %s for write\n", name);
> int wr = open(name, O_WRONLY);
> printf("wr = %d\n", wr);
> close(wr);
>
> void *thret;
> pthread_join(thr, &thret);
>
> unlink(name);
> }
Thanks for the report. I could reproduce the issue and found the culprit.
commit 31bf91f867c5fadd7deb408cf06fe3af8e86bb74
Author: Mark Geisert <[email protected]>
Date: Wed May 27 22:42:44 2026 -0700
Cygwin: Ensure unused fd available for open()
The existing logic for open() assumes an fd is always available in
the fdtable for a created file. This leads to a situation where, if
there is no fd available due to the OPEN_MAX limit being hit, the
file is created but cannot be referenced by a Cygwin fd.
Move the fd reservation code to an earlier location within open().
Reported-by: Christian Franke <[email protected]>
Addresses: https://cygwin.com/pipermail/cygwin/2026-May/259664.html
Signed-off-by: Mark Geisert <[email protected]>
Fixes: e859706578ba (* autoload.cc (NtCreateFile): Add.)
Mark, could you please have a look?
--
Takashi Yano <[email protected]>
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