Possiblly bug of cygwin1.dll

[Takashi Yano]

Hi Corinna,

On Fri, 19 Jan 2024 15:28:40 +0100
Corinna Vinschen wrote:
> On Jan 19 22:44, Takashi Yano via Cygwin wrote:
> > Hi,
> > 
> > I might find the bug of cygwin1.dll (including 3.4.x, 3.5.0 (TEST)).
> > The following test case (c++ code) causes handle leak.
> > 
> > This issue is reproducible with both g++ and clang++.
> > However, it does not happen in Linux environment.
> > So I guess this is the cygwin1.dlll bug.
> > 
> > I looked into this problem a bit, and found number of event handle
> > increases every loop.
> > 
> > I doubt pthread_mutex_xxx functions.
> > 
> > #include <future>
> > int func() { return 0; }
> > int main()
> > {
> >   for (;;) {
> >     std::future<int> f = std::async(std::launch::async, func);
> >     f.get();
> >   }
> >   return 0;
> > }
> 
> Can you create a plain C testcase from there?  It's much easier to
> debug.

I could symplify the test case:
#include <mutex>
int main()
{
  for (;;) {
    std::mutex *m = new std::mutex;
    m->lock();
    m->unlock();
    delete m;
  }
  return 0;
}

And I tried to observe the pthread_mutex_xxx() call. Then found the
test case does like:

#include <pthread.h>
int main()
{
  for (;;) {
    pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
    pthread_mutex_lock(&m);
    pthread_mutex_unlock(&m);
  }
  return 0;
}

POSIX states pthread_mutex_t can be initialized with
PTREAD_MUTEX_INITIALZER when it is STATICALLY allocated.

In this case, m is not static. So it seems that this is
a bug of libstdc++. However, the plain c code above works
in Linux without problems even with non-static mutex m.

I guess it is very difficult to make the plain c code above
work in cygwin, because cygwin can not know when cygwin can
discard the mutex resources...

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>