Another pipe-related problem?

[Ken Brown]

On 11/9/2021 5:16 PM, Ken Brown via Cygwin wrote:
> On 11/9/2021 9:11 AM, Ken Brown via Cygwin wrote:
>> On 11/9/2021 5:55 AM, Henry S. Thompson wrote:
>>> As you may know, the XEmacs situation is complicated.  The old source
>>> repo (bitbucket.org/xemacs) no longer exists.  There's a fork that's
>>> still being maintained, but it's not widely publicised.  That's the
>>> one I'm working with -- are you aware of this.
>>
>> I was aware that the bitbucket repo didn't exist, because I tried to get the 
>> sources there.  But I didn't know about the fork.  Please point me to it, or 
>> just make a tarball available to me somehow.
>>
>>> Here are the immediate contexts from the sources for the xemacs
>>> sources in the above backtrace, might be enough to check your
>>> hypothesis:
>>>
>>> sysdep.c:
>>>
>>>    retry_read_1 (int fildes, void *buf, size_t nbyte, int allow_quit)
>>>    {
>>>      ssize_t rtnval;
>>>
>>>      while ((rtnval = read (fildes, buf, nbyte)) == -1
>>>             && (errno == EINTR))                         <<<<<<<<<<<<<<<<<<<<
>>>        {
>>>          if (allow_quit)
>>>            QUIT;
>>>        }
>>>      return rtnval;
>>>    }
>> I'll have to reproduce the hang myself in order to test this (or maybe you 
>> could test it), but I now have a new guess: If the read call above keeps 
>> failing with EINTR, then we're in an infinite loop.  This could happen because 
>> of the following code in fhandler_pipe::raw_read:
>>
>>    DWORD waitret = cygwait (read_mtx, timeout);
>>    switch (waitret)
>>      {
>>      case WAIT_OBJECT_0:
>>        break;
>>      case WAIT_TIMEOUT:
>>        set_errno (EAGAIN);
>>        len = (size_t) -1;
>>        return;
>>      default:
>>        set_errno (EINTR);
>>        len = (size_t) -1;
>>        return;
>>      }
>>
>> Takashi, is EINTR really the appropriate errno in the default case?  Isn't 
>> cygwait supposed to handle signals?
> 
> I was able to build XEmacs and reproduce the problem.  My guess was wrong, 
> though my question to Takashi still stands.  I think the infinite loop is 
> actually caused by a bug in fhandler_pipe::raw_read that only affects 
> non-blocking pipes (which is what we have in XEmacs).
> 
> Consider the following code in fhandler_pipe::raw_read:
> 
>        status = NtReadFile (get_handle (), evt, NULL, NULL, &io, ptr,
>                 len1, NULL, NULL);
>        if (evt && status == STATUS_PENDING)      <<<<<<<<<<<<<<<<<<<<<<<<<<<<
>      {
>        waitret = cygwait (evt, INFINITE, cw_cancel | cw_sig);
> [...]
>      }
> 
> In the non-blocking case, evt == NULL, but we still might have status == 
> STATUS_PENDING.  We then should wait on get_handle() to let NtReadFile finish. 
> By not waiting, we end up using a garbage value from io.Information, leading to 
> an infinite loop in drain_signal_event_pipe.
> 
> I'll try to fix this.

BTW, a quick glance at raw_write suggests that there might be a similar bug 
there, but I'll have to look more closely.

Ken