Another pipe-related problem?
Ken Brown
kbrown@cornell.edu
Wed Nov 10 02:53:35 GMT 2021
On 11/9/2021 5:20 PM, Ken Brown via Cygwin wrote:
> 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.
Nope, that doesn't seem to be the issue. Even after fixing this, I still see an
infinite loop. Probably NtReadFile finishes quickly enough that io.Information
is in fact valid by the time we test it. Back to the drawing board.
> 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
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