On Sun, 5 Sep 2021 08:15:23 +0900
Takashi Yano wrote:
> Hi Ken,
> 
> On Sat, 4 Sep 2021 10:04:12 -0400
> Ken Brown wrote:
> > On 9/4/2021 8:37 AM, Takashi Yano wrote:
> > > On Sat, 4 Sep 2021 21:02:58 +0900
> > > Takashi Yano wrote:
> > >> Hi Corinna, Ken,
> > >>
> > >> On Fri, 3 Sep 2021 09:27:37 -0400
> > >> Ken Brown wrote:
> > >>> On 9/3/2021 8:22 AM, Takashi Yano wrote:
> > >>>> POSIX says:
> > >>>>       The value returned may be less than nbyte if the number of bytes left
> > >>>>       in the file is less than nbyte, if the read() request was interrupted
> > >>>>       by a signal, or if the file is a pipe or FIFO or special file and has
> > >>>>                                                                         ~~~
> > >>>>       fewer than nbyte bytes immediately available for reading.
> > >>>>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >>>>
> > >>>> https://pubs.opengroup.org/onlinepubs/009604599/functions/read.html
> > >>>>
> > >>>> If it is turned over, read() should read all data immediately available,
> > >>>> I think.
> > >>>
> > >>> I understand the reasoning now, but I think your patch isn't quite right.  As it
> > >>> stands, if the call to NtQueryInformationFile fails but total_length != 0,
> > >>> you're trying to read again without knowing that there's data in the pipe.
> > >>>
> > >>> Also, I think you need the following:
> > >>>
> > >>> diff --git a/winsup/cygwin/fhandler_pipe.cc b/winsup/cygwin/fhandler_pipe.cc
> > >>> index ef7823ae5..46bb96961 100644
> > >>> --- a/winsup/cygwin/fhandler_pipe.cc
> > >>> +++ b/winsup/cygwin/fhandler_pipe.cc
> > >>> @@ -348,8 +348,13 @@ fhandler_pipe::raw_read (void *ptr, size_t& len)
> > >>>        CloseHandle (evt);
> > >>>      if (status == STATUS_THREAD_SIGNALED)
> > >>>        {
> > >>> -      set_errno (EINTR);
> > >>> -      len = (size_t) -1;
> > >>> +      if (total_len == 0)
> > >>> +       {
> > >>> +         set_errno (EINTR);
> > >>> +         len = (size_t) -1;
> > >>> +       }
> > >>> +      else
> > >>> +       len = total_len;
> > >>>        }
> > >>>      else if (status == STATUS_THREAD_CANCELED)
> > >>>        pthread::static_cancel_self ();
> > >>
> > >> Thanks for your advice. I fixed the issue and attached new patch.
> > >>
> > >> On Fri, 3 Sep 2021 17:37:13 +0200
> > >> Corinna Vinschen wrote:
> > >>> Hmm, I see the point, but we might have another problem with that.
> > >>>
> > >>> We can't keep the mutex while waiting on the pending read, and there
> > >>> could be more than one pending read running at the time.  if so,
> > >>> chances are extremly high, that the data written to the buffer gets
> > >>> split like this:
> > >>>
> > >>>     reader 1		               reader 2
> > >>>
> > >>>     calls read(65536)                   calls read(65536)
> > >>>
> > >>>     calls NtReadFile(16384 bytes)
> > >>>                                         calls NtReadFile(16384 bytes)
> > >>>
> > >>> writer writes 65536 bytes
> > >>>
> > >>>     wakes up and gets 16384 bytes
> > >>>                                         wakes up and gets 16384 bytes
> > >>>     gets the mutex, calls
> > >>>     NtReadFile(32768) which
> > >>>     returns immediately with
> > >>>     32768 bytes added to the
> > >>>     caller's buffer.
> > >>>
> > >>> so the buffer returned to reader 1 is 49152 bytes, with 16384 bytes
> > >>> missing in the middle of it, *without* the reader knowing about that
> > >>> fact.  If reader 1 gets the first 16384 bytes, the 16384 bytes have
> > >>> been read in a single call, at least, so the byte order is not
> > >>> unknowingly broken on the application level.
> > >>>
> > >>> Does that make sense?
> > >>
> > >> Why can't we keep the mutex while waiting on the pending read?
> > >> If we can keep the mutex, the issue above mentioned does not
> > >> happen, right?
> > >>
> > >> What about the patch attached? This keeps the mutex while read()
> > >> but I do not see any defects so far.
> > 
> > LGTM.
> > 
> > If Corinna agrees, I have a couple of suggestions.
> > 
> > 1. With this patch, we can no longer have more than one pending ReadFile.  So 
> > there's no longer a need to count read handles, and the problem with select is 
> > completely fixed as long as the number of bytes requested is less than the pipe 
> > buffer size.
> > 
> > 2. raw_read is now reading in chunks, like raw_write.  For readability of the 
> > code, I think it would be better to make the two functions as similar as 
> > possible.  For example, you could replace the do/while loop by a 
> > while(total_len<orig_len) loop.  And you could even use similar names for the 
> > variables, e.g., nbytes instead of total_len, or vice versa.
> 
> Thanks for the suggestion. I have rebuilt the patch.
> Please see the patch attached.

This patch seems to fail to adopt to current git head of topic/pipe
branch. I rebuilt the patch to fit current top/pipe.

Please see the patch attached.

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