bug#18396: 24.3.1; On windows, process-send-string can freeze Emacs

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 3 Sep 2014 20:43:07 +0200
> From: Jorgen Schaefer <forcer@forcix.cx>
> Cc: 18396@debbugs.gnu.org
> 
> > Using "/" as the default directory on Windows is a bad idea, as that
> > is not a fully-qualified absolute file name.
> 
> What would be the equivalent for "out of the way and not blocking any
> mount point" (or equivalent) on Windows?

I might have a suggestion, if you explain what "out of the way" and
"not blocking any mount points" mean.

> > Looks like the write to the pipe never returned.  This could be
> > because the pipe is full and is not being read from the other end
> > (Windows pipes have 4K buffers, and you show above more than 6K of
> > data).
> 
> That is quite likely the explanation. The Python process does the
> equivalent of a REPL, reading one RPC call, evaluating it, and writing
> the response. If in the duration of that evaluation Emacs sends more
> than 4k of data, it will hang. If the response is larger than 4k,
> Python in turn will hang. Resulting in a deadlock.
> 
> Am I missing something?

I'd expect Python to continue reading from the pipe once it evaluated
one call and sent back the response.  It should see that more input is
available and continue reading.

Could this be an end-of-line format issue?  Are you sure the commands
used from Emacs side produce Windows-style CRLF EOLs?  Or maybe they
do, but Python expects Unix-style newline-only EOLs (maybe it's a
Cygwin or MSYS Python, for example)?  A wrong EOL format might cause
Python to fail to realize it was handed a full line of input.

> Does Emacs have a chance to check for a pipe to be writable before
> doing so? The whole process blocking like this feels a bit weird.

I don't know how to do such a check with pipes on Windows.  More
importantly, how would that help?  The pipe will fill up anyway, and
the communications with Python will stop.  Being able to interrupt
with C-g vs killing the subprocess is not such a big win, IMO.

> > The obvious way: attach a debugger to Emacs and see where it is hung
> > or waiting.  It is important to ask the user to produce backtraces
> > from all the threads, because at least 2 threads are involved in
> > interaction with a subprocess on MS-Windows.
> 
> Thanks. I'll ask, though I'm not sure if the user has a debugger
> available.

If the user doesn't have GDB, he/she can download one from the MinGW
site.  I think using a debugger is the only way to understand what
happens here.