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Emacs deadlocks on sub-process reads and writes
From: |
speech . free |
Subject: |
Emacs deadlocks on sub-process reads and writes |
Date: |
Sat, 18 Oct 2014 18:48:43 -0700 (PDT) |
User-agent: |
G2/1.0 |
I am debugging a Clang based code completer (irony-mode). Unfortunately the
transfer between Emacs and the Clang sub-process is slow on Windows. We are
experiencing frequent freeze-ups due to what looks like a deadlock. The
deadlock happens under the following conditions:
1. Emacs reading a large transfer from the sub-process (2+MB, around 2 seconds
to finish on Windows after setting w32-pipe-read-delay to 0);
2. While 1. is ongoing Emacs side send a new request to the sub-process. The
request is regular source file sized -- it includes the buffer content being
edited -- sufficient to cause a block on the write but nothing too big.
When these two conditions are met, Emacs UI reliably locks up until the
sub-process is killed. The error message complaining about writing to a dead
process after killing the sub-process shows that the Emacs was deadlocked
waiting for the write to finish. The process buffer is half filled showing that
the process filter function was no longer called. The write to the sub-process,
which blocks the UI, can't proceed as it exceeds the size of the small write
buffer. The sub-process will not read on its side until it finishes the
transfer to Emacs. But for some reason Emacs also stopped reading from the pipe.
After poking around the Emacs code base a little bit it looks to me that when
you call process-send-* in lisp Emacs eventually calls send_process in
process.c. If the send can't complete it calls wait_reading_process_output,
which calls read_and_dispose_of_process_output, which if there is new text
received, calls the lisp process-filter. The logic in
wait_reading_process_output is too complicated for me to understand if somehow
it may decide not to call read_process_output, causing a deadlock. So I am
asking for advice here.
A previous related question asked about transfer speed on Windows
https://groups.google.com/forum/#!topic/gnu.emacs.help/Y0MYAz8WZUU I believe
the slowness is the reason why this manifests itself on Windows but it may also
appear on Linux under the right conditions. For a very long thread of
discussion on this issue see https://github.com/Sarcasm/irony-mode/issues/131
Please feel free to forward to any Emacs list that may be relevant to solving
this issue.
- Emacs deadlocks on sub-process reads and writes,
speech . free <=