bug#18772: 24.3; Emacs deadlocks on subprocess write when read is pending

[Haitao Zhang]

I have reliably experienced UI freezes under the following conditions:
1) A large read from a subprocess is in progress (2+MBs, around 2
seconds on Windows with w32-pipe-read-delay set to 0).
2) A mid-sized write is sent to the subprocess, from a not very large source
code buffer but large enough to block the write. The subprocess will not
read from its side until it finishes the transfer started in 1.

Killing the subprocess unfreezes Emacs. The error message indicates that
Emacs blocked on the write, complaing about the process being dead while
writing. The process buffer, normally mopped up by the process filter,
is filled with text midway through the transfer, indicating that the process
filter has stopped getting called. Finally locking out the offending
blocking write while the read is still in progress has ensured that the
deadlocks no longer happen. Success of the workaround gives me
confidence that this is an Emacs bug or feature.

Reading through the Emacs source files, it appears that when user calls
process-send-* in elisp eventually Emacs calls send_process in
process.c. If the send blocks it calls wait_reading_process_output,
which in turn should read pending output. Apparently in my case this is
not happening and a deadlock occured instead. My use case has large and
large enough transfers going both ways which may be different from
typical use patterns. Also on Windows reading from the pipe is slow even
when the w32-pipe-read-delay is set to 0 which gives more opportunities
for the deadlock to happen. I note again that this happens reliably
instead of intermittently once the conditions listed above are met.

I hope this bug report will help you in improving Emacs.

Haitao Zhang


If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
c:/alt/emacs/emacs-24.3/etc/DEBUG.

In GNU Emacs 24.3.1 (i386-mingw-nt6.2.9200)
 of 2013-03-17 on MARVIN
Windowing system distributor `Microsoft Corp.', version 6.2.9200
Configured using:
 `configure --with-gcc (4.7) --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include
 -ID:/devel/emacs/libs/libiconv-1.13.1-1-dev/include
 -ID:/devel/emacs/libs/libxml2-2.7.8/include/libxml2'

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252
  default enable-multibyte-characters: t

Major mode: C++/l

Minor modes in effect:
  irony-mode: t
  global-company-mode: t
  company-mode: t
  autopair-global-mode: t
  autopair-mode: t
  delete-selection-mode: t
  cua-mode: t
  recentf-mode: t
  global-auto-revert-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Recent input:
<wheel-up> <down-mouse-1> <mouse-1> <return> <tab>
i f SPC ( c a c A-H-s-  h e <tab> p <tab> . A-H-s- 
f <backspace> F <backspace> <backspace> . A-H-s-  <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> g e t E <return> SPC =
= SPC " z i p <menu-bar> <buffer> C-a <menu-bar> <buffer>
C-a <up> <up> <up> <up> <up> <up> <up> C-a C-k C-k
<up> <up> <up> <up> C-v <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> C-k C-k <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<down> <down> C-y <up> <tab> <right> <right> <right>
<right> <right> <right> <right> <right> <right> <right>
<right> <right> <right> <right> <right> <right> <right>
<right> <right> <right> <right> <right> <right> <right>
<right> <right> <right> <right> <right> <right> <right>
<backspace> ! <right> <right> <right> <right> <right>
<right> <right> <right> <right> <left> <return> <tab>
<down> <down> C-a C-k C-k <up> <up> C-y <up> <up> <up>
<up> <return> <down> <down> <down> <right> <right>
<right> <right> <down> <down> <down> <down> <down>
<left> <left> <left> <left> <left> <left> <right> <right>
<left> <right> C-k C-k C-k C-k C-k C-k C-k C-k C-k
C-k C-k C-k C-k <up> <up> <up> <up> <up> <up> <down>
<down> <up> <up> <up> <down> C-y <up> <up> <up> <up>
<up> <up> <right> <right> <right> <right> <right> <right>
<right> <left> E d g a A-H-s-  <down> <down> <return>
( u r i <right> SPC & & SPC <help-echo> <help-echo>
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
<help-echo> <menu-bar> <help-menu> <emacs-manual-bug>
<help-echo> <down-mouse-1> <mouse-2> <wheel-down> <double-wheel-down>
<wheel-down> <double-wheel-down> q <help-echo> <help-echo>
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
<menu-bar> <help-menu> <send-emacs-bug-report>

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