[Bug 1558236] Re: /usr/bin/script freezes and eats all CPU, has to be killed, which kills shell and programs inside

[gouri]

# Workaround? Tried once, works.

TL;DR: reading the slave pts from another process unfreezes and allows
continued operation


  cd /proc/24621/fd

  ls -al

total 0
dr-x------ 2 stephane stephane  0 avril  5 16:39 .
dr-xr-xr-x 9 stephane stephane  0 avril  5 15:13 ..
lrwx------ 1 stephane stephane 64 avril  5 16:56 0 -> /dev/pts/8
lrwx------ 1 stephane stephane 64 avril  5 16:56 1 -> /dev/pts/8
lrwx------ 1 stephane stephane 64 avril  5 16:39 2 -> /dev/pts/8
lrwx------ 1 stephane stephane 64 avril  5 16:56 3 -> /dev/ptmx
lrwx------ 1 stephane stephane 64 avril  5 16:56 4 -> /dev/pts/20
lrwx------ 1 stephane stephane 64 avril  5 16:56 5 -> anon_inode:[signalfd]
l-wx------ 1 stephane stephane 64 avril  5 16:56 6 -> /home/stephane/typescript (deleted)

So, pts/8 is the master, pts/20 is the slave.

  echo foo >/dev/pts/8

-> This shows "foo" in the terminal window.

  head -n 1 /dev/pts/8

^C

This blocks my shell, so I do Ctrl-C to regain.

  echo foo >/dev/pts/20

This blocks my shell, so I do Ctrl-C to regain.

^Cbash: echo: erreur d'écriture : Appel système interrompu

This one is interesting:

  head -n 1 /dev/pts/20

It does not block, and works around the bug immediately.

I'll try again.


** Tags added: has-workaround regression-release reproducible

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https://bugs.launchpad.net/bugs/1558236

Title:
  /usr/bin/script freezes and eats all CPU, has to be killed, which
  kills shell and programs inside

Status in util-linux package in Ubuntu:
  New

Bug description:
  # Context

  I've been using /usr/bin/script quite a lot for about 4 years (since
  about Ubuntu 12.04, maybe before). It always worked without problem.

  I'm using several script instance most of the time, in several
  terminals.

  Reproducible: randomly, once every few days.

  Problem happens since Ubuntu 16.04. Never seen before (12.04 12.10
  13.04 13.10 14.04 14.10 15.04 15.10).

  # Expected

  Continued operation, terminal running programs normally, little CPU
  used.

  # Observed

  Suddenly, terminal freezes. 
  Other tabs of same terminal window not affected.
  Top shows script consumes 100% CPU.

  Strace shows an endless stream of this line:

  poll([{fd=4, events=POLLIN}], 1, 10)    = 1 ([{fd=4, revents=POLLIN}])

  /proc/*/fd shows what file descriptor it is:

  total 0
  dr-x------ 2 stephane stephane  0 mars  16 20:03 .
  dr-xr-xr-x 9 stephane stephane  0 mars  16 20:03 ..
  lrwx------ 1 stephane stephane 64 mars  16 20:15 0 -> /dev/pts/17 (deleted)
  lrwx------ 1 stephane stephane 64 mars  16 20:15 1 -> /dev/pts/17 (deleted)
  lrwx------ 1 stephane stephane 64 mars  16 20:03 2 -> /dev/pts/17 (deleted)
  lrwx------ 1 stephane stephane 64 mars  16 20:15 3 -> /dev/ptmx
  lrwx------ 1 stephane stephane 64 mars  16 20:15 4 -> /dev/pts/21
  lrwx------ 1 stephane stephane 64 mars  16 20:15 5 -> anon_inode:[signalfd]
  l-wx------ 1 stephane stephane 64 mars  16 20:15 6 -> /somewhere/mylogfile.txt
  l-wx------ 1 stephane stephane 64 mars  16 20:15 7 -> /somewhere/mylogfile_timings.txt

  What can we do?

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: bsdutils 1:2.27.1-6ubuntu1
  ProcVersionSignature: Ubuntu 4.4.0-13.29-generic 4.4.5
  Uname: Linux 4.4.0-13-generic x86_64
  ApportVersion: 2.20-0ubuntu3
  Architecture: amd64
  CurrentDesktop: XFCE
  Date: Wed Mar 16 20:04:02 2016
  ExecutablePath: /usr/bin/script
  InstallationDate: Installed on 2016-02-03 (41 days ago)
  InstallationMedia: Xubuntu 16.04 LTS "Xenial Xerus" - Alpha amd64 (20160203)
  SourcePackage: util-linux
  UpgradeStatus: No upgrade log present (probably fresh install)

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