[Bug 1822776] Re: Apply Bash 4.4.20 to fix cpu spinning on built-in wait

[Robie Basak]

** Tags added: bitesize server-next

** Changed in: bash (Ubuntu)
     Assignee: (unassigned) => Bryce Harrington (bryce)

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Title:
  Apply Bash 4.4.20 to fix cpu spinning on built-in wait

Status in bash package in Ubuntu:
  New

Bug description:
  Bash pre-4.4.20 has a bug in its PID hash table that causes spin-loops
  when spawning sub processes and waiting for them. There is a fix:

  https://ftp.gnu.org/gnu/bash/bash-4.4-patches/bash44-020

  Our application started being affected (locking up) by this since
  migrating from Ubuntu 14.04 to 18.04. Ubuntu 14.04 has bash 4.3.11(1),
  Ubuntu 18.04 has bash 4.4.19 (that is, when running 'bash --version',
  because of their unusual versions as patches, apt shows it as
  4.4.18-2ubuntu1).

  The 4.4-020 version needs to be included. I think it's actually quite
  critical.

  The Bash bug report mentions longer running loops, but it seems hash
  collisions are the cause, meaning it's just a matter of chance,
  influenced by how fast PIDs are cycled on the machine.

  Edit as per the SRU procedure:

         [Impact]

  Long running bash loops that create and reap processes will crash,
  hanging at 100% CPU.

  A justification for including the fix would be that a standard
  language feature in a script language is broken, and that it's
  indeterminate when it breaks. Considering the wide spread use of bash,
  I'm surprised not more people have reported issues. My and a client
  started having issues with independently of each other very soon after
  upgrading to an affected version.

         [Test Case]

  Run this loop for a few days/weeks:

    #!/bin/bash
    while true; do
      sleep 0.5 &
      wait
    done

  
  It will cause the 'wait' statement to hang, consuming 100% after some indeterminate amount of time, dependent on how fast PIDs are cycled in the machine.

         [Regression Potential]

  Using 'apt-get source bash' to get the original source version, I
  created a deb that includes the 4.4.20 patch and have been running it
  since April 2nd. The 100% CPU spinning is solved, and no other
  regressions have been observed.

  Ubuntu 18.04 is already at 4.4.19, which is one patch level behind, so
  this involves linearly progressing to the next version (so not
  skipping patches).

         [Other Info]

  Official patch to fix, and to bump to 4.4.20:

  http://ftp.gnu.org/gnu/bash/bash-4.4-patches/bash44-020

  The newest Ubuntu tar.xz with patches I could find at:

  http://archive.ubuntu.com/ubuntu/pool/main/b/bash/

  also didn't have the 4.4.20 patch, so it seems no Ubuntu release has
  the fix yet.

  Although not completely sure, this problem seems to have been
  introduced in the 4.4 version of Bash, so in term of LTS versions,
  18.04 and up are affected.

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