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

[halfgaar]

Public bug reported:

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, also for older Ubuntus.

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.

** Affects: bash (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1822776

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, also for older Ubuntus.

  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.

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