[Bug 1781169] Re: frontend locking

[Julian Andres Klode]

** Description changed:

- frontend locking fixes locking for dpkg frontends. Implementation is in
- dpkg git and apt support is coming soon.
+ [Impact]
+ apt acquires /var/lib/dpkg/lock but has to release it while running dpkg, offering a short window for other clients to acquire the lock. This implements /var/lib/dpkg/lock-frontend which will be acquired earlier and not released by apt for dpkg to run.
+ 
+ dpkg will try to acquire the frontend lock as well, unless told not to
+ do so via an environment variable, which apt does; hence concurrent runs
+ of dpkg without other locking can't happen.
+ 
+ [Test case]
+ Run strace apt install $something and check that lock-frontend is acquired at the beginning and not released until the end.
+ 
+ [Regression potential]
+ Frontend locking failures are in English, as there are no translations yet. More locks also mean more potential for locking to go wrong across programs, but since we don't have any waiting locks we at least won't deadlock.

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https://bugs.launchpad.net/bugs/1781169

Title:
  frontend locking

Status in apt package in Ubuntu:
  Fix Released
Status in dpkg package in Ubuntu:
  Fix Released
Status in apt source package in Xenial:
  Triaged
Status in dpkg source package in Xenial:
  New
Status in apt source package in Bionic:
  Triaged
Status in dpkg source package in Bionic:
  New

Bug description:
  [Impact]
  apt acquires /var/lib/dpkg/lock but has to release it while running dpkg, offering a short window for other clients to acquire the lock. This implements /var/lib/dpkg/lock-frontend which will be acquired earlier and not released by apt for dpkg to run.

  dpkg will try to acquire the frontend lock as well, unless told not to
  do so via an environment variable, which apt does; hence concurrent
  runs of dpkg without other locking can't happen.

  [Test case]
  Run strace apt install $something and check that lock-frontend is acquired at the beginning and not released until the end.

  [Regression potential]
  Frontend locking failures are in English, as there are no translations yet. More locks also mean more potential for locking to go wrong across programs, but since we don't have any waiting locks we at least won't deadlock.

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