[Bug 1776218] Update Released

[Łukasz Zemczak]

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https://bugs.launchpad.net/bugs/1776218

Title:
  New style hook support not robust on removal

Status in apt package in Ubuntu:
  Fix Released
Status in apt source package in Bionic:
  Fix Released

Bug description:
  [Impact]
  The new style (json-rpc) hooks are great. However when using them in snapd we noticed the following problem. When shipping a hook in /etc/apt/apt.conf.d/20snapd.conf like:
  ```
  AptCli::Hooks::Install { "[ ! -f /usr/bin/snap ] || /usr/bin/snap advise-snap --from-apt || true"; };
  ```
  this works fine.

  However when the snapd package is removed apt fails with:
  ```
  ...
  Purging configuration files for snapd (1337.2.32.8) ...
  Final directory cleanup
  Discarding preserved snap namespaces
  Removing extra snap-confine apparmor rules
  Removing snapd cache
  Removing snapd state

  E: Could not read response to hello message from hook [ ! -f /usr/bin/snap ] || /usr/bin/snap advise-snap --from-apt || true: Connection reset by peer
  E: Could not read message separator line after handshake from [ ! -f /usr/bin/snap ] || /usr/bin/snap advise-snap --from-apt || true: Connection reset by peer
  quiet: end of output.
  ```

  I.e. if the hook is not there apt still expectes a handshake.

  [Test case]
  cat > rootdir/etc/apt/apt.conf.d/99-json-hooks << EOF
         AptCli::Hooks::Install:: "true";
         AptCli::Hooks::Search:: "true";
  EOF                             
  and run an install.

  This test and some more error handling tests are part of the test
  suite, too, so autopkgtest covers that for us.

  [Regression potential]
  Hooks that do not respond to the initial handshake and just exit with 0 are silently ignored. 

  [Other info]
  There are some more changes in the patch that fix other cases of error handling: As can be seen above, there are 2 error messages for one error, because the code did not abort early, but tried to carry on, reading more data. It now goes directly to the end.

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