[Bug 1771858] Re: /snap/bin not in default PATH for units, snapd should ship system-environment-generators to inject /snap/bin into $PATH

[Michael Vogt]

When we disabled the generator in snapd we added a regression test (tests/main/snap-system-env):
https://github.com/snapcore/snapd/pull/6470/files

With the current systemd (not the one in -proposed) I get:
```
grep error: pattern not found, got:
LANG=C.UTF-8
PATH=/sbin:/usr/sbin:/bin:/usr/bin:/snap/bin
```
which is expected and shows that the generator is broken.

When running the same regression test with the systemd in -proposed I
get no error so the update looks fine (as far as our test case goes). I
heard some concerns from xnox about systems with initrd that need to be
explored too.

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https://bugs.launchpad.net/bugs/1771858

Title:
  /snap/bin not in default PATH for units, snapd should ship system-
  environment-generators to inject /snap/bin into $PATH

Status in snapd package in Ubuntu:
  Confirmed
Status in systemd package in Ubuntu:
  Fix Released
Status in snapd source package in Xenial:
  Confirmed
Status in systemd source package in Xenial:
  Confirmed
Status in snapd source package in Bionic:
  Confirmed
Status in systemd source package in Bionic:
  Fix Committed
Status in snapd source package in Cosmic:
  Fix Released
Status in systemd source package in Cosmic:
  Fix Released

Bug description:
  [Impact]

   * This means that software installed via snap isn't transparently
  available for units to use.  As snaps are first-class citizens in
  Ubuntu, we should update the PATH.

   * When a generator started to be provided by systemd, it was
  recognized that $PATH is not correctly set, nonetheless, due to an
  environment bug that systemd generators run in.

  [Testcase]

  # make snapd-env-generator executable if it is not
  $ sudo chmod +x /usr/lib/systemd/system-environment-generators/snapd-env-generator

  # reboot, then test the effect of the fix
  $ systemd-run /usr/bin/env
  $ journalctl -e | grep PATH

  Output should contain /snap/bin

  Output should contain a complete and a valid PATH, i.e.
  PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin" or similar.

  [Regression Potential]

   * snapd generator was already fixed separately to cause no harm, when
  running under a broken systemd. With the corrected environment,
  generators will now run with a correct PATH out of the box. A slight
  change of PATH will be observed by all generators, when running in
  containers/initramfs-less boots. However most generators will not be
  affected as they typically do not execute external binaries.

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