[2.7] JUJU_HOOK_NAME is only present in debug-hooks sessions

[Dmitrii Shcherbakov]

JUJU_HOOK_NAME is only added to tmux sessions created via debug-hooks.

Contrary to that, JUJU_{ACTION, FUNCTION}_NAME is always set for functions and they are not handled by debug-hooks at all.

Docs suggest that JUJU_HOOK_NAME is "available to charms during hook execution." which is incorrect:
https://discourse.jujucharms.com/t/juju-environment-variables/1162

It is possible to extract a hook name from JUJU_CONTEXT_ID but its format (<unit_name>-<hook_name>-<rand-int-63>) is not publicly documented:

https://github.com/juju/juju/blob/juju-2.7.0/worker/uniter/runner/context/contextfactory.go#L242
https://github.com/juju/juju/blob/juju-2.7.0/worker/uniter/runner/context/contextfactory.go#L154-L158

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The reason why JUJU_HOOK_NAME is needed is to give charm frameworks an identification mechanism for what exactly gets executed: an action or a hook for a regular event.

While currently the presence/lack of JUJU_{ACTION,FUNCTION}_NAME and the value of argv[0] provide the same information, if a single entrypoint style of execution (hooks/process-event, functions/process-function) will be supported by Juju, argv[0] will no longer be possible to rely on.

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Another reason to consider this is that Python charms can only rely on argv[0] on Linux distributions - not Windows. For Windows Python places a full path after symlink resolution into argv[0].

Example: for hooks/install -> ..\lib\charm.py, argv[0] will be <disk-letter>:\<charm_dir_path>\lib\charm.py

https://github.com/python/cpython/blob/3.6/Python/sysmodule.c#L2161-L2259 (3.6)
https://github.com/python/cpython/blob/3.7/Python/pathconfig.c#L280-L393 (3.7)

The Windows-specific replacement logic:
https://github.com/python/cpython/blob/3.7/Python/pathconfig.c#L352-L360

[Dmitrii Shcherbakov (dmitriis)]

Suggestion: https://github.com/juju/juju/pull/10969

[Gustavo Niemeyer (niemeyer)]

Given we have a soft agreement to introduce a new hook that runs when any other hook would run, we'll need that variable anyway. We can look at both changes in the same work.

[Richard Harding (rharding)]

Agree with Gustavo on tying it to the new "process_event" hook. I've targeted the bug and will make sure this goes into the spec and notes for that feature that the team is kicking off work on tomorrow.

[Heather Lanigan (hmlanigan)]

2.7: https://github.com/juju/juju/pull/11228