Interaction between `fallback_run_target` and `run_targets` is undefined

[danth]

What

It is possible to define a run target under run_targets called fallback, which interferes with the run target generated by the fallback_run_target option.

lifecycle/score/launch_manager/src/daemon/src/process_group_manager/process_group_manager.cpp

Lines 344 to 345 in 2822719

	// the fallback state doesn't have a name in the config, so we use
	// "fallback", it doesn't actually matter...

It is not clear what should happen in this case.

This mistake actually appears in our own integration tests:

lifecycle/tests/integration/process_wrong_binary_failure/process_wrong_binary_failure.json

Line 116 in 2822719

"fallback": {

Acceptance Criteria (DoD)

• Behaviour is clearly defined

How

A few different ideas:

1. Delete the fallback_run_target option, tell users to configure a run target called fallback directly.
2. Make fallback_run_target take only a string, which names an existing run target to be used.
3. Generate an error if a run target called fallback appears under run_targets.
4. Replace the hardcoded name with a dynamically chosen one, to avoid the conflict.

[NicolasFussberger]

I am not sure why the translation from "fallback_run_target" -> "fallback" got introduced and if it solved any problem.

From a user perspective, the fallback_run_target is referenced in the config with the term "fallback_run_target":

      "recovery_action": {
          "switch_run_target": {
              "run_target": "fallback_run_target"
          }
      },

Therefore, I think users will expect to see the same name appearing in logs.

The term "fallback_run_target" is already a reserved name and it is validated that no other run target with this name is configured.

If there is nothing speaking against this, I would propose to change the constant "fallback"->"fallback_run_target" as a first step.

Long-term I like Option 2 as it works the same way as "initial_run_target".
We could even introduce this as a backwards-compatible config change I guess.