[Elastic-Agent] Do not use unversioned home with watcher binary path

[michalpristas]

What does this PR do?

The problem is when agent tries to invoke watcher which checks upgraded agent for failures and at the same time unversioned home path is set.

Usually agent took its home path which is {top_path}/data/elastic-agent-{hash}/elastic-agent and invoked watch sub-command on binary at that path.

When unversioned path is set it tries to run watcher from {top_path}/data/elastic-agent and as so it results in binary not found.

Fixes: #25371

Why is it important?

Reduce noise created by failng attempt to run this. This is just a case when running container sub-command which tempers paths.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

cc @michel-laterman

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #25423 updated

• Start Time: 2021-04-30T06:34:59.676+0000

• Duration: 86 min 20 sec

• Commit: 8a044fc

Test stats 🧪

Test	Results
Failed	0
Passed	6781
Skipped	23
Total	6804

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	6781
Skipped	23
Total	6804

[michalpristas]

@blakerouse should i do this or should we move binary/create symlink in the setPath func

[ruflin]

@michalpristas Can you reference the issue this is coming from?

[ruflin]

I saw the name topPath in the past? What does it exactly mean?

[michalpristas]

with the structure we have topPath is the top of agent directory structure e.g. /etc/Elastic/Agent
we also use a homePath which points to a home of a certain agent version e.g /etc/Elastic/Agent/data/elastic-agent-abc123/

[ruflin]

As a follow up, we should start to think on how we can test these things in an automated way.

[blakerouse]

The only time unversioned path is used is in the container. Self-upgrade is not supported in the container anyway, so could we just disable the watcher completely instead?

[blakerouse]

This change does seem okay and should still go in.

But related to my previous comment, when will the watcher run in a container?

[michalpristas]

@blakerouse it will not, the only case when it's running is when specifically told to using DEV flag [allowUpgrade part]