feat: Keep the binary in the plugin instead of build dir

[JanSF]

This fixes Issue #76
Follow up to #127

I didn't have a full understanding of where the binary was getting saved, so these changes are required to actually keep from having to download the binary every time.

Prior to this change, the binary was downloaded to the builds directory, /var/lib/buildkite-agent/builds/... on my machine, which gets its changes cleaned up between every run. Instead we want to save the binary in the plugins directory, /etc/buildkite-agent/plugins/... on my machine, so that it persists between runs. I followed the same pattern as the pr commenter buildkite plugin.

[toote]

In addition to the 2 comments, I would also suggest allowing it to be configured in case users want to change that to something else with an option (maybe something like download-folder) that defaults to the value of BUILDKITE_PLUGINS_PATH. What do you think?

[JanSF]

If you like, we could make it configurable with a env var that would take priority if set like BUILDKITE_PLUGIN_MONOREPO_DIFF_EXECUTABLE_DIR.

[toote]

If you like, we could make it configurable with a env var that would take priority if set like BUILDKITE_PLUGIN_MONOREPO_DIFF_EXECUTABLE_DIR.

Yeah, I suggested something like that but recommended calling it donwload-folder instead to make it more obvious that it is where it will be downloaded to (so write permissions are necessary)

[toote]

this variable will get set if you add the download-folder option to the plugin itself (which will also provide the best place to document the functionality)

[JanSF]

I think you're suggesting that we should make download-folder an option in the pipeline.yml usage of the plugin. Maybe there's something I don't understand, but wouldn't that require us to parse the yaml in Bash, which is what the Go binary is actually built for?
The reason I went with an env var is because those would be accessible in Bash.

[toote]

Options passed to the plugin are also available as bash variables by the agent. Which means that a download-folder option for this plugin will be available as BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD_FOLDER to its hooks... so it would not require any further changes to the code you created, just the documentation and the plugin.yml file :)

[JanSF]

Oh, that's great!

[JanSF]

I think I've addressed your feedback here. Is there anything else I should change?

[toote]

Just a minor issue with the linter and I think that this can be approved and merged :)

[toote]

This should correct the linting issues:

Suggested change

	steps:
	- label: "Triggering pipelines"
	plugins:
	- monorepo-diff#v1.5.2:
	download_folder: "/var/buildkite-agent"
	steps:
	- label: "Triggering pipelines"
	plugins:
	- monorepo-diff#v1.6.1:
	download_folder: "/var/buildkite-agent"
	watch:
	- path: "bar-service/"
	config:
	key: echo-step
	command: "echo deploy-bar"