NPM Exec ValueSource

[adam-enko]

Implement ValueSource for executing npm.

This is a very rough first draft.

See #334 for motivations.

Example usage:

// build.gradle.kts

val npmExec = providers.of(NpmExecSource::class) { /* ... */ }
val result = npmExec.get()

Limitations

npm must be installed before NpmExecSource can be used.

Currently npm is installed using a Task, so NpmExecSourcewill fail unless the task runs first. This is not ideal, because I want to (eventually) useNpmExecSource` in the configuration phase should not depend on the execution of a task.

This can be remedied by installing npm in ValueSource in the Configuration phase. I think this should be done in a separate PR, and NpmExecSource can be released as experimental.

No caching

Each NpmExecSource will recompute the required exec properties independently. This could be slow, or expensive.

Gradle does not yet provide a tool to cache configuration-time work https://github.com/gradle/gradle/issues/

We could implement a custom mechanism for caching the exec properties.

Duplicated code

I copied this existing logic from VariantComputer and NpmExecRunner into NpmExecSource. This was to avoid the complicated provider mapping logic, which is not necessary inside of a ValueSource, because the values can be computed directly.

This means the initial implementation duplicates a lot of code.

I anticipate this will be deduplicated once the tasks are converted to use a 'ValueSource first' approach.

TODO

• Add docs
• Add experimental opt-in annotation

#334