Support using Gradle Build Cache with Yarn tasks

[mark-vieira]

This pull request enables the ability for users of Yarn install and setup tasks to enable Gradle Build Cache support. This relies on relaxing incremental build checking to use only relative path checking so that task outputs creating on one machine (or workspace directory) can be reused somewhere else. Older versions of Gradle will behave the same and ignore @PathSensitive annotations.

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This change is 

[srs]

Hi. Thanks for the pull. But I noticed the build's are failing. Possible to fix that?

[mark-vieira]

Sorry about that. The issue is that converting to using the input annotations enforced validation checks that the inputs API does not perform. Specifically, ensuring that input files exist. My guess is that this is only breaking the tests but I expect someone out in the wild might have a similar configuration (not using yarn.lock).

I've relaxed the validations to emulate with the old behavior was, which is just ignoring that input if the file doesn't exist. I'd suggest doing something more robust in the future but this should at least keep existing plugin users working.

[anuraaga]

@mark-vieira I noticed this when updating the version of gradle-node-plugin - cool change. Just wondering, since this doesn't add @CacheableTask, does that mean it's expected for users to define their own yarn task to use build cache, e.g.

@CacheableTask
class CacheableYarnInstallTask extends YarnInstallTask {
}
project.tasks.create('cacheableYarn', CacheableYarnInstallTask.class);

[mark-vieira]

Adding @CacheableTask would cause those tasks to get cached by default, which isn't necessarily what folks want. What you've put above is a perfectly fine solution. You can also do this in an ad-hoc manner.

yarn.outputs.cacheIf { true } 

[anuraaga]

@mark-vieira Ah, didn't realize cacheIf works even for tasks without the annotation, learned a great trick!