Change the PathGlobs, SnapshotSubset, and UrlToFetch intrinsics to return Digest

[Eric-Arellano]

The engine knows how to go from Digest -> Snapshot, but not the other way around. By changing these three intrinsics, rules have much more flexibility.

Now, the claim in https://www.pantsbuild.org/v2.0/docs/rules-api-file-system#core-abstractions-digest-and-snapshot that "The core building block is a Digest" is true.

--

This also renames SnapshotSubet to DigestSubset, given that it takes as input Digest and returns a Digest.

[Eric-Arellano]

There are three legit test failures in fs_test.py that await Get(Snapshot, PathGlobs) no longer correctly resolves all .dirs. I can't reproduce this issue when testing with some files in our repo, so I suspect that the problem is due to a symlinked folder in the test.

Any help would be much appreciated. I think this is a real bug on the code to go from Digest -> Snapshot. https://travis-ci.com/github/pantsbuild/pants/jobs/364916874#L673

[Eric-Arellano]

There are three legit test failures

Nevermind, the original was wrong. There's a bug in the original implementation.

Given this code:

    sn = await Get(Snapshot, PathGlobs(["src/python/pants/util/*.py"]))
    print(sn.dirs)
    sn2 = await Get(Snapshot, Digest, sn.digest)
    print(sn2.dirs)

We get:

()
('src', 'src/python', 'src/python/pants', 'src/python/pants/util')

[coveralls]

Coverage remained the same at 0.0% when pulling 5020012 on Eric-Arellano:return-snapshot into 9ca7b52 on pantsbuild:master.

[benjyw]

LGTM, but I'd recommend getting a review from someone more versed in this before merging.

[stuhood]

Thanks!