stop roundtrip snapshotting in rsc compiles

[cosmicexplorer]

Problem

The RscCompile task currently will drop the output_directory_digest of any hermetic compiles it performs, instead relying on materializing output files to .pants.d, then re-snapshotting them. This is slow, and the slowdown (possibly) scales quadratically with the depth of the dependency graph.

Solution

• Make ctx.rsc_jar_file (for RscCompileContext) into a ClasspathEntry instead of just a path.
• Return the output_directory_digest from self._runtool (if performed hermetically, and write it to ctx.rsc_jar_file.

Result

The rsc compile task is probably faster!

[stuhood]

Rather than passing the classpath as input_files, it should now be possible to pass it as classpath_entries and assume digests like zinc does. That will allow for removing the input_files arg post #7837.

[stuhood]

It's possible that rather than overriding this method, you should consider modifying self._set_directory_digests_for_valid_target_classpath_directories instead, to support populating digests for all contexts...?

[stuhood]

It's important to always actually call capture_snapshots, because it's possible that the Store no longer holds the Digest, in which case we need to re-snapshot to capture it.

So recommend to call like:

cp_globs = PathGlobsAndRoot(
  PathGlobs((fast_relpath(cp.path, get_buildroot()),)),
  text_type(get_buildroot(),
  Digest.load(os.path.dirname(cp.path)),
))
cp_snapshot, = self.context._scheduler.capture_snapshots((cp_globs,))
cp_digest = cp_snapshot.directory_digest

Also, the Digest needs to be stored explicitly:

cp_digest.dump(..)

See

pants/src/python/pants/backend/jvm/tasks/resolve_shared.py

Lines 50 to 59 in d72fb81

	snapshots = self.context._scheduler.capture_snapshots(
	tuple(
	PathGlobsAndRoot(
	PathGlobs([jar]),
	get_buildroot(),
	Digest.load(jar),
	) for jar in jar_paths
	))
	for snapshot, jar_path in zip(snapshots, jar_paths):
	snapshot.directory_digest.dump(jar_path)

[illicitonion]

Obsoleted by #7858 and #7861 :) Thanks for starting this off!