Do we need a global RESOLVER_FOR_URI_SCHEME?

[lukpueh]

The ITE-4 resolver interface provides a method to return the right resolver for a given artifact uri, e.g.:

Resolver.for_uri("file:path/to/foo") returns the FileResolver instance that was previously registered in the global RESOLVER_FOR_URI_SCHEME dictionary under the "file" scheme.

This design is above all useful to separate resolver configuration from usage, and to implement uri parsing and default dispatching behind the interface.

However, given that we currently configure and use resolvers in the same function, the design seems more complex and intransparent than useful:

in-toto/in_toto/runlib.py

Lines 162 to 192 in 4581e4b

	# Configure resolver with resolver specific arguments
	# FIXME: This should happen closer to the user boundary, where
	# resolver-specific config arguments are passed and global state is managed.
	RESOLVER_FOR_URI_SCHEME[FileResolver.SCHEME] = FileResolver(
	exclude_patterns,
	base_path,
	follow_symlink_dirs,
	normalize_line_endings,
	lstrip_paths,
	)
	# Configure resolver for OSTree
	RESOLVER_FOR_URI_SCHEME[OSTreeResolver.SCHEME] = OSTreeResolver(base_path)
	# Aggregate artifacts per resolver
	resolver_for_uris = defaultdict(list)
	for artifact in artifacts:
	resolver = Resolver.for_uri(artifact)
	resolver_for_uris[resolver].append(artifact)
	# Hash artifacts in a batch per resolver
	# FIXME: The behavior may change if we hash each artifact individually,
	# because the left-prefix duplicate check in FileResolver only works for the
	# artifacts hashed in one batch.
	for resolver, uris in resolver_for_uris.items():
	artifact_hashes.update(resolver.hash_artifacts(uris))
	# Clear resolvers to not preserve global state change beyond this function.
	# FIXME: This also clears resolver registered elsewhere. For now we
	# assume that we only modify RESOLVER_FOR_URI_SCHEME in this function.
	RESOLVER_FOR_URI_SCHEME.clear()

Unless we detach resolver configuration from resolver usage, I think, it would be easier to keep a local resolver map and also dispatch locally.