Fix handling of paths that leave and re-enter the project root via symlinks

[robhogan]

Summary:

Background

The internal implementation of TreeFS uses a tree of maps representing file path segments, with a "root" node at the project root. For paths outside the project root, we traverse through '..' nodes, so that ../outside traverses from the project root through '..' and 'outside'. Basing the representation around the project root (as opposed to a volume root) minimises traversal through irrelevant paths and makes the cache portable.

Problem

However, because this map of maps is a simple (acyclic) tree, a '..' node has no entry for one of its logical (=on disk) children - the node closer to the project root. With a project root /foo/bar, the project-root-relative path ../bar cannot be traversed, because '..' is a key of 'bar' and not vice-versa.

This mostly isn't a problem because '../bar' is not a normal path - normalisation collapses this down to '':

metro/packages/metro-file-map/src/lib/RootPathUtils.js

Lines 142 to 148 in 6856d00

	// Internal: Tries to collapse sequences like `../root/foo` for root
	// `/project/root` down to the normal 'foo'.
	#tryCollapseIndirectionsInSuffix(
	fullPath: string, // A string ending with the relative path to process
	startOfRelativePart: number, // Index of the start of part to process
	implicitUpIndirections: number, // 0=root-relative, 1=dirname(root)-relative...
	): ?string {

Observable bug

For lookups, if instead of a literal indirection we have a symlink 'link-to-parent/bar', normalisation cannot (and should not) collapse away the symlink - so after dereferencing the symlink during traversal to '..' and joining it with the remaining bar we are unable to lookup '../bar'. For the module resolver this might appear as a failed existence check, and cause a resolution failure.

This fix

When dereferencing a symlink as part of _lookupByNormalPath, we now join the symlink target to the remaining subpath using a (mostly pre-exisitng) utility function that is aware of the project root, and can collapse away the project root segments. This leaves a normalised, root-relative target path that's guaranteed not to leave and re-enter the project root.

Changelog:

- **[Fix]**: Fix some paths being unresolvable when traversing a symlink that points to an ancestor of the project root.

Reviewed By: huntie

Differential Revision: D57719224

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This pull request was exported from Phabricator. Differential Revision: D57719224

[facebook-github-bot]

This pull request has been merged in 65fd022.