TreeFS: Return traversed symlinks and first missing segments in lookup results

[robhogan]

Summary: A module resolution should be invalidated whenever the (non-)existence of a path checked during resolution may have changed. To implement this, we'll need more information from the FileSystem APIs.

Existence

A positive existence check is straightforwardly invalidated when either:

• The resolved realPath is deleted (we already watch for this), or
• Any symlink traversed on the way to the real path is deleted or modified. We will need to track symlinks traversed by their real paths.

Non-existence

For non-existence, note that it is not sufficient to simply watch for file events whose path matches the path originally looked up, because file events are emitted for real paths, and the checked path may resolve through one or more symlinks.

We will require:

• The first missing real path segment encountered (because we can watch for additions at this path, but not necessarily any of its descendants, should a symlink be created at this path), and
• The symlinks traversed to get to that first missing segment (because changes to these invalidate which real path we should watch for an addition).

This diff

This adds links, the set of traversed links by their canonical paths, and missing (for non-existence) - the first non-navigable path segment by its canonical path.

Changelog: Internal

Reviewed By: huntie

Differential Revision: D52391404

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D52391404

[facebook-github-bot]

This pull request has been merged in facebook/metro@451e2a1f174981cf7150fa0d7c70a6992b9150bb.