resolutionCache getDirectoryToWatchFailedLookupLocation always sets recursive watcher for entire project

[mprobst]

getDirectoryToWatchFailedLookupLocation in resolutionCache.ts checks if a failed lookup location is in the project's root dir, and if so, watches the entire project root, recursively (intentionally, it has a comment // Always watch root directory recursively).

For projects containing potentially very many subdirectories (just large projects, but also projects with large node_modules subtrees), this will read thousands of directories and create corresponding thousands of watchers.

I wonder if we could do better, but I don't understand the thinking that went into this. Was the idea that just having a recursive watch on the root is more effective than watching lots of more precise locations?

I'd consider only adding a watch for the closest non-existing file from the lookup location, e.g. for a location /foo/bar/baz/index.ts not being found, with baz/ not existing, a watch on /foo/bar would be sufficient, and create fewer watches and file reads than watching every file in the project recursively.

[sheetalkamat]

This is fixed in #25811 note that we would still watch sub directory since watching each loacation or watching directory of each location lakes too much time on big projects.