feature request for the filesync package: https://github.com/moby/buildkit/tree/master/session/filesync

We've been using the SyncedDir API with a lot of success to add additional filters on the build context.

https://pkg.go.dev/github.com/moby/buildkit/session/filesync#SyncedDir

In particular, the Map func(string, *fstypes.Stat) bool function has been really helpful.

One thing that we've wanted to be able to do is also tell the filesync Provider "don't even bother stat'ing this directory, there are no files to include there". Something like MapSkipDir func(string, *fstypes.Stat) bool.

Would you be open to an API like this? We'd be happy to add it if it seems reasonable.

There's already some optimizations in the underlying library to skip directories: https://github.com/tonistiigi/fsutil/blob/master/walker.go#L209
but it's a bit fragile, and too easy to accidentally kick down the slow path.

Sorry if this is all a bit pointy-headed and esoteric, happy to explain more if it's not clear.

Motivating issue: tilt-dev/tilt#5745