Watchman
A couple of days ago we announced Watchman on the Facebook Engineering blog.
Watchman watches files and records information about them as they change. You can arrange to trigger build or test steps in response to changes in matching files, but the main the reason that we built it was so that we can instantaneously query file status for a set of files.
Watchman maintains a view of the filesystem that is kept in sync using kernel filesystem notification facilities. This view is indexed so that we can quickly return information about the watched portions of the filesystem and also query the set of files that changed since a given point in time.