RFR: 8293067: (fs) Implement WatchService using system library (macOS)

[Brian Burkhalter]

On Oct 3, 2022, at 4:34 AM, Maxim Kartashev <mkartashev at openjdk.org<mailto:mkartashev at openjdk.org>> wrote:

On Mon, 3 Oct 2022 10:35:44 GMT, Alan Bateman <alanb at openjdk.org<mailto:alanb at openjdk.org>> wrote:

The suggestion was to explore doing this at a low level with kqueue so that we don't have callbacks.

Vladimir Kempik had a draft implementation based on kqueue that, AFAIR, passed all the existing tests; he may be interested in getting his work merged into the mainline.

As I mentioned, I wrote a prototype based on kqueue and it also (usually) passed all the tests but had problems shutting down. It also sometimes ran out of file descriptors. Unless my testing was wrong, I think that using kqueue here is likely to run into the exhausted file descriptor situation frequently if the `ENTRY_MODIFY` event is watched. `ENTRY_CREATE` and `ENTRY_DELETE` could be detected by watching the directory file descriptor alone and rescanning the directory, but modifying a contained file alone will not generate any event on the directory descriptor, hence watching for `ENTRY_MODIFY` requires an open descriptor for each file in the directory.
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