8029516: (fs) WatchKey cancel unreliable on Windows
Alan.Bateman at oracle.com
Thu Sep 4 16:58:50 UTC 2014
This is a bug is the Windows implementation of WatchService.
The Windows implementation is based on ReadDirectoryChangesW. It is used
asynchronously with a background thread, called the Poller, to service
the events as they are queued to an I/O completion port. It's one thread
per WatchService and there can be many directories registered with the
The cancel method is tricky. It's done by the Poller thread on behalf of
the caller of cancel and works by closing the handle to the directory.
Unfortunately this isn't quite right as it causes the outstanding
ReadDirectoryChangesW operation to abort and attempt to set an event in
the OVERLAPPED structure that might be freed at around that point
To fix this requires the Poller to cancel the outstanding
ReadDirectoryChangesW and wait for it to complete before close the
directory and releasing the resources. It unfortunately requires an
additional (unnamed) event per watched directory but overall the changes
aren't too bad. I've used the opportunity to clean up a few small things
The awkward part is creating a test that is anyway useful, it is very
machine and load dependent. The test that I have included will duplicate
it periodically when running on a loaded system, say when running tests
concurrently. All the existing tests pass on Windows 32-bit and 64-bit.
The webrev with the changes is here:
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