Marco Trevisan authored Feb 06, 2020 and Bastien Nocera committed Mar 17, 2020

If a device is currently verifying, identifying or enrolling we may want the
user to stop the operation before we actually release the device.

Otherwise we may end-up in trying to close (failing) the internal device,
while fprintd is still considering the device active, causing a dead-lock
(the device can't be released, but neither claimed again or stop the current
action).

In fact calling Claim() -> EnrollStart() -> Release(), we would fail with
the error

  net.reactivated.Fprint.Error.Internal:
  Release failed with error: The device is still busy with another
  operation, please try again later. (36)"

However, if we try to call VerifyStop, after this error, we'd fail because
for the fprintd logic, the device is not claimed anymore, but actually
closed, and we'd need to claim it again, but... That would still cause an
internal error.

To avoid this, in case Relase() is called cancel the ongoing operation,
and wait until it's done before completing the release call.