The new behavior eliminates a couple unpleasant edge cases in which it
would never flush:
* if all recording stops, whatever was unflushed would stay that way
* if every recording attempt produces a 0-duration recording (such as if the
camera sends only one frame and thus no PTS delta can be calculated),
the list of recordings to flush would continue to grow
I moved the clocks member from LockedDatabase to Database to make this happen,
so the new DatabaseGuard (replacing a direct MutexGuard<LockedDatabase>) can
access it before acquiring the lock.
I also made the type of clock a type parameter of Database (and so several
other things throughout the system). This allowed me to drop the Arc<>, but
more importantly it means that the Clocks trait doesn't need to stay
object-safe. I plan to take advantage of that shortly.
* separate these out into a new file, writer.rs, as dir.rs was getting
unwieldy.
* extract traits for the parts of SampleFileDir and std::fs::File they needed;
set up mock implementations.
* move clock.rs to a new base crate to be accessible from the db crate.
* add tests that exercise all the retry paths.
* bugfix: account for the new recording's bytes when calculating how much to
delete.
* bugfix: when retrying an unlink failure in collect_garbage, we shouldn't
warn about all the recordings no longer existing. Do this by retrying each
step rather than the whole procedure again.
* avoid double-panic scenarios, which I hit while tweaking the mocks. These
are quite annoying to debug as Rust doesn't print information about either
panic. I ended up using lldb to get a backtrace. Better to be cautious about
what we're doing when already panicking.
* give more context on raw::insert_recording errors, which I hit as well while
tweaking the new tests.
