There was a nominal split between the tests in layered_repository.rs and
repository.rs, such that tests specific to the layered implementation were
supposed to be in layered_repository.rs, and tests that should work with
any implementation of the traits were supposed to be in repository.rs.
In practice, the line was quite muddled. With minor tweaks, many of the
tests in layered_repository.rs should work with other implementations too,
and vice versa. And in practice we only have one implementation, so it's
more straightforward to gather all unit tests in one place.
The SyncQueue consisted of a tokio mpsc channel, and an atomic counter
to keep track of how many items there are in the channel. Updating the
atomic counter was racy, and sometimes the consumer would decrement
the counter before the producer had incremented it, leading to integer
wraparound to usize::MAX. Calling Vec::with_capacity(usize::MAX) leads
to a panic.
To fix, replace the channel with a VecDeque protected by a Mutex, and
a condition variable for signaling. Now that the queue is now
protected by standard blocking Mutex and Condvar, refactor the
functions touching it to be sync, not async.
A theoretical downside of this is that the calls to push items to the
queue and the storage sync thread that drains the queue might now need
to wait, if another thread is busy manipulating the queue. I believe
that's OK; the lock isn't held for very long, and these operations are
made in background threads, not in the hot GetPage@LSN path, so
they're not very latency-sensitive.
Fixes#1719. Also add a test case.
