AtomicLsn is a wrapper around AtomicU64 that has load() and store()
members that are cheap (on x86, anyway) and can be safely used in any
context.
This commit uses AtomicLsn in the page cache, and fixes up some
downstream code that manually implemented LSN formatting.
There's also a bugfix to the logging in wait_lsn, which prints the
wrong lsn value.
SeqWait can use any type that is Ord + Debug + Copy. Debug is not
strictly necessary, but allows us to keep the panic message if a caller
wants the sequence number to go backwards.
This type is a zero-cost wrapper for a u64, meant to help code
communicate with precision what that value means.
It implements Display and Debug. Display "{}" will format as
"1234ABCD:5678CDEF" while Debug will format as Lsn{1234567890}.
Clippy pointed out that `drop(waiters)` didn't do anything, because
there was a misplaced ";" causing `waiters` to be a unit type `()`.
This change makes it do what was intended: the lock should be dropped
first, then the wakeups should be processed.
SeqWait adds a way to .await the arrival of some sequence number.
It provides wait_for(num) which is an async fn, and advance(num) which
is synchronous.
This should be useful in solving the page cache deadlocks, and may be
useful in other areas too.
This implementation still uses a Mutex internally, but only for a brief
critical section. If we find this code broadly useful and start to care
more about executor stalls due to unfair thread scheduling, there might
be ways to make it lock-free.
