When other tower-batch tasks drop, wake any tasks that are waiting for
a semaphore permit. Otherwise, tower-batch can hang.
We currently pin tower in our workspace to:
d4d1c67 hedge: use auto-resizing histograms (tower-rs/tower#484)
Copy tower/src/semaphore.rs from that commit, to pick up
tower-rs/tower#480.
This change is mostly mechanical, with the exception of the changes to the
`tower-batch` middleware. This middleware was adapted from `tower::buffer`,
and the `tower::buffer` code was changed to implement its own bounded queue,
because Tokio 0.3 removed the `mpsc::Sender::poll_send` method. See
ddc64e8d4d
for more context on the Tower changes. To match Tower as closely as possible
in order to be able to upstream `tower-batch`, those changes are copied from
`tower::Buffer` to `tower-batch`.
teor
Henry de Valence