Implement the Drain::flush method

[Techcable]

Sends a message to the logging thread, then blocks until the logger acknowledges it.
Contrast this to the busy-loop approach in #36

Pinned to the branch used in PR slog-rs/slog#349, so cannot be released until that PR is accepted.

Fixes issue #35

[vorner]

As I'm reading through it, I wonder if there might be a less code-intensive and more readable approach. How about putting some kind of backchannel into the Flush message (I'd say a semaphore with 0 tokens in it, the worker thread provides one after flushing, the caller waits for it; except there don't seem to be any semaphores in Rust 😇). That way it would be clear whose flush got satisfied, there would be less work around poisoned mutexes, etc.

[dpc]

Increasing message size might negatively affect perf. of the main codepath.

[Techcable]

As I'm reading through it, I wonder if there might be a less code-intensive and more readable approach. How about putting some kind of backchannel into the Flush message (I'd say a semaphore with 0 tokens in it, the worker thread provides one after flushing, the caller waits for it; except there don't seem to be any semaphores in Rust 😇). That way it would be clear whose flush got satisfied, there would be less work around poisoned mutexes, etc.

Thank you for the suggestion! I will look for a simpler approach.

Increasing message size might negatively affect perf. of the main codepath.

You may be right about this, but an AsyncMsg is already a pretty large struct.

[Techcable]

Idea for a simpler implementation: Use a two-way channel or queue dedicated to flushing and acknowledgment of flushing.

EDIT: A zero-capacity channel may be what we want. According to crossbeam-channel:

A special case is zero-capacity channel, which cannot hold any messages. Instead, send and receive operations must appear at the same time in order to pair up and pass the message over:

[dpc]

Probably a proper way to do flush ack using only stdlib is to use CondVar paired with a Mutex<u64>. The u64 acts as a counter. Before flush the sender reads the current counter value, releases the lock, sends flush notification, then wait on the condvar for the counter to change. Backend thread increments the counter and notify_all after every flush. This should be correct and CondVar is a very bare-bone sync primitive so it's light and fast.

[Techcable]

Probably a proper way to do flush ack using only stdlib is to use CondVar paired with a Mutex<u64>. The u64 acts as a counter. Before flush the sender reads the current counter value, releases the lock, sends flush notification, then wait on the condvar for the counter to change. Backend thread increments the counter and notify_all after every flush. This should be correct and CondVar is a very bare-bone sync primitive so it's light and fast.

This is similar to what I am doing right now, using an enum for the state. Your version is better since the enum currently doesn't have a stamp and blocks if there is an existing flush rather than queuing it. I also suspect your idea would be simpler to implement.