fix: do a best-effort transmit on connection close

[colinmarc]

Description

If we're shutting down ungracefully, we can make a best-effort attempt to write the CONNECTION_CLOSE to the socket and notify the peer. If we're in the middle of graceful shutdown, it doesn't hurt, either.

This can be taken advantage of by callers to do a quicker shutdown if they know the drain period isn't necessary. Discussion here: n0-computer/iroh#4201

[matheus23]

Thank you for looking into this @colinmarc :)

Before we put in more work, I need to check back with @flub whether our ideas in n0-computer/iroh#4201 are sound & desirable.

Other than that:

• This needs a test! Ideally one that sets up a Runtime similar to the one we're using in iroh: Most importantly one that uses a TaskTracker and fairly aggressively closes all tasks (in this case those will be the connection and endpoint drivers).
• Rather than driving the transmit in close, I'd like to drive the transmit only when the ConnectionDriver is dropped. Otherwise I worry the drive_transmit from the connection driver and the close call can get in each others way. I think there's an invariant that the connection driver is the only thing that calls drive_transmit and it never misses the consecutive calls to that function. The right place for calling drive_transmit might be in the impl Drop for ConnectionRef right after the conn.implicit_close(..).

[colinmarc]

Before we put in more work, I need to check back with @flub whether our ideas in n0-computer/iroh#4201 are sound & desirable.

No worries, I just wanted to have something concrete to discuss :) This is only half of a workaround for that issue anyway.

The right place for calling drive_transmit might be in the impl Drop for ConnectionRef right after the conn.implicit_close(..).

That makes sense! Moved it to the drop for ConnectionRef. I wasn't sure if you meant inside the conditional, but that would mean we don't get this optimization for an explicit close/drop.