close sockets eagerly in Lwt_io.establish_server

[c-cube]

Currently, at

lwt/src/unix/lwt_io.ml

Line 1416 in ebd0a99

~close:(fun () -> Lazy.force close) fd);

, it seems that establish_server doesn't close the FDs itself. A server receiving lots of short-lived connections quickly blocks by FD starvation, unless on closes the two channels manually. I think the channels should be closed by establish_server itself by default (or at least providing an optional flag to enable it), à la with_file, etc..

[c-cube]

Actually it seems that each call to the callback blocks the entire establish_server thread, so the callback should return quickly, or use Lwt.async to do the real work, which is a recipe for FD leak. Then maybe what would be nice is establish_server_async with, roughly, the same signature as establish_server, except the callback returns a unit Lwt.t and is wrapped into the safe file-closing behavior.

[aantron]

Appears to be a duplicate of #208.