This was previously used to defer handling disconnections until the
next event loop iteration. We now achieve the same with simple use
of deferred_invoke(). :^)
This patch introduces IPC::Connection which becomes the new base class
of ClientConnection and ServerConnection. Most of the functionality
has been hoisted up to the base class since almost all of it is useful
on both sides.
This gives us the ability to send synchronous messages in both
directions, which is needed for the WebContent server process.
Unlike other servers, WebContent does not mind blocking on a response
from its client.
This stops servers from crashing when a client's socket buffer becomes
full and we can't post any more messages to it. Normally this means the
client process is hanged/spinning, but I suppose this could also happen
under severe system load.
It's unclear to me what a better solution here would be. We can't keep
buffering messages indefinitely if the client is just never going to
receive them anyway. At some point we have to cut our losses, and it
seems pretty reasonable to let the kernel socket buffer be the cutoff.
It will be the responsibility of the individual server implementations
to avoid sending messages to clients that may be unable to handle them.
Instead of always running the responsiveness timer for IPC clients,
we now only start it after sending a message. This avoids waking up
otherwise idle clients to do ping/pong busywork.
IPC::ClientConnection now tracks the time since the last time we got
a message from the client and calls a virtual function on itself after
3 seconds: may_have_become_unresponsive().
Subclasses of ClientConnection can then react to this if they like.
We use this mechanism in WindowServer to send out a friendly Ping
message to the client. If he doesn't Pong within 1 second, we mark
the client as "unresponsive" and recompose all of his windows with
a darkened appearance and amended title until he Pongs us.
This is a little on the aggressive side and we should figure out a way
to wake up less often. Perhaps this could only be done to windows the
user is currently interacting with, for example.
Anyways, this is pretty cool! :^)
