Refactor I/O and add SD_NOTIFY proxy support

Refactored all the conn_sock functionality to be more generic. It can deal
with different types of sockets, stream vs dgram, and reuses all the same
callbacks, shutdown and async functionality.

Conmon creates a notify socket which podman bind-mounts into the container,
and passes in via the spec's environment variables.  Conmon relays the
READY=1 signal.  This is similar to what runc and crun do, but doing it in
conmon and NOT passing NOTIFY_SOCKET to the OCI runtime allows us to start
up properly without runc and crun blocking on the "start" command.

It would also be trivial to add more proxied sockets, i.e. the /dev/log
proof of concept I did would now be super easy, if we wanted to revisit that.

Signed-off-by: Joseph Gooch <[email protected]>

src/conmon.c

@@ -84,6 +84,15 @@ int main(int argc, char *argv[])
 		exit(0);
 	}
 
+	if (getenv("NOTIFY_SOCKET") != NULL) {
+		setup_notify_socket(getenv("NOTIFY_SOCKET"));
+		int r = unsetenv("NOTIFY_SOCKET");
+		if (r < 0) {
+			nwarnf("Cannot unset NOTIFY_SOCKET %d", r)
+		}
+	}
+
+
 	/* Environment variables */
 	sync_pipe_fd = get_pipe_fd_from_env("_OCI_SYNCPIPE");