networking: explain isrv_run() API

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

networking/isrv.h

@@ -23,7 +23,32 @@ int isrv_register_fd(isrv_state_t *state, int peer, int fd);
 void isrv_close_fd(isrv_state_t *state, int fd);
 int isrv_register_peer(isrv_state_t *state, void *param);
 
-/* driver */
+/* Driver:
+ *
+ * Select on listen_fd for <linger_timeout> (or forever if 0).
+ *
+ * If we time out and we have no peers, exit.
+ * If we have peers, call do_timeout(peer_param),
+ * if it returns !0, peer is removed.
+ *
+ * If listen_fd is active, accept new connection ("peer"),
+ * call new_peer() on it, and if it returns 1,
+ * and add it to fds to select on.
+ * Now, select will wait for <timeout>, not <linger_timeout>
+ * (as long as we we have more than zero clients).
+ *
+ * If a peer's fd is active, we call do_rd() on it if read
+ * mask bit was set,
+ * and then do_wr() if write mask bit was also set.
+ * If either returns !0, peer is removed.
+ * Reaching this place also resets timeout counter for this peer.
+ *
+ * Note that peer must indicate that he wants to be selected
+ * for read and/or write using isrv_want_rd()/isrv_want_wr()
+ * [can be called in new_peer() or in do_rd()/do_wr()].
+ * If it never wants to be selected for write, do_wr()
+ * will never be called (can be NULL).
+ */
 void isrv_run(
 	int listen_fd,
 	int (*new_peer)(isrv_state_t *state, int fd),