connection: Added callback when the queue is cleared

This allows us to break out of the normal queue-based write loop and
handle things ourself for a while. Currently this is used to trigger
regular gossip dumps that do not proceed until the buffers have been
cleared in order to avoid memory-explosions.

lightningd/connection.c

@@ -17,8 +17,8 @@ static const u8 *daemon_conn_dequeue(struct daemon_conn *dc)
 	if (n == 0)
 		return NULL;
 	msg = dc->msg_out[0];
-	memmove(dc->msg_out, dc->msg_out + 1, sizeof(dc->msg_in[0]) * (n-1));
-	tal_resize(&dc->msg_out, n-1);
+	memmove(dc->msg_out, dc->msg_out + 1, sizeof(dc->msg_in[0]) * (n - 1));
+	tal_resize(&dc->msg_out, n - 1);
 	return msg;
 }
 
@@ -30,12 +30,15 @@ struct io_plan *daemon_conn_read_next(struct io_conn *conn,
 			    dc);
 }
 
-static struct io_plan *daemon_conn_write_next(struct io_conn *conn,
+struct io_plan *daemon_conn_write_next(struct io_conn *conn,
 				       struct daemon_conn *dc)
 {
 	const u8 *msg = daemon_conn_dequeue(dc);
 	if (msg) {
-		return io_write_wire(conn, take(msg), daemon_conn_write_next, dc);
+		return io_write_wire(conn, take(msg), daemon_conn_write_next,
+				     dc);
+	} else if (dc->msg_queue_cleared_cb) {
+		return dc->msg_queue_cleared_cb(conn, dc);
 	} else {
 		return io_out_wait(conn, dc, daemon_conn_write_next, dc);
 	}
@@ -59,6 +62,7 @@ void daemon_conn_init(tal_t *ctx, struct daemon_conn *dc, int fd,
 	dc->msg_in = NULL;
 	dc->msg_out = tal_arr(ctx, u8 *, 0);
 	dc->conn_fd = fd;
+	dc->msg_queue_cleared_cb = NULL;
 	io_new_conn(ctx, fd, daemon_conn_start, dc);
 }
 

lightningd/connection.h

@@ -20,7 +20,13 @@ struct daemon_conn {
 	struct io_conn *conn;
 
 	/* Callback for incoming messages */
-	struct io_plan *(*daemon_conn_recv)(struct io_conn *conn, struct daemon_conn *);
+	struct io_plan *(*daemon_conn_recv)(struct io_conn *conn,
+					    struct daemon_conn *);
+
+	/* Called whenever we've cleared the msg_out queue. Used to
+	 * inject things into the write loop */
+	struct io_plan *(*msg_queue_cleared_cb)(struct io_conn *conn,
+						struct daemon_conn *);
 };
 
 /**
@@ -32,13 +38,22 @@ struct daemon_conn {
  * @daemon_conn_recv: callback function to be called upon receiving a message
  */
 void daemon_conn_init(tal_t *ctx, struct daemon_conn *dc, int fd,
-		      struct io_plan *(*daemon_conn_recv)(struct io_conn *,
-							  struct daemon_conn *));
+		      struct io_plan *(*daemon_conn_recv)(
+			  struct io_conn *, struct daemon_conn *));
 /**
  * daemon_conn_send - Enqueue an outgoing message to be sent
  */
 void daemon_conn_send(struct daemon_conn *dc, u8 *msg);
 
+/**
+ * daemon_conn_write_next - Continue writing from the msg-queue
+ *
+ * Exposed here so that, if `msg_queue_cleared_cb` is used to break
+ * out of the write-loop, we can get back in.
+ */
+struct io_plan *daemon_conn_write_next(struct io_conn *conn,
+				       struct daemon_conn *dc);
+
 /**
  * daemon_conn_read_next - Read the next message
  */