cryptomsg: add helpers to determine if we're partway through msg read/write.

For message read, we do it as header then body, so we can have
io_plan_in_started(conn) false, but we're between header and body.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/cryptomsg.c

@@ -132,6 +132,8 @@ static struct io_plan *peer_decrypt_body(struct io_conn *conn,
 	struct io_plan *plan;
 	u8 *in, *decrypted;
 
+	pcs->reading_body = false;
+
 	decrypted = cryptomsg_decrypt_body(pcs->in, &pcs->cs, pcs->in);
 	if (!decrypted)
 		return io_close(conn);
@@ -198,6 +200,8 @@ static struct io_plan *peer_decrypt_header(struct io_conn *conn,
 
 	tal_free(pcs->in);
 
+	pcs->reading_body = true;
+
 	/* BOLT #8:
 	 *
 	 * * Read _exactly_ `l+16` bytes from the network buffer, let
@@ -224,6 +228,7 @@ struct io_plan *peer_read_message(struct io_conn *conn,
 	 *
 	 *  * Read _exactly_ `18-bytes` from the network buffer.
 	 */
+	pcs->reading_body = false;
 	pcs->in = tal_arr(conn, u8, 18);
 	pcs->next_in = next;
 	return io_read(conn, pcs->in, 18, peer_decrypt_header, pcs);
@@ -366,8 +371,23 @@ struct io_plan *peer_write_message(struct io_conn *conn,
 	return io_write(conn, pcs->out, tal_count(pcs->out), post, pcs);
 }
 
+/* We write in one op, so it's all or nothing. */
+bool peer_out_started(const struct io_conn *conn,
+		      const struct peer_crypto_state *cs)
+{
+	return io_plan_out_started(conn);
+}
+
+/* We read in two parts, so we might have started body. */
+bool peer_in_started(const struct io_conn *conn,
+		     const struct peer_crypto_state *cs)
+{
+	return io_plan_in_started(conn) || cs->reading_body;
+}
+
 void init_peer_crypto_state(struct peer *peer, struct peer_crypto_state *pcs)
 {
 	pcs->peer = peer;
 	pcs->out = pcs->in = NULL;
+	pcs->reading_body = false;
 }

common/cryptomsg.h

@@ -14,6 +14,9 @@ struct peer_crypto_state {
 	/* Peer who owns us: peer->crypto_state == this */
 	struct peer *peer;
 
+	/* Where we are up to in reading (we do in two parts). */
+	bool reading_body;
+
 	/* Output and input buffers. */
 	u8 *out, *in;
 	struct io_plan *(*next_in)(struct io_conn *, struct peer *, u8 *);
@@ -30,6 +33,12 @@ struct io_plan *peer_read_message(struct io_conn *conn,
 							  struct peer *,
 							  u8 *msg));
 
+/* Have we already started writing/reading a message? */
+bool peer_out_started(const struct io_conn *conn,
+		      const struct peer_crypto_state *cs);
+bool peer_in_started(const struct io_conn *conn,
+		     const struct peer_crypto_state *cs);
+
 /* Sends message: frees if taken(msg). */
 struct io_plan *peer_write_message(struct io_conn *conn,
 				   struct peer_crypto_state *cs,