MINOR: connections: Add a new xprt method, add_xprt().

Add a new method to xprt_ops, add_xprt(), that changes the underlying
xprt to the one provided, and optionally provide the old one.
diff --git a/include/types/connection.h b/include/types/connection.h
index e513994..74060b0 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -334,6 +334,7 @@
 	int (*subscribe)(struct connection *conn, void *xprt_ctx, int event_type, void *param); /* Subscribe to events, such as "being able to send" */
 	int (*unsubscribe)(struct connection *conn, void *xprt_ctx, int event_type, void *param); /* Unsubscribe to events */
 	int (*remove_xprt)(struct connection *conn, void *xprt_ctx, void *toremove_ctx, const struct xprt_ops *newops, void *newctx); /* Remove an xprt from the connection, used by temporary xprt such as the handshake one */
+	int (*add_xprt)(struct connection *conn, void *xprt_ctx, void *toadd_ctx, const struct xprt_ops *toadd_ops, void **oldxprt_ctx, const struct xprt_ops **oldxprt_ops); /* Add a new XPRT as the new xprt, and return the old one */
 };
 
 /* mux_ops describes the mux operations, which are to be performed at the
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 1d018aa..fed53d5 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -5630,6 +5630,22 @@
 	return 0;
 }
 
+/* Use the provided XPRT as an underlying XPRT, and provide the old one.
+ * Returns 0 on success, and non-zero on failure.
+ */
+static int ssl_add_xprt(struct connection *conn, void *xprt_ctx, void *toadd_ctx, const struct xprt_ops *toadd_ops, void **oldxprt_ctx, const struct xprt_ops **oldxprt_ops)
+{
+	struct ssl_sock_ctx *ctx = xprt_ctx;
+
+	if (oldxprt_ops != NULL)
+		*oldxprt_ops = ctx->xprt;
+	if (oldxprt_ctx != NULL)
+		*oldxprt_ctx = ctx->xprt_ctx;
+	ctx->xprt = toadd_ops;
+	ctx->xprt_ctx = toadd_ctx;
+	return 0;
+}
+
 /* Remove the specified xprt. If if it our underlying XPRT, remove it and
  * return 0, otherwise just call the remove_xprt method from the underlying
  * XPRT.
@@ -9842,6 +9858,7 @@
 	.subscribe = ssl_subscribe,
 	.unsubscribe = ssl_unsubscribe,
 	.remove_xprt = ssl_remove_xprt,
+	.add_xprt = ssl_add_xprt,
 	.rcv_pipe = NULL,
 	.snd_pipe = NULL,
 	.shutr    = NULL,