patch 7.4.2287
Problem:    The callback passed to ch_sendraw() is not used.
Solution:   Pass the read part, not the send part. (haya14busa, closes #1019)
diff --git a/src/channel.c b/src/channel.c
index 7488574..81c43a5 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -3456,7 +3456,7 @@
 	    EMSG2(_("E917: Cannot use a callback with %s()"), fun);
 	    return NULL;
 	}
-	channel_set_req_callback(channel, part_send,
+	channel_set_req_callback(channel, *part_read,
 				       opt->jo_callback, opt->jo_partial, id);
     }