json: Add function to duplicate a json_stream

Will be used in the next commit to fan out notifications to multiple
subscribing plugins. We can't just use `tal_dup` from outside since
the definition is hidden outside the compilation unit.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/json_stream.c

@@ -59,6 +59,17 @@ struct json_stream *new_json_stream(const tal_t *ctx, struct command *writer)
 	return js;
 }
 
+struct json_stream *json_stream_dup(const tal_t *ctx, struct json_stream *original)
+{
+	size_t num_elems = membuf_num_elems(&original->outbuf);
+	char *elems = membuf_elems(&original->outbuf);
+	struct json_stream *js = tal_dup(ctx, struct json_stream, original);
+	membuf_init(&js->outbuf, tal_dup_arr(js, char, elems, num_elems, 0),
+		    num_elems, membuf_tal_realloc);
+	membuf_added(&js->outbuf, num_elems);
+	return js;
+}
+
 bool json_stream_still_writing(const struct json_stream *js)
 {
 	return js->writer != NULL;

lightningd/json_stream.h

@@ -23,6 +23,19 @@ struct json_stream;
  */
 struct json_stream *new_json_stream(const tal_t *ctx, struct command *writer);
 
+/**
+ * Duplicate an existing stream.
+ *
+ * Mostly useful when we want to send copies of a given stream to
+ * multiple recipients, that might read at different speeds from the
+ * stream. For example this is used when construcing a single
+ * notification and then duplicating it for the fanout.
+ *
+ * @ctx: tal context for allocation.
+ * @original: the stream to duplicate.
+ */
+struct json_stream *json_stream_dup(const tal_t *ctx, struct json_stream *original);
+
 /**
  * json_stream_close - finished writing to a JSON stream.
  * @js: the json_stream.