lightningd: handle large numbers of command outputs gracefully.

Profiling shows us spending all our time in tal_arr_remove when dealing
with a giant number of output streams.  This applies both for RPC output
and plugin output.

Use linked list instead.

tests/test_coinmoves.py::test_generate_coinmoves (2,000,000, sqlite3):
	Time (from start to end of l2 node):	239 seconds **WAS 518**
	Worst latency:				56.9 seconds **WAS 353**

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

common/json_stream.h

@@ -9,6 +9,7 @@
 # include <external/jsmn/jsmn.h>
 
 #include <bitcoin/short_channel_id.h>
+#include <ccan/list/list.h>
 #include <ccan/time/time.h>
 #include <common/amount.h>
 #include <common/jsonrpc_errors.h>
@@ -36,6 +37,8 @@ struct wireaddr;
 struct wireaddr_internal;
 
 struct json_stream {
+	struct list_node list;
+
 	struct json_out *jout;
 
 	/* Who is writing to this buffer now; NULL if nobody is. */

lightningd/jsonrpc.c

@@ -96,8 +96,8 @@ struct json_connection {
 
 	/* Our json_streams (owned by the commands themselves while running).
 	 * Since multiple streams could start returning data at once, we
-	 * always service these in order, freeing once empty. */
-	struct json_stream **js_arr;
+	 * always service these in order. */
+	struct list_head jsouts;
 };
 
 /**
@@ -125,25 +125,10 @@ static struct json_stream *jcon_new_json_stream(const tal_t *ctx,
 
 	/* Wake writer to start streaming, in case it's not already. */
 	io_wake(jcon);
-
-	/* FIXME: Keep streams around for recycling. */
-	tal_arr_expand(&jcon->js_arr, js);
+	list_add_tail(&jcon->jsouts, &js->list);
 	return js;
 }
 
-static void jcon_remove_json_stream(struct json_connection *jcon,
-				    struct json_stream *js)
-{
-	for (size_t i = 0; i < tal_count(jcon->js_arr); i++) {
-		if (js != jcon->js_arr[i])
-			continue;
-
-		tal_arr_remove(&jcon->js_arr, i);
-		return;
-	}
-	abort();
-}
-
 /* jcon and cmd have separate lifetimes: we detach them on either destruction */
 static void destroy_jcon(struct json_connection *jcon)
 {
@@ -1156,13 +1141,16 @@ static struct io_plan *stream_out_complete(struct io_conn *conn,
 static struct io_plan *start_json_stream(struct io_conn *conn,
 					 struct json_connection *jcon)
 {
+	struct json_stream *js;
+
 	/* If something has created an output buffer, start streaming. */
-	if (tal_count(jcon->js_arr)) {
+	js = list_top(&jcon->jsouts, struct json_stream, list);
+	if (js) {
 		size_t len;
-		const char *p = json_out_contents(jcon->js_arr[0]->jout, &len);
+		const char *p = json_out_contents(js->jout, &len);
 		if (len)
 			log_io(jcon->log, LOG_IO_OUT, NULL, "", p, len);
-		return json_stream_output(jcon->js_arr[0], conn,
+		return json_stream_output(js, conn,
 					  stream_out_complete, jcon);
 	}
 
@@ -1186,7 +1174,7 @@ static struct io_plan *stream_out_complete(struct io_conn *conn,
 					   struct json_stream *js,
 					   struct json_connection *jcon)
 {
-	jcon_remove_json_stream(jcon, js);
+	list_del_from(&jcon->jsouts, &js->list);
 	tal_free(js);
 
 	/* Wait for more output. */
@@ -1207,7 +1195,7 @@ static struct io_plan *read_json(struct io_conn *conn,
 		log_io(jcon->log, LOG_IO_IN, NULL, "", buffer, len_read);
 
 	/* We wait for pending output to be consumed, to avoid DoS */
-	if (tal_count(jcon->js_arr) != 0) {
+	if (!list_empty(&jcon->jsouts)) {
 		return io_wait(conn, conn, read_json, jcon);
 	}
 
@@ -1262,7 +1250,7 @@ static struct io_plan *jcon_connected(struct io_conn *conn,
 	jcon = notleak(tal(conn, struct json_connection));
 	jcon->conn = conn;
 	jcon->ld = ld;
-	jcon->js_arr = tal_arr(jcon, struct json_stream *, 0);
+	list_head_init(&jcon->jsouts);
 	jcon->json_in = jsonrpc_io_new(jcon);
 	jcon->notifications_enabled = false;
 	jcon->db_batching = false;

lightningd/plugin.c

@@ -372,7 +372,7 @@ struct plugin *plugin_register(struct plugins *plugins, const char* path TAKES,
 	p->can_check = false;
 
 	p->plugin_state = UNCONFIGURED;
-	p->js_arr = tal_arr(p, struct json_stream *, 0);
+	list_head_init(&p->jsouts);
 	p->notification_topics = tal_arr(p, const char *, 0);
 	p->subscriptions = NULL;
 	p->dynamic = false;
@@ -474,8 +474,8 @@ void plugin_kill(struct plugin *plugin, enum log_level loglevel,
  */
 static void plugin_send(struct plugin *plugin, struct json_stream *stream)
 {
-	tal_steal(plugin->js_arr, stream);
-	tal_arr_expand(&plugin->js_arr, stream);
+	tal_steal(plugin, stream);
+	list_add_tail(&plugin->jsouts, &stream->list);
 	io_wake(plugin);
 }
 
@@ -816,10 +816,7 @@ static struct io_plan *plugin_write_json(struct io_conn *conn,
 
 static struct io_plan *plugin_stream_complete(struct io_conn *conn, struct json_stream *js, struct plugin *plugin)
 {
-	assert(tal_count(plugin->js_arr) > 0);
-	/* Remove js and shift all remainig over */
-	tal_arr_remove(&plugin->js_arr, 0);
-
+	list_del_from(&plugin->jsouts, &js->list);
 	/* It got dropped off the queue, free it. */
 	tal_free(js);
 
@@ -829,8 +826,11 @@ static struct io_plan *plugin_stream_complete(struct io_conn *conn, struct json_
 static struct io_plan *plugin_write_json(struct io_conn *conn,
 					 struct plugin *plugin)
 {
-	if (tal_count(plugin->js_arr)) {
-		return json_stream_output(plugin->js_arr[0], plugin->stdin_conn, plugin_stream_complete, plugin);
+	struct json_stream *js;
+
+	js = list_top(&plugin->jsouts, struct json_stream, list);
+	if (js) {
+		return json_stream_output(js, plugin->stdin_conn, plugin_stream_complete, plugin);
 	}
 
 	return io_out_wait(conn, plugin, plugin_write_json, plugin);

lightningd/plugin.h

@@ -78,10 +78,9 @@ struct plugin {
 	/* Stuff we read */
 	struct jsonrpc_io *json_in;
 
-	/* Our json_streams. Since multiple streams could start
-	 * returning data at once, we always service these in order,
-	 * freeing once empty. */
-	struct json_stream **js_arr;
+	/* Our plugin_jstream_out list. Since multiple streams could start
+	 * returning data at once, we always service these in order. */
+	struct list_head jsouts;
 
 	struct logger *log;