plugin: Do not forward plugin hook calls during shutdown

We make the current state of `lightningd` explicit so we don't have to
identify a shutdown by its side-effects. We then use this in order to prevent
the killing and freeing of plugins to continue down the chain of registered
plugins.

lightningd/lightningd.c

@@ -740,6 +740,7 @@ int main(int argc, char *argv[])
 	 * valgrind will warn us if we make decisions based on uninitialized
 	 * variables. */
 	ld = new_lightningd(NULL);
+	ld->state = LD_STATE_RUNNING;
 
 	/* Figure out where our daemons are first. */
 	ld->daemon_dir = find_daemon_dir(ld, argv[0]);
@@ -931,6 +932,7 @@ int main(int argc, char *argv[])
 	 *  shut down.
 	 */
 	assert(io_loop_ret == ld);
+	ld->state = LD_STATE_SHUTDOWN;
 
 	/* Keep this fd around, to write final response at the end. */
 	stop_fd = io_conn_fd(ld->stop_conn);

lightningd/lightningd.h

@@ -80,6 +80,11 @@ struct config {
 
 typedef STRMAP(const char *) alt_subdaemon_map;
 
+enum lightningd_state {
+	LD_STATE_RUNNING,
+	LD_STATE_SHUTDOWN,
+};
+
 struct lightningd {
 	/* The directory to find all the subdaemons. */
 	const char *daemon_dir;
@@ -262,6 +267,8 @@ struct lightningd {
 	struct list_head waitblockheight_commands;
 
 	alt_subdaemon_map alt_subdaemons;
+
+	enum lightningd_state state;
 };
 
 /* Turning this on allows a tal allocation to return NULL, rather than aborting.

lightningd/plugin_hook.c

@@ -17,6 +17,7 @@ struct plugin_hook_request {
 	void *cb_arg;
 	void *payload;
 	struct db *db;
+	struct lightningd *ld;
 };
 
 /* A link in the plugin_hook call chain (there's a joke in there about
@@ -156,6 +157,11 @@ static void plugin_hook_callback(const char *buffer, const jsmntok_t *toks,
 	bool more_plugins, cont;
 	struct plugin_hook_call_link *last;
 
+	if (r->ld->state == LD_STATE_SHUTDOWN) {
+		log_debug(r->ld->log,
+			  "Abandoning plugin hook call due to shutdown");
+		return;
+	}
 	/* Pop the head off the call chain and continue with the next */
 	last = list_pop(&r->call_chain, struct plugin_hook_call_link, list);
 	assert(last != NULL);
@@ -232,6 +238,7 @@ void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
 		ph_req->db = ld->wallet->db;
 		ph_req->payload = tal_steal(ph_req, payload);
 		ph_req->current_plugin = -1;
+		ph_req->ld = ld;
 
 		list_head_init(&ph_req->call_chain);
 		for (size_t i=0; i<tal_count(hook->plugins); i++) {