plugin: Add hooks that a plugin might register

This is the first use of the `hooks` autodata field, and it required a
dummy element in order for the section not to be dropped, it'll be
removed once we have actual hooks.

lightningd/plugin.c

@@ -19,6 +19,7 @@
 #include <lightningd/json.h>
 #include <lightningd/lightningd.h>
 #include <lightningd/notification.h>
+#include <lightningd/plugin_hook.h>
 #include <signal.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -787,6 +788,30 @@ static bool plugin_subscriptions_add(struct plugin *plugin, const char *buffer,
 	return true;
 }
 
+static bool plugin_hooks_add(struct plugin *plugin, const char *buffer,
+			     const jsmntok_t *resulttok)
+{
+	const jsmntok_t *hookstok = json_get_member(buffer, resulttok, "hooks");
+	if (!hookstok)
+		return true;
+
+	for (int i = 0; i < hookstok->size; i++) {
+		char *name = json_strdup(NULL, plugin->buffer,
+					 json_get_arr(hookstok, i));
+		if (!plugin_hook_register(plugin, name)) {
+			plugin_kill(plugin,
+				    "could not register hook '%s', either the "
+				    "name doesn't exist or another plugin "
+				    "already registered it.",
+				    name);
+			tal_free(name);
+			return false;
+		}
+		tal_free(name);
+	}
+	return true;
+}
+
 static void plugin_manifest_timeout(struct plugin *plugin)
 {
 	log_broken(plugin->log, "The plugin failed to respond to \"getmanifest\" in time, terminating.");
@@ -816,12 +841,13 @@ static void plugin_manifest_cb(const char *buffer,
 		return;
 	}
 
-	if (!plugin_opts_add(plugin, buffer, resulttok)
-	    || !plugin_rpcmethods_add(plugin, buffer, resulttok)
-	    || !plugin_subscriptions_add(plugin, buffer, resulttok))
+	if (!plugin_opts_add(plugin, buffer, resulttok) ||
+	    !plugin_rpcmethods_add(plugin, buffer, resulttok) ||
+	    !plugin_subscriptions_add(plugin, buffer, resulttok) ||
+	    !plugin_hooks_add(plugin, buffer, resulttok))
 		plugin_kill(
 		    plugin,
-		    "Failed to register options, methods, or subscriptions.");
+		    "Failed to register options, methods, hooks, or subscriptions.");
 	/* Reset timer, it'd kill us otherwise. */
 	tal_free(plugin->timeout_timer);
 }

lightningd/plugin_hook.c

@@ -1,5 +1,35 @@
 #include <lightningd/plugin_hook.h>
 
+static struct plugin_hook *plugin_hook_by_name(const char *name)
+{
+	static struct plugin_hook **hooks = NULL;
+	static size_t num_hooks;
+	if (!hooks)
+		hooks = autodata_get(hooks, &num_hooks);
+
+	for (size_t i=0; i<num_hooks; i++)
+		if (streq(hooks[i]->name, name))
+			return hooks[i];
+	return NULL;
+}
+
+bool plugin_hook_register(struct plugin *plugin, const char *method)
+{
+	struct plugin_hook *hook = plugin_hook_by_name(method);
+	if (!hook) {
+		/* No such hook name registered */
+		return false;
+	} else if (hook->plugin != NULL) {
+		/* Another plugin already registered for this name */
+		return false;
+	}
+	hook->plugin = plugin;
+	return true;
+}
+
+/* FIXME(cdecker): Remove dummy hook, once we have a real one */
+REGISTER_PLUGIN_HOOK(hello, NULL, void *, NULL, void *, NULL, void *);
+
 void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
 		       void *payload, void *cb_arg)
 {

lightningd/plugin_hook.h

@@ -73,7 +73,7 @@ void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
  */
 /* FIXME: Find a way to avoid back-to-back declaration and definition */
 #define PLUGIN_HOOK_CALL_DEF(name, payload_type, response_cb_arg_type)         \
-	static inline void plugin_hook_call_##name(                            \
+	UNNEEDED static inline void plugin_hook_call_##name(                   \
 	    struct lightningd *ld, payload_type payload,                       \
 	    response_cb_arg_type cb_arg)                                       \
 	{                                                                      \