lightningd/plugin_hook: make it possible to unregister a hook

This adds 'plugin_unregister_hook' and 'plugin_unregister_hook_all'
functions to unregister a given hook a plugin registered, or all hooks a
plugin registered for. Since hooks can only be registered once, it's
useful in the case a new plugin is added which would be prefered for
hook registration over an already loaded plugin.

lightningd/plugin_hook.c

@@ -39,6 +39,32 @@ bool plugin_hook_register(struct plugin *plugin, const char *method)
 	return true;
 }
 
+bool plugin_hook_unregister(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) {
+		/* This name is not registered */
+		return false;
+	}
+	hook->plugin = NULL;
+	return true;
+}
+
+void plugin_hook_unregister_all(struct plugin *plugin)
+{
+	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 (hooks[i]->plugin == plugin)
+			hooks[i]->plugin = NULL;
+}
+
 /**
  * Callback to be passed to the jsonrpc_request.
  *

lightningd/plugin_hook.h

@@ -102,6 +102,12 @@ void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
 
 bool plugin_hook_register(struct plugin *plugin, const char *method);
 
+/* Unregister a hook a plugin has registered for */
+bool plugin_hook_unregister(struct plugin *plugin, const char *method);
+
+/* Unregister all hooks a plugin has registered for */
+void plugin_hook_unregister_all(struct plugin *plugin);
+
 /* Special sync plugin hook for db: changes[] are SQL statements, with optional
  * final command appended. */
 void plugin_hook_db_sync(struct db *db, const char **changes, const char *final);