Introduce PluginLoaderFunc

This allows registered plugins to execute code when loaded. This will
also allow the Lua support code to be a plugin.

Closes: https://todo.sr.ht/~sircmpwn/koushin/54

plugin.go

@@ -23,9 +23,13 @@ type Plugin interface {
 	Close() error
 }
 
-var plugins []Plugin
+// PluginLoaderFunc loads plugins for the provided server.
+type PluginLoaderFunc func(*Server) ([]Plugin, error)
 
-// RegisterPlugin registers a plugin to be loaded on server startup.
-func RegisterPlugin(p Plugin) {
-	plugins = append(plugins, p)
+var pluginLoaders []PluginLoaderFunc
+
+// RegisterPluginLoader registers a plugin loader. The loader will be called on
+// server start-up and reload.
+func RegisterPluginLoader(f PluginLoaderFunc) {
+	pluginLoaders = append(pluginLoaders, f)
 }

plugin_go.go

@@ -132,3 +132,10 @@ func (p *GoPlugin) Inject(name string, f InjectFunc) {
 func (p *GoPlugin) Plugin() Plugin {
 	return &goPlugin{p}
 }
+
+// Loader returns a loader function for this plugin.
+func (p *GoPlugin) Loader() PluginLoaderFunc {
+	return func(*Server) ([]Plugin, error) {
+		return []Plugin{p.Plugin()}, nil
+	}
+}

plugins/base/plugin.go

@@ -12,5 +12,5 @@ func init() {
 	p.TemplateFuncs(templateFuncs)
 	registerRoutes(&p)
 
-	koushin.RegisterPlugin(p.Plugin())
+	koushin.RegisterPluginLoader(p.Loader())
 }

server.go

@@ -176,9 +176,16 @@ func (s *Server) parseSMTPUpstream() error {
 }
 
 func (s *Server) load() error {
-	plugins := append([]Plugin(nil), plugins...)
-	for _, p := range plugins {
-		s.e.Logger.Printf("Registered plugin '%v'", p.Name())
+	var plugins []Plugin
+	for _, load := range pluginLoaders {
+		l, err := load(s)
+		if err != nil {
+			return fmt.Errorf("failed to load plugins: %v", err)
+		}
+		for _, p := range l {
+			s.e.Logger.Printf("Loaded plugin %q", p.Name())
+		}
+		plugins = append(plugins, l...)
 	}
 
 	luaPlugins, err := loadAllLuaPlugins(s.e.Logger)