Improve plugins builder.

pkg/plugins/builder.go

@@ -15,6 +15,9 @@ import (
 
 const devPluginName = "dev"
 
+// Constructor creates a plugin handler.
+type Constructor func(context.Context, http.Handler) (http.Handler, error)
+
 // pluginContext The static part of a plugin configuration.
 type pluginContext struct {
 	// GoPath plugin's GOPATH
@@ -89,7 +92,7 @@ func NewBuilder(client *Client, plugins map[string]Descriptor, devPlugin *DevPlu
 }
 
 // Build builds a plugin.
-func (b Builder) Build(pName string, config map[string]interface{}, middlewareName string) (*Middleware, error) {
+func (b Builder) Build(pName string, config map[string]interface{}, middlewareName string) (Constructor, error) {
 	if b.descriptors == nil {
 		return nil, fmt.Errorf("plugin: no plugin definition in the static configuration: %s", pName)
 	}
@@ -99,7 +102,12 @@ func (b Builder) Build(pName string, config map[string]interface{}, middlewareNa
 		return nil, fmt.Errorf("plugin: unknown plugin type: %s", pName)
 	}
 
-	return newMiddleware(descriptor, config, middlewareName)
+	m, err := newMiddleware(descriptor, config, middlewareName)
+	if err != nil {
+		return nil, err
+	}
+
+	return m.NewHandler, err
 }
 
 // Middleware is a HTTP handler plugin wrapper.

pkg/server/middleware/middlewares.go

@@ -28,7 +28,6 @@ import (
 	"github.com/containous/traefik/v2/pkg/middlewares/stripprefix"
 	"github.com/containous/traefik/v2/pkg/middlewares/stripprefixregex"
 	"github.com/containous/traefik/v2/pkg/middlewares/tracing"
-	"github.com/containous/traefik/v2/pkg/plugins"
 	"github.com/containous/traefik/v2/pkg/server/provider"
 )
 
@@ -41,7 +40,7 @@ const (
 // Builder the middleware builder.
 type Builder struct {
 	configs        map[string]*runtime.MiddlewareInfo
-	pluginBuilder  *plugins.Builder
+	pluginBuilder  PluginsBuilder
 	serviceBuilder serviceBuilder
 }
 
@@ -50,7 +49,7 @@ type serviceBuilder interface {
 }
 
 // NewBuilder creates a new Builder.
-func NewBuilder(configs map[string]*runtime.MiddlewareInfo, serviceBuilder serviceBuilder, pluginBuilder *plugins.Builder) *Builder {
+func NewBuilder(configs map[string]*runtime.MiddlewareInfo, serviceBuilder serviceBuilder, pluginBuilder PluginsBuilder) *Builder {
 	return &Builder{configs: configs, serviceBuilder: serviceBuilder, pluginBuilder: pluginBuilder}
 }
 
@@ -351,13 +350,13 @@ func (b *Builder) buildConstructor(ctx context.Context, middlewareName string) (
 			return nil, err
 		}
 
-		m, err := b.pluginBuilder.Build(pluginType, rawPluginConfig, middlewareName)
+		plug, err := b.pluginBuilder.Build(pluginType, rawPluginConfig, middlewareName)
 		if err != nil {
 			return nil, err
 		}
 
 		middleware = func(next http.Handler) (http.Handler, error) {
-			return m.NewHandler(ctx, next)
+			return plug(ctx, next)
 		}
 	}
 

pkg/server/middleware/plugins.go

@@ -5,8 +5,14 @@ import (
 	"fmt"
 
 	"github.com/containous/traefik/v2/pkg/config/dynamic"
+	"github.com/containous/traefik/v2/pkg/plugins"
 )
 
+// PluginsBuilder the plugin's builder interface.
+type PluginsBuilder interface {
+	Build(pName string, config map[string]interface{}, middlewareName string) (plugins.Constructor, error)
+}
+
 func findPluginConfig(rawConfig map[string]dynamic.PluginConf) (string, map[string]interface{}, error) {
 	if len(rawConfig) != 1 {
 		return "", nil, errors.New("plugin: invalid configuration: no configuration or too many plugin definition")

pkg/server/routerfactory.go

@@ -6,7 +6,6 @@ import (
 	"github.com/containous/traefik/v2/pkg/config/runtime"
 	"github.com/containous/traefik/v2/pkg/config/static"
 	"github.com/containous/traefik/v2/pkg/log"
-	"github.com/containous/traefik/v2/pkg/plugins"
 	"github.com/containous/traefik/v2/pkg/server/middleware"
 	"github.com/containous/traefik/v2/pkg/server/router"
 	routertcp "github.com/containous/traefik/v2/pkg/server/router/tcp"
@@ -26,14 +25,14 @@ type RouterFactory struct {
 
 	managerFactory *service.ManagerFactory
 
-	pluginBuilder *plugins.Builder
+	pluginBuilder middleware.PluginsBuilder
 
 	chainBuilder *middleware.ChainBuilder
 	tlsManager   *tls.Manager
 }
 
 // NewRouterFactory creates a new RouterFactory.
-func NewRouterFactory(staticConfiguration static.Configuration, managerFactory *service.ManagerFactory, tlsManager *tls.Manager, chainBuilder *middleware.ChainBuilder, pluginBuilder *plugins.Builder) *RouterFactory {
+func NewRouterFactory(staticConfiguration static.Configuration, managerFactory *service.ManagerFactory, tlsManager *tls.Manager, chainBuilder *middleware.ChainBuilder, pluginBuilder middleware.PluginsBuilder) *RouterFactory {
 	var entryPointsTCP, entryPointsUDP []string
 	for name, cfg := range staticConfiguration.EntryPoints {
 		protocol, err := cfg.GetProtocol()