fix: allow starting Traefik even if plugin services have an issue

cmd/traefik/traefik.go

@@ -223,12 +223,16 @@ func setupServer(staticConfiguration *static.Configuration) (*server.Server, err
 
 	pluginBuilder, err := createPluginBuilder(staticConfiguration)
 	if err != nil {
-		return nil, err
+		log.WithoutContext().WithError(err).Error("Plugins are disabled because an error has occurred.")
 	}
 
 	// Providers plugins
 
 	for name, conf := range staticConfiguration.Providers.Plugin {
+		if pluginBuilder == nil {
+			break
+		}
+
 		p, err := pluginBuilder.BuildProvider(name, conf)
 		if err != nil {
 			return nil, fmt.Errorf("plugin: failed to build provider: %w", err)

pkg/plugins/client.go

@@ -164,7 +164,12 @@ func (c *Client) Download(ctx context.Context, pName, pVersion string) (string,
 
 	defer func() { _ = resp.Body.Close() }()
 
-	if resp.StatusCode == http.StatusOK {
+	switch resp.StatusCode {
+	case http.StatusNotModified:
+		// noop
+		return hash, nil
+
+	case http.StatusOK:
 		err = os.MkdirAll(filepath.Dir(filename), 0o755)
 		if err != nil {
 			return "", fmt.Errorf("failed to create directory: %w", err)
@@ -189,16 +194,12 @@ func (c *Client) Download(ctx context.Context, pName, pVersion string) (string,
 		}
 
 		return hash, nil
-	}
-
-	if resp.StatusCode == http.StatusNotModified {
-		// noop
-		return hash, nil
-	}
 
+	default:
 		data, _ := io.ReadAll(resp.Body)
 		return "", fmt.Errorf("error: %d: %s", resp.StatusCode, string(data))
 	}
+}
 
 // Check checks the plugin archive integrity.
 func (c *Client) Check(ctx context.Context, pName, pVersion, hash string) error {

pkg/server/middleware/middlewares.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"reflect"
 	"strings"
 
 	"github.com/containous/alice"
@@ -340,7 +341,7 @@ func (b *Builder) buildConstructor(ctx context.Context, middlewareName string) (
 	}
 
 	// Plugin
-	if config.Plugin != nil {
+	if config.Plugin != nil && !reflect.ValueOf(b.pluginBuilder).IsNil() { // Using "reflect" because "b.pluginBuilder" is an interface.
 		if middleware != nil {
 			return nil, badConf
 		}