[suggestion] Add helper interface for ProxyBalancer interface

middleware/proxy.go

@@ -72,6 +72,11 @@ type (
 		Next(echo.Context) *ProxyTarget
 	}
 
+	// TargetProvider defines an interface that gives the opportunity for balancer to return custom errors when selecting target.
+	TargetProvider interface {
+		GetTarget(echo.Context) (*ProxyTarget, error)
+	}
+
 	commonBalancer struct {
 		targets []*ProxyTarget
 		mutex   sync.RWMutex
@@ -231,7 +236,17 @@ func ProxyWithConfig(config ProxyConfig) echo.MiddlewareFunc {
 
 			req := c.Request()
 			res := c.Response()
-			tgt := config.Balancer.Next(c)
+
+			var tgt *ProxyTarget
+			provider, ok := config.Balancer.(TargetProvider)
+			if ok {
+				tgt, err = provider.GetTarget(c)
+				if err != nil {
+					return err
+				}
+			} else {
+				tgt = config.Balancer.Next(c)
+			}
 			c.Set(config.ContextKey, tgt)
 
 			if err := rewriteURL(config.RegexRewrite, req); err != nil {

middleware/proxy_test.go

@@ -18,7 +18,19 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-//Assert expected with url.EscapedPath method to obtain the path.
+type testProvider struct {
+	*commonBalancer
+}
+
+func (p *testProvider) Next(c echo.Context) *ProxyTarget {
+	return p.commonBalancer.targets[0]
+}
+
+func (p *testProvider) GetTarget(c echo.Context) (*ProxyTarget, error) {
+	return p.commonBalancer.targets[1], nil
+}
+
+// Assert expected with url.EscapedPath method to obtain the path.
 func TestProxy(t *testing.T) {
 	// Setup
 	t1 := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -31,7 +43,6 @@ func TestProxy(t *testing.T) {
 	}))
 	defer t2.Close()
 	url2, _ := url.Parse(t2.URL)
-
 	targets := []*ProxyTarget{
 		{
 			Name: "target 1",
@@ -89,6 +100,16 @@ func TestProxy(t *testing.T) {
 	body = rec.Body.String()
 	assert.Equal(t, "target 2", body)
 
+	// Provider
+	e = echo.New()
+	tp := &testProvider{commonBalancer: new(commonBalancer)}
+	tp.targets = targets
+	e.Use(Proxy(tp))
+	rec = httptest.NewRecorder()
+	e.ServeHTTP(rec, req)
+	body = rec.Body.String()
+	assert.Equal(t, "target 2", body)
+
 	// ModifyResponse
 	e = echo.New()
 	e.Use(ProxyWithConfig(ProxyConfig{