Address comments

docs/interceptors.md

@@ -0,0 +1,32 @@
+---
+id: "interceptors"
+title: "Interceptors"
+sidebar_label: "Interceptor"
+---
+
+The service constructor can use the option `twirp.WithServerInterceptors(interceptors ...twirp.Interceptor)`
+to plug in additional functionality:
+
+```go
+server := NewHaberdasherServer(svcImpl, twirp.WithInterceptor(NewLogInterceptor(logger.New(os.Stderr, "", 0))))
+
+// NewLogInterceptor logs various parts of a request using a standard Logger.
+func NewLogInterceptor(l *log.Logger) twirp.Interceptor {
+  return func(next twirp.Method) twirp.Method {
+    return func(ctx context.Context, req interface{}) (interface{}, error) {
+      l.Printf("request: %v", request)
+      resp, err := next(ctx, req)
+      if err != nil {
+        l.Printf("error: %v", err)
+        return nil, err
+      }
+      l.Printf("response: %v", resp)
+      return resp, nil
+    }
+  }
+}
+```
+
+Check out
+[the godoc for `Interceptor`](http://godoc.org/github.com/twitchtv/twirp#Interceptors)
+for more information.

interceptors.go

@@ -0,0 +1,68 @@
+// Copyright 2018 Twitch Interactive, Inc.  All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"). You may not
+// use this file except in compliance with the License. A copy of the License is
+// located at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// or in the "license" file accompanying this file. This file is distributed on
+// an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+// express or implied. See the License for the specific language governing
+// permissions and limitations under the License.
+package twirp
+
+import (
+	"context"
+)
+
+// Method is a method that matches the generic form of a Twirp-generated RPC method.
+//
+// This is used for Interceptors.
+type Method func(ctx context.Context, request interface{}) (interface{}, error)
+
+// Interceptor is an interceptor that can be installed on a client or server.
+//
+// Users can use Interceptors to intercept any RPC.
+//
+//   func LogInterceptor(l *log.Logger) twirp.Interceptor {
+//     return func(next twirp.Method) twirp.Method {
+//       return func(ctx context.Context, req interface{}) (interface{}, error) {
+//         l.Printf("request: %v", request)
+//         resp, err := next(ctx, req)
+//         if err != nil {
+//           l.Printf("error: %v", err)
+//           return nil, err
+//         }
+//         l.Printf("response: %v", resp)
+//         return resp, nil
+//       }
+//     }
+//   }
+type Interceptor func(Method) Method
+
+// ChainInterceptors chains the Interceptors.
+//
+// Returns nil if interceptors is empty.
+func ChainInterceptors(interceptors ...Interceptor) Interceptor {
+	filtered := make([]Interceptor, 0, len(interceptors))
+	for _, interceptor := range interceptors {
+		if interceptor != nil {
+			filtered = append(filtered, interceptor)
+		}
+	}
+	switch n := len(filtered); n {
+	case 0:
+		return nil
+	case 1:
+		return filtered[0]
+	default:
+		first := filtered[0]
+		return func(next Method) Method {
+			for i := len(filtered) - 1; i > 0; i-- {
+				next = filtered[i](next)
+			}
+			return first(next)
+		}
+	}
+}

middleware_test.go → interceptors_test.go

@@ -21,6 +21,12 @@ func TestChainInterceptors(t *testing.T) {
 	if chain := ChainInterceptors(); chain != nil {
 		t.Errorf("ChainInterceptors(0) expected to be nil, but was %v", chain)
 	}
+	if chain := ChainInterceptors(nil); chain != nil {
+		t.Errorf("ChainInterceptors(0) expected to be nil, but was %v", chain)
+	}
+	if chain := ChainInterceptors(nil, nil); chain != nil {
+		t.Errorf("ChainInterceptors(0) expected to be nil, but was %v", chain)
+	}
 
 	interceptor1 := func(next Method) Method {
 		return func(ctx context.Context, request interface{}) (interface{}, error) {
@@ -59,6 +65,14 @@ func TestChainInterceptors(t *testing.T) {
 			interceptors: []Interceptor{interceptor1, interceptor2, interceptor3},
 			want:         "abcx321",
 		},
+		{
+			interceptors: []Interceptor{interceptor1, interceptor2, nil, interceptor3},
+			want:         "abcx321",
+		},
+		{
+			interceptors: []Interceptor{interceptor1, interceptor1, interceptor1},
+			want:         "aaax111",
+		},
 	} {
 		response, err := ChainInterceptors(testCase.interceptors...)(method)(context.Background(), "")
 		if err != nil {