[feat] Add middleware support as discussed in #293 (#294)

* mux.Router now has a `Use` method that allows you to add middleware to request processing.

README.md

@@ -28,6 +28,7 @@ The name mux stands for "HTTP request multiplexer". Like the standard `http.Serv
 * [Registered URLs](#registered-urls)
 * [Walking Routes](#walking-routes)
 * [Graceful Shutdown](#graceful-shutdown)
+* [Middleware](#middleware)
 * [Full Example](#full-example)
 
 ---
@@ -447,6 +448,86 @@ func main() {
 }
 ```
 
+### Middleware
+
+Mux supports the addition of middlewares to a [Router](https://godoc.org/github.com/gorilla/mux#Router), which are executed in the order they are added if a match is found, including its subrouters.
+Middlewares are (typically) small pieces of code which take one request, do something with it, and pass it down to another middleware or the final handler. Some common use cases for middleware are request logging, header manipulation, or `ResponseWriter` hijacking.
+
+Mux middlewares are defined using the de facto standard type:
+
+```go
+type MiddlewareFunc func(http.Handler) http.Handler
+```
+
+Typically, the returned handler is a closure which does something with the http.ResponseWriter and http.Request passed to it, and then calls the handler passed as parameter to the MiddlewareFunc. This takes advantage of closures being able access variables from the context where they are created, while retaining the signature enforced by the receivers.
+
+A very basic middleware which logs the URI of the request being handled could be written as:
+
+```go
+func simpleMw(next http.Handler) http.Handler {
+    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+        // Do stuff here
+        log.Println(r.RequestURI)
+        // Call the next handler, which can be another middleware in the chain, or the final handler.
+        next.ServeHTTP(w, r)
+    })
+}
+```
+
+Middlewares can be added to a router using `Router.AddMiddlewareFunc()`:
+
+```go
+r := mux.NewRouter()
+r.HandleFunc("/", handler)
+r.AddMiddleware(simpleMw)
+```
+
+A more complex authentication middleware, which maps session token to users, could be written as:
+
+```go
+// Define our struct
+type authenticationMiddleware struct {
+	tokenUsers map[string]string
+}
+
+// Initialize it somewhere
+func (amw *authenticationMiddleware) Populate() {
+	amw.tokenUsers["00000000"] = "user0"
+	amw.tokenUsers["aaaaaaaa"] = "userA"
+	amw.tokenUsers["05f717e5"] = "randomUser"
+	amw.tokenUsers["deadbeef"] = "user0"
+}
+
+// Middleware function, which will be called for each request
+func (amw *authenticationMiddleware) Middleware(next http.Handler) http.Handler {
+    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+        token := r.Header.Get("X-Session-Token")
+        
+        if user, found := amw.tokenUsers[token]; found {
+        	// We found the token in our map
+        	log.Printf("Authenticated user %s\n", user)
+        	// Pass down the request to the next middleware (or final handler)
+        	next.ServeHTTP(w, r)
+        } else {
+        	// Write an error and stop the handler chain
+        	http.Error(w, "Forbidden", 403)
+        }
+    })
+}
+```
+
+```go
+r := mux.NewRouter()
+r.HandleFunc("/", handler)
+
+amw := authenticationMiddleware{}
+amw.Populate()
+
+r.AddMiddlewareFunc(amw.Middleware)
+```
+
+Note: The handler chain will be stopped if your middleware doesn't call `next.ServeHTTP()` with the corresponding parameters. This can be used to abort a request if the middleware writer wants to. Middlewares *should* write to `ResponseWriter` if they *are* going to terminate the request, and they *should not* write to `ResponseWriter` if they *are not* going to terminate it.
+
 ## Full Example
 
 Here's a complete, runnable example of a small `mux` based server:

doc.go

@@ -238,5 +238,70 @@ as well:
 	url, err := r.Get("article").URL("subdomain", "news",
 	                                 "category", "technology",
 	                                 "id", "42")
+
+Since **vX.Y.Z**, mux supports the addition of middlewares to a [Router](https://godoc.org/github.com/gorilla/mux#Router), which are executed if a
+match is found (including subrouters). Middlewares are defined using the de facto standard type:
+
+	type MiddlewareFunc func(http.Handler) http.Handler
+
+Typically, the returned handler is a closure which does something with the http.ResponseWriter and http.Request passed to it, and then calls the handler passed as parameter to the MiddlewareFunc (closures can access variables from the context where they are created).
+
+A very basic middleware which logs the URI of the request being handled could be written as:
+
+	func simpleMw(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Do stuff here
+			log.Println(r.RequestURI)
+			// Call the next handler, which can be another middleware in the chain, or the final handler.
+			next.ServeHTTP(w, r)
+		})
+	}
+
+Middlewares can be added to a router using `Router.Use()`:
+
+	r := mux.NewRouter()
+	r.HandleFunc("/", handler)
+	r.AddMiddleware(simpleMw)
+
+A more complex authentication middleware, which maps session token to users, could be written as:
+
+	// Define our struct
+	type authenticationMiddleware struct {
+		tokenUsers map[string]string
+	}
+
+	// Initialize it somewhere
+	func (amw *authenticationMiddleware) Populate() {
+		amw.tokenUsers["00000000"] = "user0"
+		amw.tokenUsers["aaaaaaaa"] = "userA"
+		amw.tokenUsers["05f717e5"] = "randomUser"
+		amw.tokenUsers["deadbeef"] = "user0"
+	}
+
+	// Middleware function, which will be called for each request
+	func (amw *authenticationMiddleware) Middleware(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			token := r.Header.Get("X-Session-Token")
+
+			if user, found := amw.tokenUsers[token]; found {
+				// We found the token in our map
+				log.Printf("Authenticated user %s\n", user)
+				next.ServeHTTP(w, r)
+			} else {
+				http.Error(w, "Forbidden", 403)
+			}
+		})
+	}
+
+	r := mux.NewRouter()
+	r.HandleFunc("/", handler)
+
+	amw := authenticationMiddleware{}
+	amw.Populate()
+
+	r.Use(amw.Middleware)
+
+Note: The handler chain will be stopped if your middleware doesn't call `next.ServeHTTP()` with the corresponding parameters. This can be used to abort a request if the middleware writer wants to.
+
 */
 package mux

middleware.go

@@ -0,0 +1,28 @@
+package mux
+
+import "net/http"
+
+// MiddlewareFunc is a function which receives an http.Handler and returns another http.Handler.
+// Typically, the returned handler is a closure which does something with the http.ResponseWriter and http.Request passed
+// to it, and then calls the handler passed as parameter to the MiddlewareFunc.
+type MiddlewareFunc func(http.Handler) http.Handler
+
+// middleware interface is anything which implements a MiddlewareFunc named Middleware.
+type middleware interface {
+	Middleware(handler http.Handler) http.Handler
+}
+
+// MiddlewareFunc also implements the middleware interface.
+func (mw MiddlewareFunc) Middleware(handler http.Handler) http.Handler {
+	return mw(handler)
+}
+
+// Use appends a MiddlewareFunc to the chain. Middleware can be used to intercept or otherwise modify requests and/or responses, and are executed in the order that they are applied to the Router.
+func (r *Router) Use(mwf MiddlewareFunc) {
+	r.middlewares = append(r.middlewares, mwf)
+}
+
+// useInterface appends a middleware to the chain. Middleware can be used to intercept or otherwise modify requests and/or responses, and are executed in the order that they are applied to the Router.
+func (r *Router) useInterface(mw middleware) {
+	r.middlewares = append(r.middlewares, mw)
+}

middleware_test.go

@@ -0,0 +1,336 @@
+package mux
+
+import (
+	"bytes"
+	"net/http"
+	"testing"
+)
+
+type testMiddleware struct {
+	timesCalled uint
+}
+
+func (tm *testMiddleware) Middleware(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		tm.timesCalled++
+		h.ServeHTTP(w, r)
+	})
+}
+
+func dummyHandler(w http.ResponseWriter, r *http.Request) {}
+
+func TestMiddlewareAdd(t *testing.T) {
+	router := NewRouter()
+	router.HandleFunc("/", dummyHandler).Methods("GET")
+
+	mw := &testMiddleware{}
+
+	router.useInterface(mw)
+	if len(router.middlewares) != 1 || router.middlewares[0] != mw {
+		t.Fatal("Middleware was not added correctly")
+	}
+
+	router.Use(mw.Middleware)
+	if len(router.middlewares) != 2 {
+		t.Fatal("MiddlewareFunc method was not added correctly")
+	}
+
+	banalMw := func(handler http.Handler) http.Handler {
+		return handler
+	}
+	router.Use(banalMw)
+	if len(router.middlewares) != 3 {
+		t.Fatal("MiddlewareFunc method was not added correctly")
+	}
+}
+
+func TestMiddleware(t *testing.T) {
+	router := NewRouter()
+	router.HandleFunc("/", dummyHandler).Methods("GET")
+
+	mw := &testMiddleware{}
+	router.useInterface(mw)
+
+	rw := NewRecorder()
+	req := newRequest("GET", "/")
+
+	// Test regular middleware call
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 1 {
+		t.Fatalf("Expected %d calls, but got only %d", 1, mw.timesCalled)
+	}
+
+	// Middleware should not be called for 404
+	req = newRequest("GET", "/not/found")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 1 {
+		t.Fatalf("Expected %d calls, but got only %d", 1, mw.timesCalled)
+	}
+
+	// Middleware should not be called if there is a method mismatch
+	req = newRequest("POST", "/")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 1 {
+		t.Fatalf("Expected %d calls, but got only %d", 1, mw.timesCalled)
+	}
+
+	// Add the middleware again as function
+	router.Use(mw.Middleware)
+	req = newRequest("GET", "/")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 3 {
+		t.Fatalf("Expected %d calls, but got only %d", 3, mw.timesCalled)
+	}
+
+}
+
+func TestMiddlewareSubrouter(t *testing.T) {
+	router := NewRouter()
+	router.HandleFunc("/", dummyHandler).Methods("GET")
+
+	subrouter := router.PathPrefix("/sub").Subrouter()
+	subrouter.HandleFunc("/x", dummyHandler).Methods("GET")
+
+	mw := &testMiddleware{}
+	subrouter.useInterface(mw)
+
+	rw := NewRecorder()
+	req := newRequest("GET", "/")
+
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 0 {
+		t.Fatalf("Expected %d calls, but got only %d", 0, mw.timesCalled)
+	}
+
+	req = newRequest("GET", "/sub/")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 0 {
+		t.Fatalf("Expected %d calls, but got only %d", 0, mw.timesCalled)
+	}
+
+	req = newRequest("GET", "/sub/x")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 1 {
+		t.Fatalf("Expected %d calls, but got only %d", 1, mw.timesCalled)
+	}
+
+	req = newRequest("GET", "/sub/not/found")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 1 {
+		t.Fatalf("Expected %d calls, but got only %d", 1, mw.timesCalled)
+	}
+
+	router.useInterface(mw)
+
+	req = newRequest("GET", "/")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 2 {
+		t.Fatalf("Expected %d calls, but got only %d", 2, mw.timesCalled)
+	}
+
+	req = newRequest("GET", "/sub/x")
+	router.ServeHTTP(rw, req)
+	if mw.timesCalled != 4 {
+		t.Fatalf("Expected %d calls, but got only %d", 4, mw.timesCalled)
+	}
+}
+
+func TestMiddlewareExecution(t *testing.T) {
+	mwStr := []byte("Middleware\n")
+	handlerStr := []byte("Logic\n")
+
+	router := NewRouter()
+	router.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	})
+
+	rw := NewRecorder()
+	req := newRequest("GET", "/")
+
+	// Test handler-only call
+	router.ServeHTTP(rw, req)
+
+	if bytes.Compare(rw.Body.Bytes(), handlerStr) != 0 {
+		t.Fatal("Handler response is not what it should be")
+	}
+
+	// Test middleware call
+	rw = NewRecorder()
+
+	router.Use(func(h http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write(mwStr)
+			h.ServeHTTP(w, r)
+		})
+	})
+
+	router.ServeHTTP(rw, req)
+	if bytes.Compare(rw.Body.Bytes(), append(mwStr, handlerStr...)) != 0 {
+		t.Fatal("Middleware + handler response is not what it should be")
+	}
+}
+
+func TestMiddlewareNotFound(t *testing.T) {
+	mwStr := []byte("Middleware\n")
+	handlerStr := []byte("Logic\n")
+
+	router := NewRouter()
+	router.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	})
+	router.Use(func(h http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write(mwStr)
+			h.ServeHTTP(w, r)
+		})
+	})
+
+	// Test not found call with default handler
+	rw := NewRecorder()
+	req := newRequest("GET", "/notfound")
+
+	router.ServeHTTP(rw, req)
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a 404")
+	}
+
+	// Test not found call with custom handler
+	rw = NewRecorder()
+	req = newRequest("GET", "/notfound")
+
+	router.NotFoundHandler = http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		rw.Write([]byte("Custom 404 handler"))
+	})
+	router.ServeHTTP(rw, req)
+
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a custom 404")
+	}
+}
+
+func TestMiddlewareMethodMismatch(t *testing.T) {
+	mwStr := []byte("Middleware\n")
+	handlerStr := []byte("Logic\n")
+
+	router := NewRouter()
+	router.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	}).Methods("GET")
+
+	router.Use(func(h http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write(mwStr)
+			h.ServeHTTP(w, r)
+		})
+	})
+
+	// Test method mismatch
+	rw := NewRecorder()
+	req := newRequest("POST", "/")
+
+	router.ServeHTTP(rw, req)
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a method mismatch")
+	}
+
+	// Test not found call
+	rw = NewRecorder()
+	req = newRequest("POST", "/")
+
+	router.MethodNotAllowedHandler = http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		rw.Write([]byte("Method not allowed"))
+	})
+	router.ServeHTTP(rw, req)
+
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a method mismatch")
+	}
+}
+
+func TestMiddlewareNotFoundSubrouter(t *testing.T) {
+	mwStr := []byte("Middleware\n")
+	handlerStr := []byte("Logic\n")
+
+	router := NewRouter()
+	router.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	})
+
+	subrouter := router.PathPrefix("/sub/").Subrouter()
+	subrouter.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	})
+
+	router.Use(func(h http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write(mwStr)
+			h.ServeHTTP(w, r)
+		})
+	})
+
+	// Test not found call for default handler
+	rw := NewRecorder()
+	req := newRequest("GET", "/sub/notfound")
+
+	router.ServeHTTP(rw, req)
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a 404")
+	}
+
+	// Test not found call with custom handler
+	rw = NewRecorder()
+	req = newRequest("GET", "/sub/notfound")
+
+	subrouter.NotFoundHandler = http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		rw.Write([]byte("Custom 404 handler"))
+	})
+	router.ServeHTTP(rw, req)
+
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a custom 404")
+	}
+}
+
+func TestMiddlewareMethodMismatchSubrouter(t *testing.T) {
+	mwStr := []byte("Middleware\n")
+	handlerStr := []byte("Logic\n")
+
+	router := NewRouter()
+	router.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	})
+
+	subrouter := router.PathPrefix("/sub/").Subrouter()
+	subrouter.HandleFunc("/", func(w http.ResponseWriter, e *http.Request) {
+		w.Write(handlerStr)
+	}).Methods("GET")
+
+	router.Use(func(h http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write(mwStr)
+			h.ServeHTTP(w, r)
+		})
+	})
+
+	// Test method mismatch without custom handler
+	rw := NewRecorder()
+	req := newRequest("POST", "/sub/")
+
+	router.ServeHTTP(rw, req)
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a method mismatch")
+	}
+
+	// Test method mismatch with custom handler
+	rw = NewRecorder()
+	req = newRequest("POST", "/sub/")
+
+	router.MethodNotAllowedHandler = http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		rw.Write([]byte("Method not allowed"))
+	})
+	router.ServeHTTP(rw, req)
+
+	if bytes.Contains(rw.Body.Bytes(), mwStr) {
+		t.Fatal("Middleware was called for a method mismatch")
+	}
+}

mux.go

@@ -63,6 +63,8 @@ type Router struct {
 	KeepContext bool
 	// see Router.UseEncodedPath(). This defines a flag for all routes.
 	useEncodedPath bool
+	// Slice of middlewares to be called after a match is found
+	middlewares []middleware
 }
 
 // Match attempts to match the given request against the router's registered routes.
@@ -79,6 +81,12 @@ type Router struct {
 func (r *Router) Match(req *http.Request, match *RouteMatch) bool {
 	for _, route := range r.routes {
 		if route.Match(req, match) {
+			// Build middleware chain if no error was found
+			if match.MatchErr == nil {
+				for i := len(r.middlewares) - 1; i >= 0; i-- {
+					match.Handler = r.middlewares[i].Middleware(match.Handler)
+				}
+			}
 			return true
 		}
 	}
@@ -147,6 +155,7 @@ func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	if !r.KeepContext {
 		defer contextClear(req)
 	}
+
 	handler.ServeHTTP(w, req)
 }