Add Use method to better support middlewares with ServeMux

servemux.go

@@ -26,6 +26,7 @@ type ServeMux struct {
 	mu  sync.RWMutex
 	m   map[string]muxEntry
 	es  []muxEntry // slice of entries sorted from longest to shortest.
+	mws []MiddlewareFunc
 }
 
 type muxEntry struct {
@@ -33,6 +34,11 @@ type muxEntry struct {
 	pattern string
 }
 
+// MiddlewareFunc is a function which receives an asynq.Handler and returns another asynq.Handler.
+// Typically, the returned handler is a closure which does something with the context and task passed
+// to it, and then calls the handler passed as parameter to the MiddlewareFunc.
+type MiddlewareFunc func(Handler) Handler
+
 // NewServeMux allocates and returns a new ServeMux.
 func NewServeMux() *ServeMux {
 	return new(ServeMux)
@@ -60,6 +66,9 @@ func (mux *ServeMux) Handler(t *Task) (h Handler, pattern string) {
 	if h == nil {
 		h, pattern = NotFoundHandler(), ""
 	}
+	for i := len(mux.mws) - 1; i >= 0; i-- {
+		h = mux.mws[i](h)
+	}
 	return h, pattern
 }
 
@@ -130,6 +139,16 @@ func (mux *ServeMux) HandleFunc(pattern string, handler func(context.Context, *T
 	mux.Handle(pattern, HandlerFunc(handler))
 }
 
+// Use appends a MiddlewareFunc to the chain.
+// Middlewares are executed in the order that they are applied to the ServeMux.
+func (mux *ServeMux) Use(mws ...MiddlewareFunc) {
+	mux.mu.Lock()
+	defer mux.mu.Unlock()
+	for _, fn := range mws {
+		mux.mws = append(mux.mws, fn)
+	}
+}
+
 // NotFound returns an error indicating that the handler was not found for the given task.
 func NotFound(ctx context.Context, task *Task) error {
 	return fmt.Errorf("handler not found for task %q", task.Type)

servemux_test.go

@@ -7,9 +7,12 @@ package asynq
 import (
 	"context"
 	"testing"
+
+	"github.com/google/go-cmp/cmp"
 )
 
-var called string
+var called string    // identity of the handler that was called.
+var invoked []string // list of middlewares in the order they were invoked.
 
 // makeFakeHandler returns a handler that updates the global called variable
 // to the given identity.
@@ -20,6 +23,17 @@ func makeFakeHandler(identity string) Handler {
 	})
 }
 
+// makeFakeMiddleware returns a middleware function that appends the given identity
+//to the global invoked slice.
+func makeFakeMiddleware(identity string) MiddlewareFunc {
+	return func(next Handler) Handler {
+		return HandlerFunc(func(ctx context.Context, t *Task) error {
+			invoked = append(invoked, identity)
+			return next.ProcessTask(ctx, t)
+		})
+	}
+}
+
 // A list of pattern, handler pair that is registered with mux.
 var serveMuxRegister = []struct {
 	pattern string
@@ -114,3 +128,43 @@ func TestServeMuxNotFound(t *testing.T) {
 		}
 	}
 }
+
+var middlewareTests = []struct {
+	typename    string   // task's type name
+	middlewares []string // middlewares to use. They should be called in this order.
+	want        string   // identifier of the handler that should be called
+}{
+	{"email:signup", []string{"logging", "expiration"}, "signup email handler"},
+	{"csv:export", []string{}, "csv export handler"},
+	{"email:daily", []string{"expiration", "logging"}, "default email handler"},
+}
+
+func TestServeMuxMiddlewares(t *testing.T) {
+	for _, tc := range middlewareTests {
+		mux := NewServeMux()
+		for _, e := range serveMuxRegister {
+			mux.Handle(e.pattern, e.h)
+		}
+		var mws []MiddlewareFunc
+		for _, s := range tc.middlewares {
+			mws = append(mws, makeFakeMiddleware(s))
+		}
+		mux.Use(mws...)
+
+		invoked = []string{} // reset to empty slice
+		called = ""          // reset to zero value
+
+		task := NewTask(tc.typename, nil)
+		if err := mux.ProcessTask(context.Background(), task); err != nil {
+			t.Fatal(err)
+		}
+
+		if diff := cmp.Diff(invoked, tc.middlewares); diff != "" {
+			t.Errorf("invoked middlewares were %v, want %v", invoked, tc.middlewares)
+		}
+
+		if called != tc.want {
+			t.Errorf("%q handler was called for task %q, want %q to be called", called, task.Type, tc.want)
+		}
+	}
+}