Generic singleflight

.github/workflows/go.yml

@@ -14,7 +14,8 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v2
       with:
-        go-version: 1.16
+        stable: 'false'
+        go-version: '1.18.0-beta1'
 
     - name: Checkout
       uses: actions/checkout@v1
@@ -31,11 +32,12 @@ jobs:
           ${{ runner.OS }}-build-
           ${{ runner.OS }}-
 
-    - name: Lint
-      uses: golangci/golangci-lint-action@v2
-      with:
-        version: v1.40.1
-        args: --timeout 10m
+    # todo: enable when go 1.18 is supported
+    # - name: Lint
+    #   uses: golangci/golangci-lint-action@v2
+    #   with:
+    #     version: v1.43.0
+    #     args: --timeout 10m
 
     - name: Vet
       if: matrix.os == 'ubuntu-latest'

go.mod

@@ -1,3 +1,3 @@
 module resenje.org/singleflight
 
-go 1.13
+go 1.18

singleflight.go

@@ -15,8 +15,8 @@ import (
 
 // Group represents a class of work and forms a namespace in
 // which units of work can be executed with duplicate suppression.
-type Group struct {
-	calls map[string]*call // lazily initialized
+type Group[K comparable, V any] struct {
+	calls map[K]*call[V] // lazily initialized
 	mu    sync.Mutex       // protects calls
 }
 
@@ -31,10 +31,10 @@ type Group struct {
 // effect the execution and returned values of others.
 //
 // The return value shared indicates whether v was given to multiple callers.
-func (g *Group) Do(ctx context.Context, key string, fn func(ctx context.Context) (interface{}, error)) (v interface{}, shared bool, err error) {
+func (g *Group[K, V]) Do(ctx context.Context, key K, fn func(ctx context.Context) (V, error)) (v V, shared bool, err error) {
 	g.mu.Lock()
 	if g.calls == nil {
-		g.calls = make(map[string]*call)
+		g.calls = make(map[K]*call[V])
 	}
 
 	if c, ok := g.calls[key]; ok {
@@ -47,7 +47,7 @@ func (g *Group) Do(ctx context.Context, key string, fn func(ctx context.Context)
 
 	callCtx, cancel := context.WithCancel(context.Background())
 
-	c := &call{
+	c := &call[V]{
 		done:    make(chan struct{}),
 		cancel:  cancel,
 		counter: 1,
@@ -64,7 +64,7 @@ func (g *Group) Do(ctx context.Context, key string, fn func(ctx context.Context)
 }
 
 // wait for function passed to Do to finish or context to be done.
-func (g *Group) wait(ctx context.Context, key string, c *call) (v interface{}, shared bool, err error) {
+func (g *Group[K, V]) wait(ctx context.Context, key K, c *call[V]) (v V, shared bool, err error) {
 	select {
 	case <-c.done:
 		v = c.val
@@ -87,7 +87,7 @@ func (g *Group) wait(ctx context.Context, key string, c *call) (v interface{}, s
 // Forget tells the singleflight to forget about a key. Future calls
 // to Do for this key will call the function rather than waiting for
 // an earlier call to complete.
-func (g *Group) Forget(key string) {
+func (g *Group[K, V]) Forget(key K) {
 	g.mu.Lock()
 	if c, ok := g.calls[key]; ok {
 		c.forgotten = true
@@ -97,9 +97,9 @@ func (g *Group) Forget(key string) {
 }
 
 // call stores information about as single function call passed to Do function.
-type call struct {
+type call[V any] struct {
 	// val and err hold the state about results of the function call.
-	val interface{}
+	val V
 	err error
 
 	// done channel signals that the function call is done.

singleflight_test.go

@@ -22,10 +22,10 @@ import (
 )
 
 func TestDo(t *testing.T) {
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 
 	want := "val"
-	got, shared, err := g.Do(context.Background(), "key", func(_ context.Context) (interface{}, error) {
+	got, shared, err := g.Do(context.Background(), "key", func(_ context.Context) (string, error) {
 		return want, nil
 	})
 	if err != nil {
@@ -40,21 +40,21 @@ func TestDo(t *testing.T) {
 }
 
 func TestDo_error(t *testing.T) {
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 	wantErr := errors.New("test error")
-	got, _, err := g.Do(context.Background(), "key", func(_ context.Context) (interface{}, error) {
-		return nil, wantErr
+	got, _, err := g.Do(context.Background(), "key", func(_ context.Context) (string, error) {
+		return "", wantErr
 	})
 	if err != wantErr {
 		t.Errorf("got error %v, want %v", err, wantErr)
 	}
-	if got != nil {
+	if got != "" {
 		t.Errorf("unexpected value %#v", got)
 	}
 }
 
 func TestDo_multipleCalls(t *testing.T) {
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 
 	want := "val"
 	var counter int32
@@ -68,7 +68,7 @@ func TestDo_multipleCalls(t *testing.T) {
 	for i := 0; i < n; i++ {
 		go func(i int) {
 			defer wg.Done()
-			got[i], shared[i], err[i] = g.Do(context.Background(), "key", func(_ context.Context) (interface{}, error) {
+			got[i], shared[i], err[i] = g.Do(context.Background(), "key", func(_ context.Context) (string, error) {
 				atomic.AddInt32(&counter, 1)
 				time.Sleep(100 * time.Millisecond)
 				return want, nil
@@ -95,11 +95,11 @@ func TestDo_multipleCalls(t *testing.T) {
 }
 
 func TestDo_callRemoval(t *testing.T) {
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 
 	wantPrefix := "val"
 	counter := 0
-	fn := func(_ context.Context) (interface{}, error) {
+	fn := func(_ context.Context) (string, error) {
 		counter++
 		return wantPrefix + strconv.Itoa(counter), nil
 	}
@@ -131,7 +131,7 @@ func TestDo_cancelContext(t *testing.T) {
 	done := make(chan struct{})
 	defer close(done)
 
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 
 	want := "val"
 	ctx, cancel := context.WithCancel(context.Background())
@@ -140,7 +140,7 @@ func TestDo_cancelContext(t *testing.T) {
 		cancel()
 	}()
 	start := time.Now()
-	got, shared, err := g.Do(ctx, "key", func(_ context.Context) (interface{}, error) {
+	got, shared, err := g.Do(ctx, "key", func(_ context.Context) (string, error) {
 		select {
 		case <-time.After(time.Second):
 		case <-done:
@@ -156,7 +156,7 @@ func TestDo_cancelContext(t *testing.T) {
 	if shared {
 		t.Error("the value should not be shared")
 	}
-	if got != nil {
+	if got != "" {
 		t.Errorf("unexpected value %#v", got)
 	}
 }
@@ -165,10 +165,10 @@ func TestDo_cancelContextSecond(t *testing.T) {
 	done := make(chan struct{})
 	defer close(done)
 
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 
 	want := "val"
-	fn := func(_ context.Context) (interface{}, error) {
+	fn := func(_ context.Context) (string, error) {
 		select {
 		case <-time.After(time.Second):
 		case <-done:
@@ -196,7 +196,7 @@ func TestDo_cancelContextSecond(t *testing.T) {
 	if !shared {
 		t.Error("the value should be shared")
 	}
-	if got != nil {
+	if got != "" {
 		t.Errorf("unexpected value %#v", got)
 	}
 }
@@ -205,12 +205,12 @@ func TestForget(t *testing.T) {
 	done := make(chan struct{})
 	defer close(done)
 
-	var g singleflight.Group
+	var g singleflight.Group[string, string]
 
 	wantPrefix := "val"
 	var counter uint64
 	firstCall := make(chan struct{})
-	fn := func(_ context.Context) (interface{}, error) {
+	fn := func(_ context.Context) (string, error) {
 		c := atomic.AddUint64(&counter, 1)
 		if c == 1 {
 			close(firstCall)
@@ -252,7 +252,7 @@ func TestDo_multipleCallsCanceled(t *testing.T) {
 			done := make(chan struct{})
 			defer close(done)
 
-			var g singleflight.Group
+			var g singleflight.Group[string, string]
 
 			var counter int32
 
@@ -271,7 +271,7 @@ func TestDo_multipleCallsCanceled(t *testing.T) {
 					contexts[i] = ctx
 					cancelFuncs[i] = cancel
 					mu.Unlock()
-					_, _, _ = g.Do(ctx, "key", func(ctx context.Context) (interface{}, error) {
+					_, _, _ = g.Do(ctx, "key", func(ctx context.Context) (string, error) {
 						atomic.AddInt32(&counter, 1)
 						close(fnCalled)
 						var err error
@@ -288,7 +288,7 @@ func TestDo_multipleCallsCanceled(t *testing.T) {
 
 						fnErrChan <- err
 
-						return nil, nil
+						return "", nil
 					})
 				}(i)
 			}