Change LoadMany() to return nil []error with no errors (#32)

Author: tonyghita

dataloader.go

@@ -270,11 +270,13 @@ func (l *Loader) Load(originalContext context.Context, key interface{}) Thunk {
 func (l *Loader) LoadMany(originalContext context.Context, keys []interface{}) ThunkMany {
 	ctx, finish := l.tracer.TraceLoadMany(originalContext, keys)
 
-	length := len(keys)
-	data := make([]interface{}, length)
-	errors := make([]error, length)
-	c := make(chan *ResultMany, 1)
-	wg := sync.WaitGroup{}
+	var (
+		length = len(keys)
+		data   = make([]interface{}, length)
+		errors = make([]error, length)
+		c      = make(chan *ResultMany, 1)
+		wg     sync.WaitGroup
+	)
 
 	wg.Add(length)
 	for i := range keys {
@@ -289,7 +291,18 @@ func (l *Loader) LoadMany(originalContext context.Context, keys []interface{}) T
 
 	go func() {
 		wg.Wait()
-		c <- &ResultMany{data, errors}
+
+		// errs is nil unless there exists a non-nil error.
+		// This prevents dataloader from returning a slice of all-nil errors.
+		var errs []error
+		for _, e := range errors {
+			if e != nil {
+				errs = errors
+				break
+			}
+		}
+
+		c <- &ResultMany{Data: data, Error: errs}
 		close(c)
 	}()
 

dataloader_test.go

@@ -94,7 +94,6 @@ func TestLoader(t *testing.T) {
 		ctx := context.Background()
 		future := loader.LoadMany(ctx, []interface{}{"1", "2", "3"})
 		_, err := future()
-		log.Printf("errs: %#v", err)
 		if len(err) != 3 {
 			t.Errorf("LoadMany didn't return right number of errors (should match size of input)")
 		}
@@ -108,6 +107,16 @@ func TestLoader(t *testing.T) {
 		}
 	})
 
+	t.Run("test LoadMany returns nil []error when no errors occurred", func(t *testing.T) {
+		t.Parallel()
+		loader, _ := IDLoader(0)
+		ctx := context.Background()
+		_, err := loader.LoadMany(ctx, []interface{}{"1", "2", "3"})()
+		if err != nil {
+			t.Errorf("Expected LoadMany() to return nil error slice when no errors occurred")
+		}
+	})
+
 	t.Run("test thunkmany does not contain race conditions", func(t *testing.T) {
 		t.Parallel()
 		identityLoader, _ := IDLoader(0)
@@ -491,7 +500,7 @@ func OneErrorLoader(max int) (*Loader, *[][]interface{}) {
 	var mu sync.Mutex
 	var loadCalls [][]interface{}
 	identityLoader := NewBatchedLoader(func(_ context.Context, keys []interface{}) []*Result {
-		results := make([]*Result, max, max)
+		results := make([]*Result, max)
 		mu.Lock()
 		loadCalls = append(loadCalls, keys)
 		mu.Unlock()