fix vikstrous#14 fix panic when wrong number of errors returned

Author: vikstrous

dataloadgen.go

@@ -132,16 +132,21 @@ func (l *Loader[KeyT, ValueT]) LoadThunk(ctx context.Context, key KeyT) func() (
 
 		// If the batch function returned the wrong number of responses, return an error to all callers
 		if len(batch.results) != len(batch.keys) {
-			return data, fmt.Errorf("bug in loader: %d values returned for %d keys", len(batch.results), len(batch.keys))
+			return data, fmt.Errorf("bug in fetch function: %d values returned for %d keys", len(batch.results), len(batch.keys))
 		}
 
 		if pos < len(batch.results) {
 			data = batch.results[pos]
 		}
 
 		var err error
-		if batch.errors != nil {
-			err = batch.errors[pos]
+		if len(batch.errors) != 0 {
+			if pos < len(batch.errors) {
+				err = batch.errors[pos]
+			} else {
+				err = fmt.Errorf("bug in fetch function: %d errors returned for %d keys; last error: %w", len(batch.errors), len(batch.keys), batch.errors[len(batch.errors)-1])
+			}
+
 		}
 
 		return data, err

dataloadgen_test.go

@@ -32,7 +32,7 @@ func ExampleLoader() {
 	// Output: 1
 }
 
-func TestEdgeCases(t *testing.T) {
+func TestCache(t *testing.T) {
 	ctx := context.Background()
 	var fetches [][]int
 	var mu sync.Mutex
@@ -57,33 +57,54 @@ func TestEdgeCases(t *testing.T) {
 		dataloadgen.WithWait(1*time.Millisecond),
 	)
 
-	t.Run("load function called only once when cached", func(t *testing.T) {
-		for i := 0; i < 2; i++ {
-			_, err := dl.Load(ctx, 0)
-			if err == nil {
-				t.Fatal("expected error")
-			}
-			if len(fetches) != 1 {
-				t.Fatal("wrong number of fetches", fetches)
-			}
-			if len(fetches[0]) != 1 {
-				t.Fatal("wrong number of keys in fetch request")
-			}
+	for i := 0; i < 2; i++ {
+		_, err := dl.Load(ctx, 0)
+		if err == nil {
+			t.Fatal("expected error")
 		}
-		for i := 0; i < 2; i++ {
-			r, err := dl.Load(ctx, 1)
-			if err != nil {
-				t.Fatal(err)
-			}
-			if len(fetches) != 2 {
-				t.Fatal("wrong number of fetches", fetches)
-			}
-			if len(fetches[1]) != 1 {
-				t.Fatal("wrong number of keys in fetch request")
-			}
-			if r != "1" {
-				t.Fatal("wrong data fetched", r)
-			}
+		if len(fetches) != 1 {
+			t.Fatal("wrong number of fetches", fetches)
+		}
+		if len(fetches[0]) != 1 {
+			t.Fatal("wrong number of keys in fetch request")
+		}
+	}
+	for i := 0; i < 2; i++ {
+		r, err := dl.Load(ctx, 1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(fetches) != 2 {
+			t.Fatal("wrong number of fetches", fetches)
+		}
+		if len(fetches[1]) != 1 {
+			t.Fatal("wrong number of keys in fetch request")
 		}
-	})
+		if r != "1" {
+			t.Fatal("wrong data fetched", r)
+		}
+	}
+}
+
+func TestErrors(t *testing.T) {
+	ctx := context.Background()
+	dl := dataloadgen.NewLoader(func(_ context.Context, keys []int) ([]string, []error) {
+		return []string{"1", "2", "3"}, []error{fmt.Errorf("error 1"), fmt.Errorf("error 2")}
+	},
+		dataloadgen.WithBatchCapacity(3),
+	)
+	_, err := dl.LoadAll(ctx, []int{1, 2, 3})
+	errs := err.(dataloadgen.ErrorSlice)
+	if len(errs) != 3 {
+		t.Fatalf("wrong number of errors: %d", len(errs))
+	}
+	if errs[0].Error() != "error 1" {
+		t.Fatalf("wrong error: %s", errs[0].Error())
+	}
+	if errs[1].Error() != "error 2" {
+		t.Fatalf("wrong error: %s", errs[1].Error())
+	}
+	if errs[2].Error() != "bug in fetch function: 2 errors returned for 3 keys; last error: error 2" {
+		t.Fatalf("wrong error: %s", errs[2].Error())
+	}
 }