fix(types): isolate JSON values across reused scans

types/json.go

@@ -40,16 +40,27 @@ func (j JSON[T]) Value() (driver.Value, error) {
 
 // Scan implements the Scanner interface.
 func (j *JSON[T]) Scan(value any) error {
+	var data []byte
 	switch x := value.(type) {
 	case string:
-		return json.NewDecoder(bytes.NewBuffer([]byte(x))).Decode(j)
+		data = []byte(x)
 	case []byte:
-		return json.NewDecoder(bytes.NewBuffer(x)).Decode(j)
+		data = x
 	case nil:
+		var zero T
+		j.Val = zero
 		return nil
 	default:
 		return fmt.Errorf("cannot scan type %T: %v", value, value)
 	}
+	// Destinations are reused across rows. Decode separately to avoid mutating
+	// pointers and slices already copied into previously scanned values.
+	var decoded T
+	if err := json.NewDecoder(bytes.NewReader(data)).Decode(&decoded); err != nil {
+		return err
+	}
+	j.Val = decoded
+	return nil
 }
 
 // UnmarshalJSON implements json.Unmarshaler.

types/json_scan_reuse_test.go

@@ -0,0 +1,54 @@
+package types
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestJSONScanReusedDestination(t *testing.T) {
+	type nested struct {
+		Name string   `json:"name"`
+		Tags []string `json:"tags"`
+	}
+	type record struct {
+		Nested *nested `json:"nested"`
+	}
+	var destination JSON[record]
+	if err := destination.Scan([]byte(`{"nested":{"name":"First","tags":["a","b"]}}`)); err != nil {
+		t.Fatal(err)
+	}
+	previous := destination.Val
+	if err := destination.Scan(`{"nested":{"name":"Second","tags":["c"]}}`); err != nil {
+		t.Fatal(err)
+	}
+	want := record{Nested: &nested{Name: "First", Tags: []string{"a", "b"}}}
+	if diff := cmp.Diff(want, previous); diff != "" {
+		t.Fatalf("previous row changed (-want +got):\n%s", diff)
+	}
+	if err := destination.Scan([]byte(`{}`)); err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(record{}, destination.Val); diff != "" {
+		t.Fatalf("absent fields retained (-want +got):\n%s", diff)
+	}
+	if err := destination.Scan([]byte(`{"nested":{"name":"First"}}`)); err != nil {
+		t.Fatal(err)
+	}
+	previous = destination.Val
+	if err := destination.Scan([]byte(`{"nested":{"name":"Wrong","tags":123}}`)); err == nil {
+		t.Fatal("expected invalid JSON field error")
+	}
+	if diff := cmp.Diff(previous, destination.Val); diff != "" {
+		t.Fatalf("failed scan changed destination (-want +got):\n%s", diff)
+	}
+	if destination.Val.Nested.Name != "First" {
+		t.Fatal("failed scan mutated previous row")
+	}
+	if err := destination.Scan(nil); err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(record{}, destination.Val); diff != "" {
+		t.Fatalf("nil scan retained fields (-want +got):\n%s", diff)
+	}
+}