fix: deep copy strings during scan (#499)

When decoding a document from the disk, fields of type TEXT are not copied on purpose to avoid doing unnecessary copies.
However, when the data reaches the upper layers, usually when it's about to get scanned (i.e. document.Scan, document.StructScan), strings must always get deep cloned before being returned to the user.

db_test.go

@@ -227,6 +227,47 @@ func TestPrepareThreadSafe(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestIterateDeepCopy(t *testing.T) {
+	db, err := genji.Open(":memory:")
+	assert.NoError(t, err)
+	defer db.Close()
+
+	err = db.Exec(`
+	CREATE TABLE foo (
+		a integer primary key,
+		b text not null
+	);
+
+	INSERT INTO foo (a, b) VALUES
+		(1, 'sample text 1'),
+		(2, 'sample text 2');
+	`)
+	assert.NoError(t, err)
+
+	res, err := db.Query(`SELECT * FROM foo ORDER BY a DESC`)
+	assert.NoError(t, err)
+
+	type item struct {
+		A int
+		B string
+	}
+
+	var items []*item
+	err = res.Iterate(func(d types.Document) error {
+		var i item
+		err := document.StructScan(d, &i)
+		assert.NoError(t, err)
+
+		items = append(items, &i)
+		return nil
+	})
+	assert.NoError(t, err)
+
+	require.Equal(t, len(items), 2)
+	require.Equal(t, &item{A: 2, B: "sample text 2"}, items[0])
+	require.Equal(t, &item{A: 1, B: "sample text 1"}, items[1])
+}
+
 func BenchmarkSelect(b *testing.B) {
 	for size := 1; size <= 10000; size *= 10 {
 		b.Run(fmt.Sprintf("%.05d", size), func(b *testing.B) {

document/scan.go

@@ -275,7 +275,11 @@ func scanValue(v types.Value, ref reflect.Value) error {
 		if err != nil {
 			return err
 		}
-		ref.SetString(string(types.As[string](v)))
+		// copy the string to avoid
+		// keeping a reference to the underlying buffer
+		// which could be reused
+		cp := strings.Clone(types.As[string](v))
+		ref.SetString(cp)
 		return nil
 	case reflect.Bool:
 		v, err := CastAsBool(v)