exp/sql: copy when scanning into []byte by default

Fixes #2698

R=rsc
CC=golang-dev
https://golang.org/cl/5539060

src/pkg/exp/sql/sql.go

@@ -30,6 +30,11 @@ func Register(name string, driver driver.Driver) {
 	drivers[name] = driver
 }
 
+// RawBytes is a byte slice that holds a reference to memory owned by
+// the database itself. After a Scan into a RawBytes, the slice is only
+// valid until the next call to Next, Scan, or Close.
+type RawBytes []byte
+
 // NullableString represents a string that may be null.
 // NullableString implements the ScannerInto interface so
 // it can be used as a scan destination:
@@ -760,9 +765,13 @@ func (rs *Rows) Columns() ([]string, error) {
 }
 
 // Scan copies the columns in the current row into the values pointed
-// at by dest. If dest contains pointers to []byte, the slices should
+// at by dest.
-// not be modified and should only be considered valid until the next
+//
-// call to Next or Scan.
+// If an argument has type *[]byte, Scan saves in that argument a copy
+// of the corresponding data. The copy is owned by the caller and can
+// be modified and held indefinitely. The copy can be avoided by using
+// an argument of type *RawBytes instead; see the documentation for
+// RawBytes for restrictions on its use.
 func (rs *Rows) Scan(dest ...interface{}) error {
 	if rs.closed {
 		return errors.New("sql: Rows closed")
@@ -782,6 +791,18 @@ func (rs *Rows) Scan(dest ...interface{}) error {
 			return fmt.Errorf("sql: Scan error on column index %d: %v", i, err)
 		}
 	}
+	for _, dp := range dest {
+		b, ok := dp.(*[]byte)
+		if !ok {
+			continue
+		}
+		if _, ok = dp.(*RawBytes); ok {
+			continue
+		}
+		clone := make([]byte, len(*b))
+		copy(clone, *b)
+		*b = clone
+	}
 	return nil
 }
 
@@ -838,6 +859,9 @@ func (r *Row) Scan(dest ...interface{}) error {
 	// they were obtained from the network anyway) But for now we
 	// don't care.
 	for _, dp := range dest {
+		if _, ok := dp.(*RawBytes); ok {
+			return errors.New("sql: RawBytes isn't allowed on Row.Scan")
+		}
 		b, ok := dp.(*[]byte)
 		if !ok {
 			continue

src/pkg/exp/sql/sql_test.go

@@ -76,7 +76,7 @@ func TestQuery(t *testing.T) {
 		{age: 3, name: "Chris"},
 	}
 	if !reflect.DeepEqual(got, want) {
-		t.Logf(" got: %#v\nwant: %#v", got, want)
+		t.Errorf("mismatch.\n got: %#v\nwant: %#v", got, want)
 	}
 
 	// And verify that the final rows.Next() call, which hit EOF,
@@ -86,6 +86,43 @@ func TestQuery(t *testing.T) {
 	}
 }
 
+func TestByteOwnership(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+	rows, err := db.Query("SELECT|people|name,photo|")
+	if err != nil {
+		t.Fatalf("Query: %v", err)
+	}
+	type row struct {
+		name  []byte
+		photo RawBytes
+	}
+	got := []row{}
+	for rows.Next() {
+		var r row
+		err = rows.Scan(&r.name, &r.photo)
+		if err != nil {
+			t.Fatalf("Scan: %v", err)
+		}
+		got = append(got, r)
+	}
+	corruptMemory := []byte("\xffPHOTO")
+	want := []row{
+		{name: []byte("Alice"), photo: corruptMemory},
+		{name: []byte("Bob"), photo: corruptMemory},
+		{name: []byte("Chris"), photo: corruptMemory},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("mismatch.\n got: %#v\nwant: %#v", got, want)
+	}
+
+	var photo RawBytes
+	err = db.QueryRow("SELECT|people|photo|name=?", "Alice").Scan(&photo)
+	if err == nil {
+		t.Error("want error scanning into RawBytes from QueryRow")
+	}
+}
+
 func TestRowsColumns(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)
@@ -300,6 +337,6 @@ func TestQueryRowClosingStmt(t *testing.T) {
 	}
 	fakeConn := db.freeConn[0].(*fakeConn)
 	if made, closed := fakeConn.stmtsMade, fakeConn.stmtsClosed; made != closed {
-		t.Logf("statement close mismatch: made %d, closed %d", made, closed)
+		t.Errorf("statement close mismatch: made %d, closed %d", made, closed)
 	}
 }