sql: fix potential corruption in QueryRow.Scan into a *[]byte

Fixes #2622

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5533077

src/pkg/exp/sql/fakedb_test.go

@@ -306,6 +306,8 @@ func (c *fakeConn) prepareInsert(stmt *fakeStmt, parts []string) (driver.Stmt, e
 			switch ctype {
 			case "string":
 				subsetVal = []byte(value)
+			case "blob":
+				subsetVal = []byte(value)
 			case "int32":
 				i, err := strconv.Atoi(value)
 				if err != nil {
@@ -510,9 +512,19 @@ type rowsCursor struct {
 	pos    int
 	rows   []*row
 	closed bool
+
+	// a clone of slices to give out to clients, indexed by the
+	// the original slice's first byte address.  we clone them
+	// just so we're able to corrupt them on close.
+	bytesClone map[*byte][]byte
 }
 
 func (rc *rowsCursor) Close() error {
+	if !rc.closed {
+		for _, bs := range rc.bytesClone {
+			bs[0] = 255 // first byte corrupted
+		}
+	}
 	rc.closed = true
 	return nil
 }
@@ -537,6 +549,19 @@ func (rc *rowsCursor) Next(dest []interface{}) error {
 		// for ease of drivers, and to prevent drivers from
 		// messing up conversions or doing them differently.
 		dest[i] = v
+
+		if bs, ok := v.([]byte); ok {
+			if rc.bytesClone == nil {
+				rc.bytesClone = make(map[*byte][]byte)
+			}
+			clone, ok := rc.bytesClone[&bs[0]]
+			if !ok {
+				clone = make([]byte, len(bs))
+				copy(clone, bs)
+				rc.bytesClone[&bs[0]] = clone
+			}
+			dest[i] = clone
+		}
 	}
 	return nil
 }

src/pkg/exp/sql/sql.go

@@ -803,10 +803,6 @@ type Row struct {
 // pointed at by dest.  If more than one row matches the query,
 // Scan uses the first row and discards the rest.  If no row matches
 // the query, Scan returns ErrNoRows.
-//
-// If dest contains pointers to []byte, the slices should not be
-// modified and should only be considered valid until the next call to
-// Next or Scan.
 func (r *Row) Scan(dest ...interface{}) error {
 	if r.err != nil {
 		return r.err
@@ -815,7 +811,33 @@ func (r *Row) Scan(dest ...interface{}) error {
 	if !r.rows.Next() {
 		return ErrNoRows
 	}
-	return r.rows.Scan(dest...)
+	err := r.rows.Scan(dest...)
+	if err != nil {
+		return err
+	}
+
+	// TODO(bradfitz): for now we need to defensively clone all
+	// []byte that the driver returned, since we're about to close
+	// the Rows in our defer, when we return from this function.
+	// the contract with the driver.Next(...) interface is that it
+	// can return slices into read-only temporary memory that's
+	// only valid until the next Scan/Close.  But the TODO is that
+	// for a lot of drivers, this copy will be unnecessary.  We
+	// should provide an optional interface for drivers to
+	// implement to say, "don't worry, the []bytes that I return
+	// from Next will not be modified again." (for instance, if
+	// they were obtained from the network anyway) But for now we
+	// don't care.
+	for _, dp := range dest {
+		b, ok := dp.(*[]byte)
+		if !ok {
+			continue
+		}
+		clone := make([]byte, len(*b))
+		copy(clone, *b)
+		*b = clone
+	}
+	return nil
 }
 
 // A Result summarizes an executed SQL command.

src/pkg/exp/sql/sql_test.go

@@ -21,10 +21,10 @@ func newTestDB(t *testing.T, name string) *DB {
 		t.Fatalf("exec wipe: %v", err)
 	}
 	if name == "people" {
-		exec(t, db, "CREATE|people|name=string,age=int32,dead=bool")
-		exec(t, db, "INSERT|people|name=Alice,age=?", 1)
-		exec(t, db, "INSERT|people|name=Bob,age=?", 2)
-		exec(t, db, "INSERT|people|name=Chris,age=?", 3)
+		exec(t, db, "CREATE|people|name=string,age=int32,photo=blob,dead=bool")
+		exec(t, db, "INSERT|people|name=Alice,age=?,photo=APHOTO", 1)
+		exec(t, db, "INSERT|people|name=Bob,age=?,photo=BPHOTO", 2)
+		exec(t, db, "INSERT|people|name=Chris,age=?,photo=CPHOTO", 3)
 	}
 	return db
 }
@@ -132,6 +132,16 @@ func TestQueryRow(t *testing.T) {
 	if age != 1 {
 		t.Errorf("expected age 1, got %d", age)
 	}
+
+	var photo []byte
+	err = db.QueryRow("SELECT|people|photo|name=?", "Alice").Scan(&photo)
+	if err != nil {
+		t.Fatalf("photo QueryRow+Scan: %v", err)
+	}
+	want := []byte("APHOTO")
+	if !reflect.DeepEqual(photo, want) {
+		t.Errorf("photo = %q; want %q", photo, want)
+	}
 }
 
 func TestStatementErrorAfterClose(t *testing.T) {