database/sql: make RawBytes safely usable with contexts

sql.RawBytes was added the very first Go release, Go 1. Its docs
say:

> 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.

That "only valid until the next call" bit was true at the time,
until contexts were added to database/sql in Go 1.8.

In the past ~dozen releases it's been unsafe to use QueryContext with
a context that might become Done to get an *sql.Rows that's scanning
into a RawBytes. The Scan can succeed, but then while the caller's
reading the memory, a database/sql-managed goroutine can see the
context becoming done and call Close on the database/sql/driver and
make the caller's view of the RawBytes memory no longer valid,
introducing races, crashes, or database corruption. See golang#60304
and golang#53970 for details.

This change does the minimal surgery on database/sql to make it safe
again: Rows.Scan was already acquiring a mutex to check whether the
rows had been closed, so this change make Rows.Scan notice whether
*RawBytes was used and, if so, doesn't release the mutex on exit
before returning. That mean it's still locked while the user code
operates on the RawBytes memory and the concurrent context-watching
goroutine to close the database still runs, but if it fires, it then
gets blocked on the mutex until the next call to a Rows method (Next,
NextResultSet, Err, Close).

Updates golang#60304
Updates golang#53970 (earlier one I'd missed)

Change-Id: Ie41c0c6f32c24887b2f53ec3686c2aab73a1bfff
Reviewed-on: https://go-review.googlesource.com/c/go/+/497675
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/database/sql/fakedb_test.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 )
@@ -90,6 +91,8 @@ func (cc *fakeDriverCtx) OpenConnector(name string) (driver.Connector, error) {
 type fakeDB struct {
 	name string
 
+	useRawBytes atomic.Bool
+
 	mu       sync.Mutex
 	tables   map[string]*table
 	badConn  bool
@@ -697,6 +700,8 @@ func (c *fakeConn) PrepareContext(ctx context.Context, query string) (driver.Stm
 		switch cmd {
 		case "WIPE":
 			// Nothing
+		case "USE_RAWBYTES":
+			c.db.useRawBytes.Store(true)
 		case "SELECT":
 			stmt, err = c.prepareSelect(stmt, parts)
 		case "CREATE":
@@ -800,6 +805,9 @@ func (s *fakeStmt) ExecContext(ctx context.Context, args []driver.NamedValue) (d
 	case "WIPE":
 		db.wipe()
 		return driver.ResultNoRows, nil
+	case "USE_RAWBYTES":
+		s.c.db.useRawBytes.Store(true)
+		return driver.ResultNoRows, nil
 	case "CREATE":
 		if err := db.createTable(s.table, s.colName, s.colType); err != nil {
 			return nil, err
@@ -929,6 +937,7 @@ func (s *fakeStmt) QueryContext(ctx context.Context, args []driver.NamedValue) (
 				txStatus = "transaction"
 			}
 			cursor := &rowsCursor{
+				db:        s.c.db,
 				parentMem: s.c,
 				posRow:    -1,
 				rows: [][]*row{
@@ -1025,6 +1034,7 @@ func (s *fakeStmt) QueryContext(ctx context.Context, args []driver.NamedValue) (
 	}
 
 	cursor := &rowsCursor{
+		db:        s.c.db,
 		parentMem: s.c,
 		posRow:    -1,
 		rows:      setMRows,
@@ -1067,6 +1077,7 @@ func (tx *fakeTx) Rollback() error {
 }
 
 type rowsCursor struct {
+	db        *fakeDB
 	parentMem memToucher
 	cols      [][]string
 	colType   [][]string
@@ -1141,7 +1152,7 @@ func (rc *rowsCursor) Next(dest []driver.Value) error {
 		// messing up conversions or doing them differently.
 		dest[i] = v
 
-		if bs, ok := v.([]byte); ok {
+		if bs, ok := v.([]byte); ok && !rc.db.useRawBytes.Load() {
 			if rc.bytesClone == nil {
 				rc.bytesClone = make(map[*byte][]byte)
 			}

src/database/sql/sql.go

@@ -2893,6 +2893,8 @@ type Rows struct {
 	cancel      func()      // called when Rows is closed, may be nil.
 	closeStmt   *driverStmt // if non-nil, statement to Close on close
 
+	contextDone atomic.Pointer[error] // error that awaitDone saw; set before close attempt
+
 	// closemu prevents Rows from closing while there
 	// is an active streaming result. It is held for read during non-close operations
 	// and exclusively during close.
@@ -2905,6 +2907,15 @@ type Rows struct {
 	// lastcols is only used in Scan, Next, and NextResultSet which are expected
 	// not to be called concurrently.
 	lastcols []driver.Value
+
+	// closemuScanHold is whether the previous call to Scan kept closemu RLock'ed
+	// without unlocking it. It does that when the user passes a *RawBytes scan
+	// target. In that case, we need to prevent awaitDone from closing the Rows
+	// while the user's still using the memory. See go.dev/issue/60304.
+	//
+	// It is only used by Scan, Next, and NextResultSet which are expected
+	// not to be called concurrently.
+	closemuScanHold bool
 }
 
 // lasterrOrErrLocked returns either lasterr or the provided err.
@@ -2942,7 +2953,11 @@ func (rs *Rows) awaitDone(ctx, txctx context.Context) {
 	}
 	select {
 	case <-ctx.Done():
+		err := ctx.Err()
+		rs.contextDone.Store(&err)
 	case <-txctxDone:
+		err := txctx.Err()
+		rs.contextDone.Store(&err)
 	}
 	rs.close(ctx.Err())
 }
@@ -2954,6 +2969,15 @@ func (rs *Rows) awaitDone(ctx, txctx context.Context) {
 //
 // Every call to Scan, even the first one, must be preceded by a call to Next.
 func (rs *Rows) Next() bool {
+	// If the user's calling Next, they're done with their previous row's Scan
+	// results (any RawBytes memory), so we can release the read lock that would
+	// be preventing awaitDone from calling close.
+	rs.closemuRUnlockIfHeldByScan()
+
+	if rs.contextDone.Load() != nil {
+		return false
+	}
+
 	var doClose, ok bool
 	withLock(rs.closemu.RLocker(), func() {
 		doClose, ok = rs.nextLocked()
@@ -3008,6 +3032,11 @@ func (rs *Rows) nextLocked() (doClose, ok bool) {
 // scanning. If there are further result sets they may not have rows in the result
 // set.
 func (rs *Rows) NextResultSet() bool {
+	// If the user's calling NextResultSet, they're done with their previous
+	// row's Scan results (any RawBytes memory), so we can release the read lock
+	// that would be preventing awaitDone from calling close.
+	rs.closemuRUnlockIfHeldByScan()
+
 	var doClose bool
 	defer func() {
 		if doClose {
@@ -3044,6 +3073,10 @@ func (rs *Rows) NextResultSet() bool {
 // Err returns the error, if any, that was encountered during iteration.
 // Err may be called after an explicit or implicit Close.
 func (rs *Rows) Err() error {
+	if errp := rs.contextDone.Load(); errp != nil {
+		return *errp
+	}
+
 	rs.closemu.RLock()
 	defer rs.closemu.RUnlock()
 	return rs.lasterrOrErrLocked(nil)
@@ -3237,6 +3270,11 @@ func rowsColumnInfoSetupConnLocked(rowsi driver.Rows) []*ColumnType {
 // If any of the first arguments implementing Scanner returns an error,
 // that error will be wrapped in the returned error.
 func (rs *Rows) Scan(dest ...any) error {
+	if rs.closemuScanHold {
+		// This should only be possible if the user calls Scan twice in a row
+		// without calling Next.
+		return fmt.Errorf("sql: Scan called without calling Next (closemuScanHold)")
+	}
 	rs.closemu.RLock()
 
 	if rs.lasterr != nil && rs.lasterr != io.EOF {
@@ -3248,23 +3286,50 @@ func (rs *Rows) Scan(dest ...any) error {
 		rs.closemu.RUnlock()
 		return err
 	}
-	rs.closemu.RUnlock()
+
+	if scanArgsContainRawBytes(dest) {
+		rs.closemuScanHold = true
+	} else {
+		rs.closemu.RUnlock()
+	}
 
 	if rs.lastcols == nil {
+		rs.closemuRUnlockIfHeldByScan()
 		return errors.New("sql: Scan called without calling Next")
 	}
 	if len(dest) != len(rs.lastcols) {
+		rs.closemuRUnlockIfHeldByScan()
 		return fmt.Errorf("sql: expected %d destination arguments in Scan, not %d", len(rs.lastcols), len(dest))
 	}
+
 	for i, sv := range rs.lastcols {
 		err := convertAssignRows(dest[i], sv, rs)
 		if err != nil {
+			rs.closemuRUnlockIfHeldByScan()
 			return fmt.Errorf(`sql: Scan error on column index %d, name %q: %w`, i, rs.rowsi.Columns()[i], err)
 		}
 	}
 	return nil
 }
 
+// closemuRUnlockIfHeldByScan releases any closemu.RLock held open by a previous
+// call to Scan with *RawBytes.
+func (rs *Rows) closemuRUnlockIfHeldByScan() {
+	if rs.closemuScanHold {
+		rs.closemuScanHold = false
+		rs.closemu.RUnlock()
+	}
+}
+
+func scanArgsContainRawBytes(args []any) bool {
+	for _, a := range args {
+		if _, ok := a.(*RawBytes); ok {
+			return true
+		}
+	}
+	return false
+}
+
 // rowsCloseHook returns a function so tests may install the
 // hook through a test only mutex.
 var rowsCloseHook = func() func(*Rows, *error) { return nil }
@@ -3274,6 +3339,11 @@ var rowsCloseHook = func() func(*Rows, *error) { return nil }
 // the Rows are closed automatically and it will suffice to check the
 // result of Err. Close is idempotent and does not affect the result of Err.
 func (rs *Rows) Close() error {
+	// If the user's calling Close, they're done with their previous row's Scan
+	// results (any RawBytes memory), so we can release the read lock that would
+	// be preventing awaitDone from calling the unexported close before we do so.
+	rs.closemuRUnlockIfHeldByScan()
+
 	return rs.close(nil)
 }
 

src/database/sql/sql_test.go

@@ -4385,6 +4385,64 @@ func TestRowsScanProperlyWrapsErrors(t *testing.T) {
 	}
 }
 
+// From go.dev/issue/60304
+func TestContextCancelDuringRawBytesScan(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	if _, err := db.Exec("USE_RAWBYTES"); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	r, err := db.QueryContext(ctx, "SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	numRows := 0
+	var sink byte
+	for r.Next() {
+		numRows++
+		var s RawBytes
+		err = r.Scan(&s)
+		if !r.closemuScanHold {
+			t.Errorf("expected closemu to be held")
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Logf("read %q", s)
+		if numRows == 2 {
+			cancel() // invalidate the context, which used to call close asynchronously
+		}
+		for _, b := range s { // some operation reading from the raw memory
+			sink += b
+		}
+	}
+	if r.closemuScanHold {
+		t.Errorf("closemu held; should not be")
+	}
+
+	// There are 3 rows. We canceled after reading 2 so we expect either
+	// 2 or 3 depending on how the awaitDone goroutine schedules.
+	switch numRows {
+	case 0, 1:
+		t.Errorf("got %d rows; want 2+", numRows)
+	case 2:
+		if err := r.Err(); err != context.Canceled {
+			t.Errorf("unexpected error: %v (%T)", err, err)
+		}
+	default:
+		// Made it to the end. This is rare, but fine. Permit it.
+	}
+
+	if err := r.Close(); err != nil {
+		t.Fatal(err)
+	}
+}
+
 // badConn implements a bad driver.Conn, for TestBadDriver.
 // The Exec method panics.
 type badConn struct{}