database/sql: record the context error in Rows if canceled

Previously it was intended that Rows.Scan would return
an error and Rows.Err would return nil. This was problematic
because drivers could not differentiate between a normal
Rows.Close or a context cancel close.

The alternative is to require drivers to return a Scan to return
an error if the driver is closed while there are still rows to be read.
This is currently not how several drivers currently work and may be
difficult to detect when there are additional rows.

At the same time guard the the Rows.lasterr and prevent a close
while a Rows operation is active.

For the drivers that do not have Context methods, do not check for
context cancelation after the operation, but before for any operation
that may modify the database state.

Fixes #18961

Change-Id: I49a25318ecd9f97a35d5b50540ecd850c01cfa5e
Reviewed-on: https://go-review.googlesource.com/36485Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/ctxutil.go

@@ -35,15 +35,12 @@ func ctxDriverExec(ctx context.Context, execer driver.Execer, query string, nvda
 		return nil, err
 	}
 
-	resi, err := execer.Exec(query, dargs)
-	if err == nil {
-		select {
-		default:
-		case <-ctx.Done():
-			return resi, ctx.Err()
-		}
+	select {
+	default:
+	case <-ctx.Done():
+		return nil, ctx.Err()
 	}
-	return resi, err
+	return execer.Exec(query, dargs)
 }
 
 func ctxDriverQuery(ctx context.Context, queryer driver.Queryer, query string, nvdargs []driver.NamedValue) (driver.Rows, error) {
@@ -56,16 +53,12 @@ func ctxDriverQuery(ctx context.Context, queryer driver.Queryer, query string, n
 		return nil, err
 	}
 
-	rowsi, err := queryer.Query(query, dargs)
-	if err == nil {
-		select {
-		default:
-		case <-ctx.Done():
-			rowsi.Close()
-			return nil, ctx.Err()
-		}
+	select {
+	default:
+	case <-ctx.Done():
+		return nil, ctx.Err()
 	}
-	return rowsi, err
+	return queryer.Query(query, dargs)
 }
 
 func ctxDriverStmtExec(ctx context.Context, si driver.Stmt, nvdargs []driver.NamedValue) (driver.Result, error) {
@@ -77,15 +70,12 @@ func ctxDriverStmtExec(ctx context.Context, si driver.Stmt, nvdargs []driver.Nam
 		return nil, err
 	}
 
-	resi, err := si.Exec(dargs)
-	if err == nil {
-		select {
-		default:
-		case <-ctx.Done():
-			return resi, ctx.Err()
-		}
+	select {
+	default:
+	case <-ctx.Done():
+		return nil, ctx.Err()
 	}
-	return resi, err
+	return si.Exec(dargs)
 }
 
 func ctxDriverStmtQuery(ctx context.Context, si driver.Stmt, nvdargs []driver.NamedValue) (driver.Rows, error) {
@@ -97,16 +87,12 @@ func ctxDriverStmtQuery(ctx context.Context, si driver.Stmt, nvdargs []driver.Na
 		return nil, err
 	}
 
-	rowsi, err := si.Query(dargs)
-	if err == nil {
-		select {
-		default:
-		case <-ctx.Done():
-			rowsi.Close()
-			return nil, ctx.Err()
-		}
+	select {
+	default:
+	case <-ctx.Done():
+		return nil, ctx.Err()
 	}
-	return rowsi, err
+	return si.Query(dargs)
 }
 
 var errLevelNotSupported = errors.New("sql: selected isolation level is not supported")

src/database/sql/sql.go

@@ -2071,14 +2071,21 @@ type Rows struct {
 	dc          *driverConn // owned; must call releaseConn when closed to release
 	releaseConn func(error)
 	rowsi       driver.Rows
+	cancel      func()      // called when Rows is closed, may be nil.
+	closeStmt   *driverStmt // if non-nil, statement to Close on close
 
-	// closed value is 1 when the Rows is closed.
-	// Use atomic operations on value when checking value.
-	closed    int32
-	cancel    func() // called when Rows is closed, may be nil.
-	lastcols  []driver.Value
-	lasterr   error       // non-nil only if closed is true
-	closeStmt *driverStmt // if non-nil, statement to Close on close
+	// 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.
+	//
+	// closemu guards lasterr and closed.
+	closemu sync.RWMutex
+	closed  bool
+	lasterr error // non-nil only if closed is true
+
+	// lastcols is only used in Scan, Next, and NextResultSet which are expected
+	// not not be called concurrently.
+	lastcols []driver.Value
 }
 
 func (rs *Rows) initContextClose(ctx context.Context) {
@@ -2089,7 +2096,7 @@ func (rs *Rows) initContextClose(ctx context.Context) {
 // awaitDone blocks until the rows are closed or the context canceled.
 func (rs *Rows) awaitDone(ctx context.Context) {
 	<-ctx.Done()
-	rs.Close()
+	rs.close(ctx.Err())
 }
 
 // Next prepares the next result row for reading with the Scan method. It
@@ -2099,8 +2106,19 @@ func (rs *Rows) awaitDone(ctx context.Context) {
 //
 // Every call to Scan, even the first one, must be preceded by a call to Next.
 func (rs *Rows) Next() bool {
-	if rs.isClosed() {
-		return false
+	var doClose, ok bool
+	withLock(rs.closemu.RLocker(), func() {
+		doClose, ok = rs.nextLocked()
+	})
+	if doClose {
+		rs.Close()
+	}
+	return ok
+}
+
+func (rs *Rows) nextLocked() (doClose, ok bool) {
+	if rs.closed {
+		return false, false
 	}
 	if rs.lastcols == nil {
 		rs.lastcols = make([]driver.Value, len(rs.rowsi.Columns()))
@@ -2109,23 +2127,21 @@ func (rs *Rows) Next() bool {
 	if rs.lasterr != nil {
 		// Close the connection if there is a driver error.
 		if rs.lasterr != io.EOF {
-			rs.Close()
-			return false
+			return true, false
 		}
 		nextResultSet, ok := rs.rowsi.(driver.RowsNextResultSet)
 		if !ok {
-			rs.Close()
-			return false
+			return true, false
 		}
 		// The driver is at the end of the current result set.
 		// Test to see if there is another result set after the current one.
 		// Only close Rows if there is no further result sets to read.
 		if !nextResultSet.HasNextResultSet() {
-			rs.Close()
+			doClose = true
 		}
-		return false
+		return doClose, false
 	}
-	return true
+	return false, true
 }
 
 // NextResultSet prepares the next result set for reading. It returns true if
@@ -2137,18 +2153,28 @@ func (rs *Rows) Next() bool {
 // scanning. If there are further result sets they may not have rows in the result
 // set.
 func (rs *Rows) NextResultSet() bool {
-	if rs.isClosed() {
+	var doClose bool
+	defer func() {
+		if doClose {
+			rs.Close()
+		}
+	}()
+	rs.closemu.RLock()
+	defer rs.closemu.RUnlock()
+
+	if rs.closed {
 		return false
 	}
+
 	rs.lastcols = nil
 	nextResultSet, ok := rs.rowsi.(driver.RowsNextResultSet)
 	if !ok {
-		rs.Close()
+		doClose = true
 		return false
 	}
 	rs.lasterr = nextResultSet.NextResultSet()
 	if rs.lasterr != nil {
-		rs.Close()
+		doClose = true
 		return false
 	}
 	return true
@@ -2157,6 +2183,8 @@ 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 {
+	rs.closemu.RLock()
+	defer rs.closemu.RUnlock()
 	if rs.lasterr == io.EOF {
 		return nil
 	}
@@ -2167,7 +2195,9 @@ func (rs *Rows) Err() error {
 // Columns returns an error if the rows are closed, or if the rows
 // are from QueryRow and there was a deferred error.
 func (rs *Rows) Columns() ([]string, error) {
-	if rs.isClosed() {
+	rs.closemu.RLock()
+	defer rs.closemu.RUnlock()
+	if rs.closed {
 		return nil, errors.New("sql: Rows are closed")
 	}
 	if rs.rowsi == nil {
@@ -2179,7 +2209,9 @@ func (rs *Rows) Columns() ([]string, error) {
 // ColumnTypes returns column information such as column type, length,
 // and nullable. Some information may not be available from some drivers.
 func (rs *Rows) ColumnTypes() ([]*ColumnType, error) {
-	if rs.isClosed() {
+	rs.closemu.RLock()
+	defer rs.closemu.RUnlock()
+	if rs.closed {
 		return nil, errors.New("sql: Rows are closed")
 	}
 	if rs.rowsi == nil {
@@ -2329,9 +2361,13 @@ func rowsColumnInfoSetup(rowsi driver.Rows) []*ColumnType {
 // For scanning into *bool, the source may be true, false, 1, 0, or
 // string inputs parseable by strconv.ParseBool.
 func (rs *Rows) Scan(dest ...interface{}) error {
-	if rs.isClosed() {
+	rs.closemu.RLock()
+	if rs.closed {
+		rs.closemu.RUnlock()
 		return errors.New("sql: Rows are closed")
 	}
+	rs.closemu.RUnlock()
+
 	if rs.lastcols == nil {
 		return errors.New("sql: Scan called without calling Next")
 	}
@@ -2351,20 +2387,28 @@ func (rs *Rows) Scan(dest ...interface{}) error {
 // hook through a test only mutex.
 var rowsCloseHook = func() func(*Rows, *error) { return nil }
 
-func (rs *Rows) isClosed() bool {
-	return atomic.LoadInt32(&rs.closed) != 0
-}
-
 // Close closes the Rows, preventing further enumeration. If Next is called
 // and returns false and there are no further result sets,
 // 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 !atomic.CompareAndSwapInt32(&rs.closed, 0, 1) {
+	return rs.close(nil)
+}
+
+func (rs *Rows) close(err error) error {
+	rs.closemu.Lock()
+	defer rs.closemu.Unlock()
+
+	if rs.closed {
 		return nil
 	}
+	rs.closed = true
+
+	if rs.lasterr == nil {
+		rs.lasterr = err
+	}
 
-	err := rs.rowsi.Close()
+	err = rs.rowsi.Close()
 	if fn := rowsCloseHook(); fn != nil {
 		fn(rs, &err)
 	}

src/database/sql/sql_test.go

@@ -313,9 +313,13 @@ func TestQueryContext(t *testing.T) {
 		got = append(got, r)
 		index++
 	}
-	err = rows.Err()
-	if err != nil {
-		t.Fatalf("Err: %v", err)
+	select {
+	case <-ctx.Done():
+		if err := ctx.Err(); err != context.Canceled {
+			t.Fatalf("context err = %v; want context.Canceled")
+		}
+	default:
+		t.Fatalf("context err = nil; want context.Canceled")
 	}
 	want := []row{
 		{age: 1, name: "Alice"},