data race: avoid unprotected access to sb.file

79575d8 (included in klog v1.130.0) added a small data race with regards to
accessing the syncBuffer.file field when calling Sync.

We can eliminate the entire redirectBuffer interface that led to this mistake
and instead work directly with syncBuffer for flushing and syncing.

To avoid allocating a slice inside flushAll, fixed-sized struct is
returned. However, it still ends up being allocated according to `go test
-benchmem`.

klog.go

@@ -404,13 +404,6 @@ func (t *traceLocation) Set(value string) error {
 	return nil
 }
 
-// flushSyncWriter is the interface satisfied by logging destinations.
-type flushSyncWriter interface {
-	Flush() error
-	Sync() error
-	io.Writer
-}
-
 var logging loggingT
 var commandLine flag.FlagSet
 
@@ -486,7 +479,7 @@ type settings struct {
 	// Access to all of the following fields must be protected via a mutex.
 
 	// file holds writer for each of the log types.
-	file [severity.NumSeverity]flushSyncWriter
+	file [severity.NumSeverity]io.Writer
 	// flushInterval is the interval for periodic flushing. If zero,
 	// the global default will be used.
 	flushInterval time.Duration
@@ -831,32 +824,12 @@ func (l *loggingT) printS(err error, s severity.Severity, depth int, msg string,
 	buffer.PutBuffer(b)
 }
 
-// redirectBuffer is used to set an alternate destination for the logs
-type redirectBuffer struct {
-	w io.Writer
-}
-
-func (rb *redirectBuffer) Sync() error {
-	return nil
-}
-
-func (rb *redirectBuffer) Flush() error {
-	return nil
-}
-
-func (rb *redirectBuffer) Write(bytes []byte) (n int, err error) {
-	return rb.w.Write(bytes)
-}
-
 // SetOutput sets the output destination for all severities
 func SetOutput(w io.Writer) {
 	logging.mu.Lock()
 	defer logging.mu.Unlock()
 	for s := severity.FatalLog; s >= severity.InfoLog; s-- {
-		rb := &redirectBuffer{
-			w: w,
-		}
-		logging.file[s] = rb
+		logging.file[s] = w
 	}
 }
 
@@ -868,10 +841,7 @@ func SetOutputBySeverity(name string, w io.Writer) {
 	if !ok {
 		panic(fmt.Sprintf("SetOutputBySeverity(%q): unrecognized severity name", name))
 	}
-	rb := &redirectBuffer{
-		w: w,
-	}
-	logging.file[sev] = rb
+	logging.file[sev] = w
 }
 
 // LogToStderr sets whether to log exclusively to stderr, bypassing outputs
@@ -1011,8 +981,8 @@ func (l *loggingT) exit(err error) {
 		logExitFunc(err)
 		return
 	}
-	files := l.flushAll()
-	l.syncAll(files)
+	needToSync := l.flushAll()
+	l.syncAll(needToSync)
 	OsExit(2)
 }
 
@@ -1029,10 +999,6 @@ type syncBuffer struct {
 	maxbytes uint64 // The max number of bytes this syncBuffer.file can hold before cleaning up.
 }
 
-func (sb *syncBuffer) Sync() error {
-	return sb.file.Sync()
-}
-
 // CalculateMaxSize returns the real max size in bytes after considering the default max size and the flag options.
 func CalculateMaxSize() uint64 {
 	if logging.logFile != "" {
@@ -1224,37 +1190,44 @@ func StartFlushDaemon(interval time.Duration) {
 // lockAndFlushAll is like flushAll but locks l.mu first.
 func (l *loggingT) lockAndFlushAll() {
 	l.mu.Lock()
-	files := l.flushAll()
+	needToSync := l.flushAll()
 	l.mu.Unlock()
 	// Some environments are slow when syncing and holding the lock might cause contention.
-	l.syncAll(files)
+	l.syncAll(needToSync)
 }
 
 // flushAll flushes all the logs
 // l.mu is held.
-func (l *loggingT) flushAll() []flushSyncWriter {
-	files := make([]flushSyncWriter, 0, severity.NumSeverity)
+//
+// The result is the number of files which need to be synced and the pointers to them.
+func (l *loggingT) flushAll() fileArray {
+	var needToSync fileArray
+
 	// Flush from fatal down, in case there's trouble flushing.
 	for s := severity.FatalLog; s >= severity.InfoLog; s-- {
 		file := l.file[s]
-		if file != nil {
-			_ = file.Flush() // ignore error
+		if sb, ok := file.(*syncBuffer); ok && sb.file != nil {
+			_ = sb.Flush() // ignore error
+			needToSync.files[needToSync.num] = sb.file
+			needToSync.num++
 		}
-		files = append(files, file)
 	}
 	if logging.loggerOptions.flush != nil {
 		logging.loggerOptions.flush()
 	}
-	return files
+	return needToSync
+}
+
+type fileArray struct {
+	num   int
+	files [severity.NumSeverity]*os.File
 }
 
 // syncAll attempts to "sync" their data to disk.
-func (l *loggingT) syncAll(files []flushSyncWriter) {
+func (l *loggingT) syncAll(needToSync fileArray) {
 	// Flush from fatal down, in case there's trouble flushing.
-	for _, file := range files {
-		if file != nil {
-			_ = file.Sync() // ignore error
-		}
+	for i := 0; i < needToSync.num; i++ {
+		_ = needToSync.files[i].Sync() // ignore error
 	}
 }
 

klog_test.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	stdLog "log"
 	"os"
@@ -73,7 +74,7 @@ func (f *flushBuffer) Sync() error {
 }
 
 // swap sets the log writers and returns the old array.
-func (l *loggingT) swap(writers [severity.NumSeverity]flushSyncWriter) (old [severity.NumSeverity]flushSyncWriter) {
+func (l *loggingT) swap(writers [severity.NumSeverity]io.Writer) (old [severity.NumSeverity]io.Writer) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	old = l.file
@@ -82,8 +83,8 @@ func (l *loggingT) swap(writers [severity.NumSeverity]flushSyncWriter) (old [sev
 }
 
 // newBuffers sets the log writers to all new byte buffers and returns the old array.
-func (l *loggingT) newBuffers() [severity.NumSeverity]flushSyncWriter {
-	return l.swap([severity.NumSeverity]flushSyncWriter{new(flushBuffer), new(flushBuffer), new(flushBuffer), new(flushBuffer)})
+func (l *loggingT) newBuffers() [severity.NumSeverity]io.Writer {
+	return l.swap([severity.NumSeverity]io.Writer{new(flushBuffer), new(flushBuffer), new(flushBuffer), new(flushBuffer)})
 }
 
 // contents returns the specified log value as a string.
@@ -540,14 +541,17 @@ func TestOpenAppendOnStart(t *testing.T) {
 
 	// Logging creates the file
 	Info(x)
-	_, ok := logging.file[severity.InfoLog].(*syncBuffer)
+	sb, ok := logging.file[severity.InfoLog].(*syncBuffer)
 	if !ok {
 		t.Fatal("info wasn't created")
 	}
 
 	// ensure we wrote what we expected
-	files := logging.flushAll()
-	logging.syncAll(files)
+	needToSync := logging.flushAll()
+	if needToSync.num != 1 || needToSync.files[0] != sb.file {
+		t.Errorf("Should have received exactly the file from severity.InfoLog for syncing, got instead: %+v", needToSync)
+	}
+	logging.syncAll(needToSync)
 	b, err := ioutil.ReadFile(logging.logFile)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -811,15 +815,22 @@ func BenchmarkLogs(b *testing.B) {
 		Severity: severity.FatalLog,
 	}
 	logging.logFile = testFile.Name()
-	logging.swap([severity.NumSeverity]flushSyncWriter{nil, nil, nil, nil})
+	logging.swap([severity.NumSeverity]io.Writer{nil, nil, nil, nil})
 
 	for i := 0; i < b.N; i++ {
 		Error("error")
 		Warning("warning")
 		Info("info")
 	}
-	files := logging.flushAll()
-	logging.syncAll(files)
+	needToSync := logging.flushAll()
+	sb, ok := logging.file[severity.InfoLog].(*syncBuffer)
+	if !ok {
+		b.Fatal("info wasn't created")
+	}
+	if needToSync.num != 1 || needToSync.files[0] != sb.file {
+		b.Fatalf("Should have received exactly the file from severity.InfoLog for syncing, got instead: %+v", needToSync)
+	}
+	logging.syncAll(needToSync)
 }
 
 // Test the logic on checking log size limitation.