cherrypick-1.1: util/log: Avoid infinite recursion out of disk errors cause an exit

pkg/util/log/clog.go

@@ -889,10 +889,20 @@ var logExitFunc func(error)
 // point in hanging around. l.mu is held.
 func (l *loggingT) exitLocked(err error) {
 	l.mu.AssertHeld()
+
 	// Either stderr or our log file is broken. Try writing the error to both
 	// streams in the hope that one still works or else the user will have no idea
 	// why we crashed.
-	for _, w := range []io.Writer{OrigStderr, l.file} {
+	outputs := make([]io.Writer, 2)
+	outputs[0] = OrigStderr
+	if f, ok := l.file.(*syncBuffer); ok {
+		// Don't call syncBuffer's Write method, because it can call back into
+		// exitLocked. Go directly to syncBuffer's underlying writer.
+		outputs[1] = f.Writer
+	} else {
+		outputs[1] = l.file
+	}
+	for _, w := range outputs {
 		if w == nil {
 			continue
 		}

pkg/util/log/clog_test.go

@@ -28,6 +28,7 @@ import (
 	"reflect"
 	"regexp"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -775,6 +776,36 @@ func TestFileSeverityFilter(t *testing.T) {
 	}
 }
 
+type outOfSpaceWriter struct{}
+
+func (w *outOfSpaceWriter) Write([]byte) (int, error) {
+	return 0, fmt.Errorf("no space left on device")
+}
+
+func TestExitOnFullDisk(t *testing.T) {
+	oldLogExitFunc := logExitFunc
+	logExitFunc = nil
+	defer func() { logExitFunc = oldLogExitFunc }()
+
+	var exited sync.WaitGroup
+	exited.Add(1)
+	l := &loggingT{
+		exitFunc: func(int) {
+			exited.Done()
+		},
+	}
+	l.file = &syncBuffer{
+		logger: l,
+		Writer: bufio.NewWriterSize(&outOfSpaceWriter{}, 1),
+	}
+
+	l.mu.Lock()
+	l.exitLocked(fmt.Errorf("out of space"))
+	l.mu.Unlock()
+
+	exited.Wait()
+}
+
 func BenchmarkHeader(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		buf := formatHeader(Severity_INFO, timeutil.Now(), 200, "file.go", 100, nil)