Enable log rotation by automatically closing log file (fixes #2251)

cmd/syncthing/monitor.go

@@ -28,8 +28,10 @@ var (
 )
 
 const (
-	countRestarts = 4
-	loopThreshold = 60 * time.Second
+	countRestarts         = 4
+	loopThreshold         = 60 * time.Second
+	logFileAutoCloseDelay = 5 * time.Second
+	logFileMaxOpenTime    = time.Minute
 )
 
 func monitorMain() {
@@ -37,16 +39,10 @@ func monitorMain() {
 	os.Setenv("STMONITORED", "yes")
 	l.SetPrefix("[monitor] ")
 
-	var err error
 	var dst io.Writer = os.Stdout
 
 	if logFile != "-" {
-		var fileDst io.Writer
-
-		fileDst, err = os.Create(logFile)
-		if err != nil {
-			l.Fatalln("log file:", err)
-		}
+		var fileDst io.Writer = newAutoclosedFile(logFile, logFileAutoCloseDelay, logFileMaxOpenTime)
 
 		if runtime.GOOS == "windows" {
 			// Translate line breaks to Windows standard
@@ -262,3 +258,114 @@ func restartMonitorWindows(args []string) error {
 	cmd.Stdin = os.Stdin
 	return cmd.Start()
 }
+
+// An autoclosedFile is an io.WriteCloser that opens itself for appending on
+// Write() and closes itself after an interval of no writes (closeDelay) or
+// when the file has been open for too long (maxOpenTime). A call to Write()
+// will return any error that happens on the resulting Open() call too. Errors
+// on automatic Close() calls are silently swallowed...
+type autoclosedFile struct {
+	name        string        // path to write to
+	closeDelay  time.Duration // close after this long inactivity
+	maxOpenTime time.Duration // or this long after opening
+
+	fd         io.WriteCloser // underlying WriteCloser
+	opened     time.Time      // timestamp when the file was last opened
+	closed     chan struct{}  // closed on Close(), stops the closerLoop
+	closeTimer *time.Timer    // fires closeDelay after a write
+
+	mut sync.Mutex
+}
+
+func newAutoclosedFile(name string, closeDelay, maxOpenTime time.Duration) *autoclosedFile {
+	f := &autoclosedFile{
+		name:        name,
+		closeDelay:  closeDelay,
+		maxOpenTime: maxOpenTime,
+		mut:         sync.NewMutex(),
+		closed:      make(chan struct{}),
+		closeTimer:  time.NewTimer(time.Minute),
+	}
+	go f.closerLoop()
+	return f
+}
+
+func (f *autoclosedFile) Write(bs []byte) (int, error) {
+	f.mut.Lock()
+	defer f.mut.Unlock()
+
+	// Make sure the file is open for appending
+	if err := f.ensureOpen(); err != nil {
+		return 0, err
+	}
+
+	// If we haven't run into the maxOpenTime, postpone close for another
+	// closeDelay
+	if time.Since(f.opened) < f.maxOpenTime {
+		f.closeTimer.Reset(f.closeDelay)
+	}
+
+	return f.fd.Write(bs)
+}
+
+func (f *autoclosedFile) Close() error {
+	f.mut.Lock()
+	defer f.mut.Unlock()
+
+	// Stop the timer and closerLoop() routine
+	f.closeTimer.Stop()
+	close(f.closed)
+
+	// Close the file, if it's open
+	if f.fd != nil {
+		return f.fd.Close()
+	}
+
+	return nil
+}
+
+// Must be called with f.mut held!
+func (f *autoclosedFile) ensureOpen() error {
+	if f.fd != nil {
+		// File is already open
+		return nil
+	}
+
+	// We open the file for write only, and create it if it doesn't exist.
+	flags := os.O_WRONLY | os.O_CREATE
+	if f.opened.IsZero() {
+		// This is the first time we are opening the file. We should truncate
+		// it to better emulate an os.Create() call.
+		flags |= os.O_TRUNC
+	} else {
+		// The file was already opened once, so we should append to it.
+		flags |= os.O_APPEND
+	}
+
+	fd, err := os.OpenFile(f.name, flags, 0644)
+	if err != nil {
+		return err
+	}
+
+	f.fd = fd
+	f.opened = time.Now()
+	return nil
+}
+
+func (f *autoclosedFile) closerLoop() {
+	for {
+		select {
+		case <-f.closeTimer.C:
+			// Close the file when the timer expires.
+			f.mut.Lock()
+			if f.fd != nil {
+				f.fd.Close() // errors, schmerrors
+				f.fd = nil
+			}
+			f.mut.Unlock()
+
+		case <-f.closed:
+			return
+		}
+	}
+}