Added windows version of TryLock

moved AlreadyLocked to general file so it's the same instance for all

filemutex.go

@@ -0,0 +1,5 @@
+package filemutex
+
+import "errors"
+
+var AlreadyLocked = errors.New("lock already acquired")

filemutex_flock.go

@@ -35,8 +35,6 @@ func (m *FileMutex) Lock() error {
 	return nil
 }
 
-var AlreadyLocked = errors.New("lock already acquired")
-
 func (m *FileMutex) TryLock() error {
 	if err := syscall.Flock(m.fd, syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
 		if errno, ok := err.(syscall.Errno); ok {

filemutex_windows.go

@@ -16,7 +16,8 @@ var (
 )
 
 const (
-	lockfileExclusiveLock = 2
+	lockfileFailImmediately = 1
+	lockfileExclusiveLock   = 2
 )
 
 func lockFileEx(h syscall.Handle, flags, reserved, locklow, lockhigh uint32, ol *syscall.Overlapped) (err error) {
@@ -58,6 +59,19 @@ func New(filename string) (*FileMutex, error) {
 	return &FileMutex{fd: fd}, nil
 }
 
+func (m *FileMutex) TryLock() error {
+	var ol syscall.Overlapped
+	if err := lockFileEx(m.fd, lockfileFailImmediately|lockfileExclusiveLock, 0, 1, 0, &ol); err != nil {
+		if errno, ok := err.(syscall.Errno); ok {
+			if errno == syscall.Errno(0x21) {
+				return AlreadyLocked
+			}
+		}
+		return err
+	}
+	return nil
+}
+
 func (m *FileMutex) Lock() error {
 	var ol syscall.Overlapped
 	if err := lockFileEx(m.fd, lockfileExclusiveLock, 0, 1, 0, &ol); err != nil {