Ensure only a single instance of after-lock runs at a time

cmd/after-lock/after-lock.go

@@ -2,19 +2,50 @@ package main
 
 import (
 	"after-lock/afterlock"
+	"after-lock/lockfile"
+	"fmt"
+	"runtime/debug"
+	"os"
 )
 
 const (
 	initialDelay = 3
 	loopDelay = 6
 	keyboardDeviceNode = "/dev/input/by-id/usb-Dell_Dell_USB_Entry_Keyboard-event-kbd"
+	lockfilePath = "/tmp/after-lock.lock"
+	printStackTraces = false
 )
 
 
 func main() {
+	defer handleErrors()
+
+	lock := grabExclusiveProcessLock()
+	defer lock.Unlock()
+
 	al := afterlock.New(keyboardDeviceNode)
 	al.InitialDelay = initialDelay
 	al.LoopDelay = loopDelay
 
 	al.Start()
+}
+
+func handleErrors() {
+	if err := recover(); err != nil {
+		fmt.Printf("Crashed with unexpected error: %v!\n", err)
+		if printStackTraces {
+			fmt.Printf("Stack trace:\n\n")
+			debug.PrintStack()
+		}
+		os.Exit(1)
+	}
+}
+
+func grabExclusiveProcessLock() *lockfile.Lockfile {
+	lf := lockfile.New(lockfilePath)
+	err := lf.Lock()
+	if err != nil {
+		panic(err)
+	}
+	return lf
 }

lockfile/lockfile.go

@@ -0,0 +1,49 @@
+package lockfile
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+
+type Lockfile struct {
+	path string
+	fd int
+}
+
+func New(path string) *Lockfile {
+	return &Lockfile{path: path}
+}
+
+func (lf *Lockfile) Lock() error {
+	fd, err := syscall.Open(lf.path, syscall.O_CREAT | syscall.O_RDWR, 0600)
+	if err != nil {
+		return err
+	}
+	lf.fd = fd
+	err = syscall.Flock(lf.fd, syscall.LOCK_EX | syscall.LOCK_NB)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (lf *Lockfile) Unlock() error {
+	if lf.fd == 0 {
+		panic(errors.New("Lockfile was not locked"))
+	}
+	err := syscall.Flock(lf.fd, syscall.LOCK_UN)
+	if err != nil {
+		return err
+	}
+	err = syscall.Close(lf.fd)
+	if err != nil {
+		return err
+	}
+	os.Remove(lf.path)
+	if err != nil {
+		return err
+	}
+	return nil
+}

lockfile/lockfile_test.go

@@ -0,0 +1,71 @@
+package lockfile_test
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"after-lock/lockfile"
+)
+
+
+func tmpFile() string {
+	tf, err := ioutil.TempFile("", "lockfile_test*")
+	if err != nil {
+		panic(err)
+	}
+	return tf.Name()
+}
+
+func TestNotLockedUnlockPanics(t *testing.T) {
+	defer func() {
+        if r := recover(); r == nil {
+            t.Errorf("Unlock() did not panic")
+        }
+    }()
+
+	tf := tmpFile()
+	defer os.Remove(tf)
+
+
+	err := lockfile.New(tf).Unlock()
+	if err != nil {
+		panic(err)
+	}
+}
+
+func TestSecondLockFails(t *testing.T) {
+	tf := tmpFile()
+	defer os.Remove(tf)
+
+	fl1 := lockfile.New(tf)
+	err := fl1.Lock()
+	if err != nil {
+		panic(err)
+	}
+	defer fl1.Unlock()
+
+	fl2 := lockfile.New(tf)
+	err = fl2.Lock()
+	if err == nil {
+		t.Errorf("Second Lock() did not fail")
+	}
+}
+
+func TestLockUnlock(t *testing.T) {
+	tf := tmpFile()
+	defer os.Remove(tf)
+
+	fl := lockfile.New(tf)
+	err := fl.Lock()
+	if err != nil {
+		panic(err)
+	}
+	err = fl.Unlock()
+	if err != nil {
+		panic(err)
+	}
+	if _, err := os.Stat(tf); !os.IsNotExist(err) {
+		t.Errorf("Lockfile %s still exists", tf)
+	}
+}