Split out process check as separate function.

This will allow the checks to be changed for different operating systems.

Issue #260 is related to this, but this does not change any current behaviour.

lock_unix.go

@@ -3,8 +3,12 @@
 package restic
 
 import (
+	"os"
 	"os/user"
 	"strconv"
+	"syscall"
+
+	"github.com/restic/restic/debug"
 )
 
 // uidGidInt returns uid, gid of the user as a number.
@@ -22,3 +26,23 @@ func uidGidInt(u user.User) (uid, gid uint32, err error) {
 	gid = uint32(gi)
 	return
 }
+
+// checkProcess will check if the process retaining the lock
+// exists and responds to SIGHUP signal.
+// Returns true if the process exists and responds.
+func (l Lock) processExists() bool {
+	proc, err := os.FindProcess(l.PID)
+	if err != nil {
+		debug.Log("Lock.Stale", "error searching for process %d: %v\n", l.PID, err)
+		return false
+	}
+	defer proc.Release()
+
+	debug.Log("Lock.Stale", "sending SIGHUP to process %d\n", l.PID)
+	err = proc.Signal(syscall.SIGHUP)
+	if err != nil {
+		debug.Log("Lock.Stale", "signal error: %v, lock is probably stale\n", err)
+		return false
+	}
+	return true
+}