Flesh out the mmutex binary wrapper and test for existence of lockfile

mmutex/lib/flock_test.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package lib
+
+import (
+        "fmt"
+        "io/ioutil"
+        "os"
+        "testing"
+
+        . "github.com/smartystreets/goconvey/convey"
+
+        . "github.com/luci/luci-go/common/testing/assertions"
+)
+
+func TestAcquireExclusiveLock(t *testing.T) {
+        lockDir, err := ioutil.TempDir("/tmp", "")
+        if err != nil {
+                t.Error("received error creating temporary testing directory")
+        }
+        defer os.Remove(lockDir)
+        path := lockDir + ".lock"
+
+        Convey("AcquireExclusiveLock errors if lock file doesn't exist", t, func(c C) {
+                So(AcquireExclusiveLock(path), ShouldErrLike, fmt.Sprintf("cannot acquire lock on %s because file does not exist", path))

[iannucci, 2017/07/12 01:34:59]

tbh, "file does not exist" is enough of a substring for this test :)

[charliea (OOO until 10-5), 2017/07/12 02:17:17]

I went with "lock file does not exist"

+        })
+}