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

mmutex/lib/flock_test.go

Index: mmutex/lib/flock_test.go
diff --git a/mmutex/lib/flock_test.go b/mmutex/lib/flock_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..d3b848e4ff90cbf8e49015eb39ca959ae2415f9a
--- /dev/null
+++ b/mmutex/lib/flock_test.go
@@ -0,0 +1,28 @@
+// 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 (
+	"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("", "")
+	if err != nil {
+		t.Error("received error creating temporary testing directory")
+	}
+	defer os.Remove(lockDir)
+	path := lockDir + "test.lock"
+
+	Convey("AcquireExclusiveLock errors if lock file doesn't exist", t, func(c C) {
+		So(AcquireExclusiveLock(path), ShouldErrLike, "lock file does not exist")
+	})
+}