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

mmutex/lib/flock.go

Index: mmutex/lib/flock.go
diff --git a/mmutex/lib/flock.go b/mmutex/lib/flock.go
new file mode 100644
index 0000000000000000000000000000000000000000..24c6c2f8045e1834da37eb9f5c9ce056c7bcc66f
--- /dev/null
+++ b/mmutex/lib/flock.go
@@ -0,0 +1,21 @@
+// 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 (
+	"os"
+
+	"github.com/luci/luci-go/common/errors"
+)
+
+// TODO(charliea): Add timeout parameter to this function.
+func AcquireExclusiveLock(path string) error {
+	_, err := os.OpenFile(path, os.O_APPEND, 0666)
+	if os.IsNotExist(err) {
+		return errors.Reason("lock file does not exist", path).Err()
+	}
+
+	panic("implement lock acquisition")
+}