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

mmutex/lib/flock.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"
+        "os"
+)
+
+// 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 fmt.Errorf("cannot acquire lock on %s because file does not exist", path)

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

recommend use of luci-go/common/errors (they collect stack traces and other
nifty things):

  errors.Reason("cannot acquire lock on %s because file does not exist",
path).Err()

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

Done.

+        }
+
+        panic("implement lock acquisition")
+}