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..591a329bf8923c89487df7d30f897d8b3343a5ee
--- /dev/null
+++ b/mmutex/lib/flock.go
@@ -0,0 +1,20 @@
+// 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")
+}