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 {
+        if _, err := os.Stat(path); os.IsNotExist(err) {

[iannucci, 2017/07/11 20:52:15]

tbh, I would probably not stat, I'd just bail on ENOENT from the actual lock
invocation

[charliea (OOO until 10-5), 2017/07/11 23:26:16]

The actual lock implementation accepts an already-open file handle rather than a
path, so I instead bailed on the OpenFile failure (where the ENOENT first pops
up)

+                return fmt.Errorf("cannot acquire lock on %s because file does not exist", path)
+        }
+
+        // TODO(charliea): Acquire the actual lock.

[iannucci, 2017/07/11 20:52:14]

can also `panic("implement lock acquisition")`

[charliea (OOO until 10-5), 2017/07/11 23:26:16]

Done.

+        return nil
+}