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..6a6a0ee9c1351629556cb1313ec756c96dfb447a
--- /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 {
+	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
+}