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

mmutex/cmd/mmutex/exclusive.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 main
+
+import (
+        "fmt"
+        "os"
+
+        "github.com/maruel/subcommands"
+
+        "github.com/luci/luci-go/mmutex/lib"
+)
+
+var cmdExclusive = &subcommands.Command{
+        UsageLine: "exclusive -- <command>",
+        ShortDesc: "acquires an exclusive lock before running the command",
+        CommandRun: func() subcommands.CommandRun {
+                return &cmdExclusiveRun{}
+        },
+}
+
+type cmdExclusiveRun struct {
+        subcommands.CommandRunBase
+}
+
+func (c *cmdExclusiveRun) Run(a subcommands.Application, args []string, env subcommands.Env) int {
+        // TODO(charliea): Make sure that `args` has length greater than zero.
+
+        if err := lib.AcquireExclusiveLock(lockFilePath); err != nil {
+                fmt.Fprintf(os.Stderr, "error acquiring exclusive lock: %s\n", err)
+                return 1
+        }
+
+        return 0
+}