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

mmutex/cmd/mmutex/exclusive.go

Index: mmutex/cmd/mmutex/exclusive.go
diff --git a/mmutex/cmd/mmutex/exclusive.go b/mmutex/cmd/mmutex/exclusive.go
new file mode 100644
index 0000000000000000000000000000000000000000..2c77b738170a6c1d085a8417af58cc4ea71cd378
--- /dev/null
+++ b/mmutex/cmd/mmutex/exclusive.go
@@ -0,0 +1,37 @@
+// 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
+}