Make mmutex actually acquire and release file locks

mmutex/cmd/mmutex/main.go

 // Copyright 2017 The LUCI Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 package main
 
 import (
         "os"
+        "path"
 
         "github.com/maruel/subcommands"
 )
 
-// TODO(charliea): Compute this path from $MMUTEX_LOCK_DIR rather than using a constant.
-const lockFilePath = "/tmp/lock"
+// TODO(charliea): Compute this path from $MMUTEX_LOCK_DIR rather than making it fixed.
+var LockFilePath = path.Join(os.TempDir(), "mmutex.lock")
 
 var application = &subcommands.DefaultApplication{
         Name: "mmutex",
         Title: `'Maintenance Mutex' - Global mutex to isolate maintenance tasks.
 
 mmutex is a command line tool that helps prevent maintenance tasks from running
 during user tasks. The tool does this by way of a global lock file that users
 must acquire before running their tasks.
 
 Clients can use this tool to request that their task be run with one of two
@@ skipping 11 matching lines @@
   https://github.com/luci/luci-go/tree/master/mmutex`,
         Commands: []*subcommands.Command{
                 cmdExclusive,
                 subcommands.CmdHelp,
         },
 }
 
 func main() {
         os.Exit(subcommands.Run(application, nil))
 }