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

mmutex/cmd/mmutex/main.go

Index: mmutex/cmd/mmutex/main.go
diff --git a/mmutex/cmd/mmutex/main.go b/mmutex/cmd/mmutex/main.go
index d61b202902035f014f3edb4f856c6365c1465583..2335bd2fb36b96c007750495ef117237357df5ec 100644
--- a/mmutex/cmd/mmutex/main.go
+++ b/mmutex/cmd/mmutex/main.go
@@ -4,6 +4,42 @@
 
 package main
 
+import (
+	"os"
+
+	"github.com/maruel/subcommands"
+)
+
+// TODO(charliea): Compute this path from $MMUTEX_LOCK_DIR rather than using a constant.
+const lockFilePath = "/tmp/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
+types of access to the system:
+
+  * Exclusive access guarantees that no other callers have any access
+    exclusive or shared) to the resource while the specified command is run.
+  * Shared access guarantees that only other callers with shared access
+    will have access to the resource while the specified command is run.
+
+In short, exclusive access guarantees a task is run alone, while shared access
+tasks may be run alongside other shared access tasks.
+
+The source for mmutex lives at:
+  https://github.com/luci/luci-go/tree/master/mmutex`,
+	Commands: []*subcommands.Command{
+		cmdExclusive,
+		subcommands.CmdHelp,
+	},
+}
+
 func main() {
-	// TODO(charliea): Write the command line interface.
+	os.Exit(subcommands.Run(application, nil))
 }