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..416c9565cbbac9b442ac8f442bb48e5b43d862d9 100644
--- a/mmutex/cmd/mmutex/main.go
+++ b/mmutex/cmd/mmutex/main.go
@@ -4,6 +4,31 @@
 
 package main
 
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"github.com/luci/luci-go/mmutex/lib"
+)
+
+// TODO(charliea): Compute this path from $MMUTEX_LOCK_DIR rather than using a constant.
+const lockFilePath = "/tmp/lock"
+
 func main() {
-	// TODO(charliea): Write the command line interface.
+	var isExclusive = flag.Bool("x", false, "Runs the command after acquiring an exclusive lock")

[iannucci, 2017/07/11 20:52:14]

we will probably want to convert this to use the luci-go cli library (like
https://chromium.googlesource.com/infra/infra/+/master/go/src/infra/tools/led...).

Then the syntax could look like:

  mmutex shared -- program args
  mmutex exclusive -- program args

[charliea (OOO until 10-5), 2017/07/11 23:26:16]

Done. Started with just "exclusive" for now, given that it's the only one with a
stub of an implementation.

+	var isShared = flag.Bool("s", false, "Runs the command after acquiring a shared lock")
+	flag.Parse()
+
+	if (*isExclusive && *isShared) || (!*isExclusive && !*isShared) {
+		fmt.Fprintf(os.Stderr, "mmutex must be run in either exclusive mode (-x) or shared mode (-s), but not both.\\n")
+		os.Exit(1)
+	}
+
+	if *isExclusive {
+		if err := lib.AcquireExclusiveLock(lockFilePath); err != nil {
+			fmt.Fprintf(os.Stderr, "error acquiring exclusive lock: %s\n", err)
+			os.Exit(1)
+		}
+	}
 }