Implement the 'shared' subcommand for mmutex

mmutex/lib/exit_code.go

Index: mmutex/lib/exit_code.go
diff --git a/mmutex/lib/exit_code.go b/mmutex/lib/exit_code.go
new file mode 100644
index 0000000000000000000000000000000000000000..22da0c855b27bca32f25c7c8847e87a6fc80aa19
--- /dev/null
+++ b/mmutex/lib/exit_code.go
@@ -0,0 +1,29 @@
+// 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 lib
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+func GetExitCode(err *exec.ExitError) int {
+	if status, ok := err.Sys().(syscall.WaitStatus); ok {
+		return status.ExitStatus()
+	} else {
+		// Unexpected: Sys() should be a syscall.WaitStatus on all platforms we care about.
+		panic(err)
+	}
+}