Implement the 'shared' subcommand for mmutex

mmutex/lib/exit_code.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 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)
+        }
+}