vpython: Add application entry point.

vpython/application/support.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package application
+
+import (
+        "github.com/luci/luci-go/vpython/python"
+
+        "github.com/luci/luci-go/common/errors"
+
+        "golang.org/x/net/context"
+)
+
+var appKey = "github.com/luci/luci-go/vpython/application.A"
+
+func withApplication(c context.Context, a *A) context.Context {
+        return context.WithValue(c, &appKey, a)
+}
+
+func getApplication(c context.Context) *A {
+        return c.Value(&appKey).(*A)
+}
+
+func run(c context.Context, fn func(context.Context) error) int {
+        err := fn(c)
+
+        switch t := errors.Unwrap(err).(type) {
+        case nil:
+                return 0
+
+        case python.Error:
+                return int(t)
+
+        default:
+                errors.Log(c, err)
+                return 1
+        }
+}