vpython: Add application entry point.

vpython/application/support.go

Index: vpython/application/support.go
diff --git a/vpython/application/support.go b/vpython/application/support.go
new file mode 100644
index 0000000000000000000000000000000000000000..87ac6b8dd6dcd9cde2ded310870e4d0bac042cbb
--- /dev/null
+++ b/vpython/application/support.go
@@ -0,0 +1,37 @@
+// 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/common/errors"
+
+	"golang.org/x/net/context"
+)
+
+var appKey = "github.com/luci/luci-go/vpython/application.A"
+
+func withConfig(c context.Context, cfg *Config) context.Context {
+	return context.WithValue(c, &appKey, cfg)
+}
+
+func getConfig(c context.Context) *Config {
+	return c.Value(&appKey).(*Config)
+}
+
+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 ReturnCodeError:
+		return int(t)
+
+	default:
+		errors.Log(c, err)
+		return 1
+	}
+}