Add filters for user service.

service/user/context.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package user
 
 import (
         "golang.org/x/net/context"
 )
 
 type key int
 
-var serviceKey key
+var (
+        serviceKey       key
+        serviceFilterKey key = 1
+)
 
 // Factory is the function signature for factory methods compatible with
 // SetFactory.
 type Factory func(context.Context) Interface
 
-// Get pulls the user service implementation from context or nil if it
-// wasn't set.
-func Get(c context.Context) Interface {
+// Filter is the function signature for a filter user implementation. It
+// gets the current user implementation, and returns a new user implementation
+// backed by the one passed in.
+type Filter func(context.Context, Interface) Interface
+
+// getUnfiltered gets gets the Interface implementation from context without
+// any of the filters applied.
+func getUnfiltered(c context.Context) Interface {
         if f, ok := c.Value(serviceKey).(Factory); ok && f != nil {
                 return f(c)
         }
         return nil
 }
 
+func getCurFilters(c context.Context) []Filter {
+        curFiltsI := c.Value(serviceFilterKey)
+        if curFiltsI != nil {
+                return curFiltsI.([]Filter)
+        }
+        return nil
+}
+
+// Get pulls the user service implementation from context or nil if it
+// wasn't set.
+func Get(c context.Context) Interface {
+        ret := getUnfiltered(c)
+        if ret == nil {
+                return nil
+        }
+        for _, f := range getCurFilters(c) {
+                ret = f(c, ret)
+        }
+        return ret
+}
+
 // SetFactory sets the function to produce user.Interface instances,
 // as returned by the Get method.
 func SetFactory(c context.Context, f Factory) context.Context {
         return context.WithValue(c, serviceKey, f)
 }
 
 // Set sets the user service in this context. Useful for testing with a quick
 // mock. This is just a shorthand SetFactory invocation to set a factory which
 // always returns the same object.
 func Set(c context.Context, u Interface) context.Context {
         return SetFactory(c, func(context.Context) Interface { return u })
 }
+
+// AddFilters adds Interface filters to the context.
+func AddFilters(c context.Context, filts ...Filter) context.Context {
+        if len(filts) == 0 {
+                return c
+        }
+        cur := getCurFilters(c)
+        newFilts := make([]Filter, 0, len(cur)+len(filts))
+        newFilts = append(newFilts, getCurFilters(c)...)
+        newFilts = append(newFilts, filts...)
+        return context.WithValue(c, serviceFilterKey, newFilts)
+}