Add filters for user service.

service/user/context.go

Index: service/user/context.go
diff --git a/service/user/context.go b/service/user/context.go
index c86a830b7c2da737c6b1a654324e62c5da11bb63..21061667e825d2daa91f650b699b345b983fb8f7 100644
--- a/service/user/context.go
+++ b/service/user/context.go
@@ -10,21 +10,50 @@ import (
 
 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 {
@@ -37,3 +66,15 @@ func SetFactory(c context.Context, f Factory) context.Context {
 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)
+}