impl/cloud: Add support for "memcached" memcache.

impl/cloud/context.go

 // Copyright 2016 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 cloud
 
 import (
+        "github.com/luci/gae/impl/dummy"
+        ds "github.com/luci/gae/service/datastore"
+        "github.com/luci/gae/service/mail"
+        mc "github.com/luci/gae/service/memcache"
+        "github.com/luci/gae/service/module"
+        "github.com/luci/gae/service/taskqueue"
+        "github.com/luci/gae/service/user"
+
         "cloud.google.com/go/datastore"
+        "github.com/bradfitz/gomemcache/memcache"
 
         "golang.org/x/net/context"
 )
 
-// Use installs the cloud services implementation into the supplied Context.
+// Config is a full-stack cloud service configuration. A user can selectively
+// populate its fields, and services for the populated fields will be installed
+// in the Context and available.
 //
-// This includes:
-//      - github.com/luci/gae/service/info
-//      - github.com/luci/gae/service/datastore
-//
-// This is built around the ability to use cloud datastore.
-func Use(c context.Context, client *datastore.Client) context.Context {
-        return UseDS(useInfo(c), client)
+// Because the "impl/cloud" service collection is a composite set of cloud
+// services, the user can choose services based on their configuration.
+type Config struct {
+        // DS is the cloud datastore client. If populated, the datastore service will
+        // be installed.
+        DS *datastore.Client
+
+        // MC is the memcache service client. If populated, the memcache service will
+        // be installed.
+        MC *memcache.Client
 }
 
-// UseDS installs the cloud datastore implementation into the supplied Context.
-func UseDS(c context.Context, client *datastore.Client) context.Context {
-        cds := cloudDatastore{
-                client: client,
+// Use installs the Config into the supplied Context. Services will be installed
+// based on the fields that are populated in Config.
+//
+// Any services that are missing will have "impl/dummy" stubs installed. These
+// stubs will panic if called.
+func (cfg Config) Use(c context.Context) context.Context {
+        // Dummy services that we don't support.
+        c = mail.Set(c, dummy.Mail())
+        c = module.Set(c, dummy.Module())
+        c = taskqueue.SetRaw(c, dummy.TaskQueue())
+        c = user.Set(c, dummy.User())
+
+        c = useInfo(c)
+
+        // datastore service
+        if cfg.DS != nil {
+                cds := cloudDatastore{
+                        client: cfg.DS,
+                }
+                c = cds.use(c)
+        } else {
+                c = ds.SetRaw(c, dummy.Datastore())
         }
-        return cds.use(c)
+
+        // memcache service
+        if cfg.MC != nil {
+                mc := memcacheClient{
+                        client: cfg.MC,
+                }
+                c = mc.use(c)
+        } else {
+                c = mc.SetRaw(c, dummy.Memcache())
+        }
+
+        return c
 }