Refactor distributor API so that methods always get the Quest_Desc too.

dm/appengine/distributor/config.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 distributor
 
 import (
+        "fmt"
+
         "golang.org/x/net/context"
 
         "github.com/golang/protobuf/proto"
+        "github.com/luci/gae/service/info"
         "github.com/luci/gae/service/taskqueue"
+        "github.com/luci/luci-go/common/gcloud/pubsub"
+        dm "github.com/luci/luci-go/dm/api/service/v1"
 )
 
 // Config represents the configuration for a single instance of a given
 // distributor implementation at a given point in time (e.g. version).
 type Config struct {
         // DMHost is the host for the DM API. This may be used by the distributor
         // implementation to pass to jobs so that they can call back into DM's api.
         DMHost string
 
         // Name is the name of this distributor configuration. This is always the
         // fully-resolved name of the configuration (i.e. aliases are dereferenced).
         Name string
 
         // Version is the version of the distributor configuration retrieved from
         // luci-config.
         Version string
 
         // Content is the actual parsed implementation-specific configuration.
         Content proto.Message
 }
 
 // EnqueueTask allows a Distributor to enqueue a TaskQueue task that will be
 // handled by the Distributor's HandleTaskQueueTask method.
 func (cfg *Config) EnqueueTask(c context.Context, tsk *taskqueue.Task) error {
         tsk.Path = handlerPath(cfg.Name)
         return taskqueue.Get(c).Add(tsk, "")
 }
+
+// PrepareTopic returns a pubsub topic that notifications should be sent to, and
+// is meant to be called from the D.Run method.
+//
+// It returns the full name of the topic and a token that will be used to route
+// PubSub messages back to the Distributor. The publisher to the topic must be
+// instructed to put the token into the 'auth_token' attribute of PubSub
+// messages. DM will know how to route such messages to D.HandleNotification.
+func (cfg *Config) PrepareTopic(c context.Context, eid *dm.Execution_ID) (topic pubsub.Topic, token string, err error) {
+        topic = pubsub.NewTopic(info.Get(c).TrimmedAppID(), notifyTopicSuffix)

[Vadim Sh., 2016/09/20 00:24:26]

consider caching the flag that this work has been done already, e.g. here's how
luci-scheduler does it:
https://github.com/luci/luci-go/blob/master/scheduler/appengine/engine/engine...

[iannucci, 2016/09/20 00:51:28]

I think it will make sense to do this when DM eventually does the topic creation
and permission adjustment. Right now it's just concatenating strings together
:).

+        if err := topic.Validate(); err != nil {
+                panic(fmt.Errorf("failed to validate Topic %q: %s", topic, err))
+        }
+        token, err = encodeAuthToken(c, eid, cfg.Name)
+        return
+}