swarming: add transaction_id to tasks.new request

appengine/swarming/swarming_rpcs.py

Index: appengine/swarming/swarming_rpcs.py
diff --git a/appengine/swarming/swarming_rpcs.py b/appengine/swarming/swarming_rpcs.py
index 980d133c7704edf0eebe6b60e493e1b00b24d338..b9044311b5d254b75cedda2e8df659bda0b4de8c 100644
--- a/appengine/swarming/swarming_rpcs.py
+++ b/appengine/swarming/swarming_rpcs.py
@@ -236,6 +236,15 @@ class NewTaskRequest(messages.Message):
   # Will be but into "userdata" fields of PubSub message.
   pubsub_userdata = messages.StringField(11)
 
+  # Global ID of the transaction to create a new task request.
+  # If specified and a task request with the same transaction id was created
+  # recently, the existing request will be returned as is.
+  # If two NewTaskRequests use the same transaction ID, requests must have all

[M-A Ruel, 2017/05/10 13:41:25]

Hey, thinking about it, what about if all attributes are the same, why not use
the hash of the request minus created_ts and using (expiration_ts-created_ts) as
a relative value? Then no need for an arbitrary ID.

Right now we use TaskRequest.properties_hash for task result deduping but what
I'm thinking about is TaskRequest.request_hash for task request deduping. This
would be much more generic and would be transparent to the user.

Give me a day to think about it. I think it'd work quite fine, as all the task
retry logic we have already use a different task name.

+  # attributes equal.
+  #
+  # Transaction ID can be used to cancel this task request.
+  transaction_id = messages.StringField(12)
+
 
 class TaskRequest(messages.Message):
   """Description of a task request as registered by the server."""
@@ -325,6 +334,7 @@ class CancelResponse(messages.Message):
   """Result of a request to cancel a task."""
   ok = messages.BooleanField(1)
   was_running = messages.BooleanField(2)
+  task_id = messages.StringField(3)
 
 
 class TasksCancelResponse(messages.Message):