swarming: add transaction_id to tasks.new request

appengine/swarming/server/task_scheduler.py

Index: appengine/swarming/server/task_scheduler.py
diff --git a/appengine/swarming/server/task_scheduler.py b/appengine/swarming/server/task_scheduler.py
index 0d79b0562b7585b3d569635a0055360d48f56c91..be6ce197c40e6e043cc11955800636de418a367e 100644
--- a/appengine/swarming/server/task_scheduler.py
+++ b/appengine/swarming/server/task_scheduler.py
@@ -12,6 +12,7 @@ import logging
 import math
 import random
 
+from google.appengine.api import memcache
 from google.appengine.ext import ndb
 
 from components import auth
@@ -474,14 +475,16 @@ def exponential_backoff(attempt_num):
   return min(max_wait, math.pow(1.5, min(attempt_num, 10) + 1))
 
 
-def schedule_request(request, secret_bytes, check_acls=True):
+def schedule_request(
+    request, secret_bytes, check_acls=True, memcache_key_for_task_id=None):
   """Creates and stores all the entities to schedule a new task request.
 
   Checks ACLs first. Raises auth.AuthorizationError if caller is not authorized
   to post this request.
 
   The number of entities created is 3: TaskRequest, TaskToRun and
-  TaskResultSummary.
+  TaskResultSummary. If memcache_key_for_task_id is not None, sets it to the
+  task_id right after creation.
 
   All 4 entities in the same entity group (TaskReqest, TaskToRun,
   TaskResultSummary, SecretBytes) are saved as a DB transaction.
@@ -566,6 +569,8 @@ def schedule_request(request, secret_bytes, check_acls=True):
     # be run but the client will not know about it.
     datastore_utils.insert(request, get_new_keys,
         extra=filter(bool, [task, result_summary, secret_bytes]))
+    if memcache_key_for_task_id:
+      memcache.set(memcache_key_for_task_id, result_summary.task_id)
     logging.debug('New request %s', result_summary.task_id)
 
   # Get parent task details if applicable.