Enforce users to specify a 'pool' dimension when triggering a task.

appengine/swarming/server/task_request.py

Index: appengine/swarming/server/task_request.py
diff --git a/appengine/swarming/server/task_request.py b/appengine/swarming/server/task_request.py
index 19b4c78c0bf31403bbb4989a96d821a45473e156..c09d30270397c5cb93e931cf68fd4d0edf902bcd 100644
--- a/appengine/swarming/server/task_request.py
+++ b/appengine/swarming/server/task_request.py
@@ -433,11 +433,14 @@ class TaskRequest(ndb.Model):
     elif self.priority == 0:
       raise datastore_errors.BadValueError(
           'priority 0 can only be used for terminate request')
-    self.tags.append('priority:%s' % self.priority)
-    self.tags.append('user:%s' % self.user)
-    for key, value in self.properties.dimensions.iteritems():
-      self.tags.append('%s:%s' % (key, value))
-    self.tags = sorted(set(self.tags))
+
+    if not self.properties.dimensions:
+      raise datastore_errors.BadValueError('dimensions must be specified')
+    dim_keys = self.properties.dimensions.keys()
+    if 'pool' not in dim_keys and 'id' not in dim_keys:
+      raise datastore_errors.BadValueError(
+          'At least one of \'id\' or \'pool\' must be used as dimensions')
+
     if (self.pubsub_topic and
         not pubsub.validate_full_name(self.pubsub_topic, 'topics')):
       raise datastore_errors.BadValueError(
@@ -449,6 +452,12 @@ class TaskRequest(ndb.Model):
       raise datastore_errors.BadValueError(
           'pubsub_userdata requires pubsub_topic')
 
+    self.tags.append('priority:%s' % self.priority)
+    self.tags.append('user:%s' % self.user)
+    for key, value in self.properties.dimensions.iteritems():
+      self.tags.append('%s:%s' % (key, value))
+    self.tags = sorted(set(self.tags))
+
 
 def _new_request_key():
   """Returns a valid ndb.Key for this entity.