Bug 1381613 - Populate a repository-less index route for "trunk" repos; r=dustin

There are a few places where we walk commit ancestry looking for things
attached to a specific revision. Because the repository name is attached
to the index path and because a revision can exist in multiple
repositories, we often have to perform N index lookups to find a result
for a specific revision. This is inefficient.

To facilitate faster index lookups by revision, we introduce a new route
that doesn't contain the repository name. In theory, we should be able
to do this globally - for all repos. However, the configuration of
tasks can vary significantly by repo. So e.g. a linux64 build on
"central" is sufficiently different from a linux64 build on "beta" or
"release." For that reason, this commit takes the conservative
approach and only defines a shared route for repositories with a similar
configuration: the "trunk" repositories.

MozReview-Commit-ID: 8rIgUbzW4eL

taskcluster/taskgraph/transforms/task.py

@@ -15,6 +15,7 @@ import os
 import time
 from copy import deepcopy
 
+from taskgraph.util.attributes import TRUNK_PROJECTS
 from taskgraph.util.treeherder import split_symbol
 from taskgraph.transforms.base import TransformSequence
 from taskgraph.util.schema import validate_schema, Schema
@@ -476,6 +477,12 @@ V2_ROUTE_TEMPLATES = [
     "index.gecko.v2.{project}.revision.{head_rev}.{product}.{job-name}",
 ]
 
+# {central, inbound, autoland} write to a "trunk" index prefix. This facilitates
+# walking of tasks with similar configurations.
+V2_TRUNK_ROUTE_TEMPLATES = [
+    "index.gecko.v2.trunk.revision.{head_rev}.{product}.{job-name}",
+]
+
 V2_NIGHTLY_TEMPLATES = [
     "index.gecko.v2.{project}.nightly.latest.{product}.{job-name}",
     "index.gecko.v2.{project}.nightly.{build_date}.revision.{head_rev}.{product}.{job-name}",
@@ -820,9 +827,18 @@ def add_generic_index_routes(config, task):
                                             time.gmtime(config.params['build_date']))
     subs['product'] = index['product']
 
+    project = config.params.get('project')
+
     for tpl in V2_ROUTE_TEMPLATES:
         routes.append(tpl.format(**subs))
 
+
+    # Additionally alias all tasks for "trunk" repos into a common
+    # namespace.
+    if project and project in TRUNK_PROJECTS:
+        for tpl in V2_TRUNK_ROUTE_TEMPLATES:
+            routes.append(tpl.format(**subs))
+
     return task