Do not schedule downstream jobs without upstream jobs

Downstream tasks (Buildbot test and talos builders) that do not have an upstream task
(Buildbot build builder) is because the build that triggers it is executed in the TaskCluster
CI rather than on the Buildbot CI. For instance, Linux64 talos jobs are executed on Buildbot
while the Linux64 builds are generated by TaskCluster. The decision task schedules the
Buildbot downstream jobs via the Buildbot bridge.

In this change we change determine_trigger_objective to ignore try to schedule Buildbot builders
that do not have a Buildbot upstream builder but a TaskCluster upstream task.

Author: armenzg

mozci/mozci.py

@@ -173,12 +173,18 @@ def determine_trigger_objective(revision, buildername,
     * Files, if needed, to trigger such builder
 
     """
+    # Upstream builders return their ownself
+    build_buildername = determine_upstream_builder(buildername)
+
+    if build_buildername is None:
+        LOG.info("We believe {} is a builder triggered via Buildbot bridge "
+                 "and we won't schedule anything.".format(buildername))
+        return None, None, None
+
     builder_to_trigger = None
     files = None
     repo_name = query_repo_name_from_buildername(buildername)
 
-    build_buildername = determine_upstream_builder(buildername)
-
     if VALIDATE and not valid_builder(build_buildername):
         raise MozciError("Our platforms mapping system has failed.")
 

test/test_platforms.py

@@ -38,6 +38,19 @@
     get_builder_extra_properties,
 )
 
+@patch('mozci.platforms.fetch_allthethings_data')
+def test_no_buildbot_upstream_builder(fetch_allthethings_data):
+    fetch_allthethings_data.return_value = ALLTHETHINGS
+    assert determine_upstream_builder(
+        "Ubuntu HW 12.04 x64 autoland talos g2-e10s") is None
+
+@patch('mozci.platforms.fetch_allthethings_data')
+def test_has_buildbot_upstream_builder(fetch_allthethings_data):
+    fetch_allthethings_data.return_value = ALLTHETHINGS
+    assert determine_upstream_builder(
+        "Rev7 MacOSX Yosemite 10.10.5 mozilla-esr52 debug test cppunit") == \
+        'OS X 10.7 64-bit mozilla-esr52 leak test build'
+
 
 class TestIsDownstream(unittest.TestCase):