Bug 1356122 - Run Stylo tests in sequential mode r=aki

taskcluster/ci/test/test-platforms.yml

@@ -80,6 +80,14 @@ linux64-stylo/opt:
     test-sets:
         - stylo-tests
         - talos
+linux64-stylo-sequential/debug:
+    build-platform: linux64-stylo/debug
+    test-sets:
+        - stylo-sequential-tests
+linux64-stylo-sequential/opt:
+    build-platform: linux64-stylo/opt
+    test-sets:
+        - stylo-sequential-tests
 
 # QR builds just run a subset right now. Note that the tests in this
 # test set are further restricted in tests.yml to run on certain projects

taskcluster/ci/test/test-sets.yml

@@ -79,6 +79,11 @@ stylo-tests:
     - mochitest-style
     - mochitest-chrome-style
 
+stylo-sequential-tests:
+    - crashtest
+    - reftest-stylo
+    - mochitest-style
+
 qr-tests:
     - cppunit
     - crashtest

taskcluster/ci/test/tests.yml

@@ -1103,6 +1103,7 @@ reftest-stylo:
     run-on-projects:
         by-test-platform:
             linux64-stylo/opt: [ 'stylo', 'autoland', 'mozilla-central', 'try' ] # not on mozilla-inbound
+            linux64-stylo-sequential/opt: [ 'mozilla-central']
             default: built-projects
     e10s:
         # run only e10s tests for this platform see bug 1343301
@@ -1121,6 +1122,14 @@ reftest-stylo:
                     mozilla-central: true
                     mozilla-inbound: true
                     default: true
+            linux64-stylo-sequential/opt:
+                by-project:
+                    mozilla-central: true
+                    default: true
+            linux64-stylo-sequential/debug:
+                by-project:
+                    mozilla-central: true
+                    default: true 
     mozharness:
         script: desktop_unittest.py
         no-read-buildbot-config: true

taskcluster/taskgraph/transforms/tests.py

@@ -644,7 +644,8 @@ def parallel_stylo_tests(config, tests):
     parallel traversal in the style system."""
 
     for test in tests:
-        if not test['test-platform'].startswith('linux64-stylo/'):
+        if (not test['test-platform'].startswith('linux64-stylo/')) and \
+           (not test['test-platform'].startswith('linux64-stylo-sequential/')):
             yield test
             continue
 
@@ -655,9 +656,15 @@ def parallel_stylo_tests(config, tests):
             yield test
             continue
 
-        test['mozharness'].setdefault('extra-options', [])\
-                          .append('--parallel-stylo-traversal')
-        yield test
+        # Bug 1356122 - Run Stylo tests in sequential mode
+        if test['test-platform'].startswith('linux64-stylo-sequential/'):
+            yield test
+
+        if test['test-platform'].startswith('linux64-stylo/'):
+            # add parallel stylo tests
+            test['mozharness'].setdefault('extra-options', [])\
+                              .append('--parallel-stylo-traversal')
+            yield test
 
 
 @transforms.add