Add support for appending list values in mixins.

Introduce a new $mixin_append key to mixins. Values in the $mixin_append
dictionary must be lists, and will be appended to any existing items in
the corresponding key of the test specification.

Bug: 879806
Change-Id: Ibd51c679df1944bcc9f34a4acba47ee39a5f871d
Reviewed-on: https://chromium-review.googlesource.com/c/1306352
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603705}

testing/buildbot/generate_buildbot_json.py

@@ -756,9 +756,6 @@ class BBJSONGenerator(object):
     is then applied to every dimension set in the test.
 
     """
-    # TODO(martiniss): Maybe make lists extend, possibly on a case by case
-    # basis. Motivation is 'args', where you want a common base, and then
-    # different mixins adding different sets of args.
     new_test = copy.deepcopy(test)
     mixin = copy.deepcopy(mixin)
 
@@ -777,6 +774,23 @@ class BBJSONGenerator(object):
       new_test['swarming'].update(swarming_mixin)
       del mixin['swarming']
 
+    if '$mixin_append' in mixin:
+      # Values specified under $mixin_append should be appended to existing
+      # lists, rather than replacing them.
+      mixin_append = mixin['$mixin_append']
+      for key in mixin_append:
+        new_test.setdefault(key, [])
+        if not isinstance(mixin_append[key], list):
+          raise BBGenErr(
+              'Key "' + key + '" in $mixin_append must be a list.')
+        if not isinstance(new_test[key], list):
+          raise BBGenErr(
+              'Cannot apply $mixin_append to non-list "' + key + '".')
+        new_test[key].extend(mixin_append[key])
+      if 'args' in mixin_append:
+        new_test['args'] = self.maybe_fixup_args_array(new_test['args'])
+      del mixin['$mixin_append']
+
     new_test.update(mixin)
 
     return new_test

testing/buildbot/generate_buildbot_json_unittest.py

@@ -2145,6 +2145,36 @@ SWARMING_MIXINS = """\
 }
 """
 
+SWARMING_MIXINS_APPEND = """\
+{
+  'builder_mixin': {
+    '$mixin_append': {
+      'args': [ '--mixin-argument' ],
+    },
+  },
+}
+"""
+
+SWARMING_MIXINS_APPEND_NOT_LIST = """\
+{
+  'builder_mixin': {
+    '$mixin_append': {
+      'args': 'I am not a list',
+    },
+  },
+}
+"""
+
+SWARMING_MIXINS_APPEND_TO_SWARMING = """\
+{
+  'builder_mixin': {
+    '$mixin_append': {
+      'swarming': [ 'swarming!' ],
+    },
+  },
+}
+"""
+
 SWARMING_MIXINS_DUPLICATED = """\
 {
   'builder_mixin': {
@@ -2296,6 +2326,27 @@ BUILDER_MIXIN_NON_SWARMING_WATERFALL_OUTPUT = """\
 }
 """
 
+BUILDER_MIXIN_APPEND_ARGS_WATERFALL_OUTPUT = """\
+{
+  "AAAAA1 AUTOGENERATED FILE DO NOT EDIT": {},
+  "AAAAA2 See generate_buildbot_json.py to make changes": {},
+  "Fake Tester": {
+    "gtest_tests": [
+      {
+        "args": [
+          "--c_arg",
+          "--mixin-argument"
+        ],
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "foo_test"
+      }
+    ]
+  }
+}
+"""
+
 TEST_MIXIN_WATERFALL_OUTPUT = """\
 {
   "AAAAA1 AUTOGENERATED FILE DO NOT EDIT": {},
@@ -2530,6 +2581,39 @@ class MixinTests(unittest.TestCase):
           '<snip>',
         ])
 
+  def test_mixin_append_args(self):
+    fbb = FakeBBGen(FOO_GTESTS_BUILDER_MIXIN_WATERFALL,
+                    FOO_TEST_SUITE_WITH_ARGS,
+                    EMPTY_PYL_FILE,
+                    SWARMING_MIXINS_APPEND,
+                    LUCI_MILO_CFG)
+    fbb.files['chromium.test.json'] = BUILDER_MIXIN_APPEND_ARGS_WATERFALL_OUTPUT
+    fbb.check_output_file_consistency(verbose=True)
+    self.assertFalse(fbb.printed_lines)
+
+  def test_mixin_append_mixin_field_not_list(self):
+    fbb = FakeBBGen(FOO_GTESTS_BUILDER_MIXIN_WATERFALL,
+                    FOO_TEST_SUITE_WITH_ARGS,
+                    EMPTY_PYL_FILE,
+                    SWARMING_MIXINS_APPEND_NOT_LIST,
+                    LUCI_MILO_CFG)
+    with self.assertRaisesRegexp(
+        generate_buildbot_json.BBGenErr,
+        'Key "args" in \$mixin_append must be a list.'):
+      fbb.check_output_file_consistency(verbose=True)
+    self.assertFalse(fbb.printed_lines)
+
+  def test_mixin_append_test_field_not_list(self):
+    fbb = FakeBBGen(FOO_GTESTS_BUILDER_MIXIN_WATERFALL,
+                    FOO_TEST_SUITE,
+                    EMPTY_PYL_FILE,
+                    SWARMING_MIXINS_APPEND_TO_SWARMING,
+                    LUCI_MILO_CFG)
+    with self.assertRaisesRegexp(
+        generate_buildbot_json.BBGenErr,
+        'Cannot apply \$mixin_append to non-list "swarming".'):
+      fbb.check_output_file_consistency(verbose=True)
+    self.assertFalse(fbb.printed_lines)
 
 if __name__ == '__main__':
   unittest.main()