[presubmit] Use vpython3 on bots without vpython2

Bug: 1401307
Change-Id: Ie81608be8e8ec64ed569b0367f1426b4b2ab8b70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4110968
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

recipes/README.recipes.md

@@ -758,7 +758,7 @@ Raises:
 — **def [initialize](/recipes/recipe_modules/osx_sdk/api.py#48)(self):**
 ### *recipe_modules* / [presubmit](/recipes/recipe_modules/presubmit)
 
-[DEPS](/recipes/recipe_modules/presubmit/__init__.py#13): [bot\_update](#recipe_modules-bot_update), [depot\_tools](#recipe_modules-depot_tools), [gclient](#recipe_modules-gclient), [git](#recipe_modules-git), [tryserver](#recipe_modules-tryserver), [recipe\_engine/context][recipe_engine/recipe_modules/context], [recipe\_engine/cq][recipe_engine/recipe_modules/cq], [recipe\_engine/json][recipe_engine/recipe_modules/json], [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/properties][recipe_engine/recipe_modules/properties], [recipe\_engine/resultdb][recipe_engine/recipe_modules/resultdb], [recipe\_engine/step][recipe_engine/recipe_modules/step]
+[DEPS](/recipes/recipe_modules/presubmit/__init__.py#13): [bot\_update](#recipe_modules-bot_update), [depot\_tools](#recipe_modules-depot_tools), [gclient](#recipe_modules-gclient), [git](#recipe_modules-git), [tryserver](#recipe_modules-tryserver), [recipe\_engine/buildbucket][recipe_engine/recipe_modules/buildbucket], [recipe\_engine/context][recipe_engine/recipe_modules/context], [recipe\_engine/cq][recipe_engine/recipe_modules/cq], [recipe\_engine/json][recipe_engine/recipe_modules/json], [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/properties][recipe_engine/recipe_modules/properties], [recipe\_engine/resultdb][recipe_engine/recipe_modules/resultdb], [recipe\_engine/step][recipe_engine/recipe_modules/step]
 
 PYTHON_VERSION_COMPATIBILITY: PY2+3
 
@@ -768,7 +768,7 @@ PYTHON_VERSION_COMPATIBILITY: PY2+3
 
 Returns a presubmit step.
 
-— **def [execute](/recipes/recipe_modules/presubmit/api.py#120)(self, bot_update_step, skip_owners=False, run_all=False):**
+— **def [execute](/recipes/recipe_modules/presubmit/api.py#130)(self, bot_update_step, skip_owners=False, run_all=False):**
 
 Runs presubmit and sets summary markdown if applicable.
 
@@ -779,7 +779,7 @@ Args:
 Returns:
   a RawResult object, suitable for being returned from RunSteps.
 
-— **def [prepare](/recipes/recipe_modules/presubmit/api.py#68)(self, root_solution_revision=None):**
+— **def [prepare](/recipes/recipe_modules/presubmit/api.py#78)(self, root_solution_revision=None):**
 
 Sets up a presubmit run.
 

recipes/recipe_modules/presubmit/__init__.py

@@ -15,6 +15,7 @@ DEPS = [
   'depot_tools',
   'gclient',
   'git',
+  'recipe_engine/buildbucket',
   'recipe_engine/context',
   'recipe_engine/cq',
   'recipe_engine/json',

recipes/recipe_modules/presubmit/api.py

@@ -30,7 +30,17 @@ class PresubmitApi(recipe_api.RecipeApi):
 
     name = kwargs.pop('name', 'presubmit')
     with self.m.depot_tools.on_path():
-      cmd = ['vpython', self.presubmit_support_path]
+      exe = 'vpython'
+
+      # Use only vpython3 on bots that don't have vpython2 on the path any
+      # longer.
+      # TODO(https://crbug.com/1401307): Switch this to vpython3 premanently
+      # and remove py3 part below.
+      experiments = self.m.buildbucket.build.input.experiments
+      if 'luci.buildbucket.omit_python2' in experiments:
+        exe = 'vpython3'
+
+      cmd = [exe, self.presubmit_support_path]
       cmd.extend(args)
       cmd.extend(['--json_output', self.m.json.output()])
       if self.m.resultdb.enabled:

recipes/recipe_modules/presubmit/tests/execute.py

@@ -69,6 +69,14 @@ def GenTests(api):
                           ['--skip_canned', 'CheckOwners']) +
          api.post_process(post_process.DropExpectation))
 
+  yield (api.test('vpython3') + api.runtime(is_experimental=False) +
+         api.buildbucket.try_build(
+            project='infra', experiments=['luci.buildbucket.omit_python2']) +
+         api.post_process(post_process.StepCommandContains, 'presubmit',
+                          ['vpython3']) +
+         api.post_process(post_process.StatusSuccess) +
+         api.post_process(post_process.DropExpectation))
+
   yield (
       api.test('timeout') + api.runtime(is_experimental=False) +
       api.buildbucket.try_build(project='infra') +