bot_update (+recipe_module): treat patch download as infra failure.

scripts/slave/recipe_modules/bot_update/api.py

Index: scripts/slave/recipe_modules/bot_update/api.py
diff --git a/scripts/slave/recipe_modules/bot_update/api.py b/scripts/slave/recipe_modules/bot_update/api.py
index 17add359bb5ed1c2bf0014f9ff93322e6b925705..c5d47e5985b411970e08925c0e1d82846472b4b1 100644
--- a/scripts/slave/recipe_modules/bot_update/api.py
+++ b/scripts/slave/recipe_modules/bot_update/api.py
@@ -232,10 +232,13 @@ class BotUpdateApi(recipe_api.RecipeApi):
       # the checkout.
       # If there is a patch failure, emit another step that said things failed.
       if step_result.json.output.get('patch_failure'):
-        # TODO(phajdan.jr): Differentiate between failure to download the patch
-        # and failure to apply it. The first is an infra failure, the latter
-        # a definite patch failure.
-        self.m.tryserver.set_patch_failure_tryjob_result()
+        return_code = step_result.json.output.get('return_code')
+        if return_code == 3:
+          # This is download failure, hence an infra failure.
+          self.m.step.InfraFailure('Failed to download the patch')
+        else:
+          # This is actual patch failure.
+          self.m.tryserver.set_patch_failure_tryjob_result()
         self.m.python.failing_step(
             'Patch failure', 'Check the bot_update step for details')