tryserver module: fix in set_failure_hash

set_failure_hash assumes that (StepFailure is raised) => (some steps
were ran), which is not a correct implication.
Check that there was a step before.

R=iannucci@chromium.org
BUG=

Change-Id: I36ad187584cb42696676e3339f6a77fc77dfd3b1
Reviewed-on: https://chromium-review.googlesource.com/412994
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

recipe_modules/tryserver/api.py

@@ -284,14 +284,17 @@ class TryserverApi(recipe_api.RecipeApi):
     except self.m.step.StepFailure as e:
       self.add_failure_reason(e.reason)
 
-      failure_hash = hashlib.sha1()
-      failure_hash.update(self.m.json.dumps(self._failure_reasons))
-
-      step_result = self.m.step.active_result
-      step_result.presentation.properties['failure_hash'] = \
-          failure_hash.hexdigest()
-
-      raise
+      try:
+        step_result = self.m.step.active_result
+      except ValueError:
+        step_result = None
+      if step_result:
+        failure_hash = hashlib.sha1()
+        failure_hash.update(self.m.json.dumps(self._failure_reasons))
+        step_result.presentation.properties['failure_hash'] = (
+            failure_hash.hexdigest())
+
+      raise e
 
   def get_footers(self, patch_text=None):
     """Retrieves footers from the patch description.

recipe_modules/tryserver/example.expected/set_failure_hash_with_no_steps.json

@@ -0,0 +1,8 @@
+[
+  {
+    "name": "$result",
+    "reason": "boom!",
+    "recipe_result": null,
+    "status_code": 1
+  }
+]

recipe_modules/tryserver/example.py

@@ -15,6 +15,10 @@ DEPS = [
 
 
 def RunSteps(api):
+  if api.properties.get('set_failure_hash_with_no_steps'):
+    with api.tryserver.set_failure_hash():
+      raise api.step.StepFailure('boom!')
+
   api.path['checkout'] = api.path['start_dir']
   if api.properties.get('patch_text'):
     api.step('patch_text test', [
@@ -106,3 +110,6 @@ def GenTests(api):
              'parse description (2)',
              api.json.output({'Foo': ['bar']}))
   )
+
+  yield (api.test('set_failure_hash_with_no_steps') +
+         api.properties(set_failure_hash_with_no_steps=True))