Run the linter on CloudBuild CI. (#141)

Github integration with CloudBuild does not copy over the
.git folder, thus to compute the diff we have to perform
a git clone as the first step.

Fixes #127

cloudbuild.yaml

@@ -13,6 +13,26 @@
 # limitations under the License.
 
 steps:
+- id: Initialize git
+  name: gcr.io/cloud-builders/git
+  entrypoint: /bin/bash
+  args:
+  - -exc
+  - |
+    # Cloud Build x GitHub integration uses source archives to fetch
+    # the source, rather than Git source fetching, and as a consequence
+    # does not include the .git/ directory. As a workaround, we clone
+    # the repository to grab the .git directory.
+    git clone 'https://github.com/google/clusterfuzz' tmp
+    git -C tmp fetch origin "$COMMIT_SHA"
+    git -C tmp checkout -qf FETCH_HEAD
+    mv tmp/.git .git
+    rm -rf tmp
+- id: Lint
+  name: 'gcr.io/clusterfuzz-images/ci'
+  args: ['python', 'butler.py', 'lint']
+  env:
+    - 'GOOGLE_CLOUDBUILD=1'
 - name: 'gcr.io/clusterfuzz-images/ci'
   args: ['setup']
 - name: 'gcr.io/clusterfuzz-images/ci'

src/local/butler/lint.py

@@ -21,7 +21,11 @@
 
 def execute(_):
   """Lint changed code."""
-  _, output = common.execute('git diff --name-only FETCH_HEAD')
+  if "GOOGLE_CLOUDBUILD" in os.environ:
+    # Explicitly compare against master if we're running on the CI
+    _, output = common.execute('git diff --name-only master FETCH_HEAD')
+  else:
+    _, output = common.execute('git diff --name-only FETCH_HEAD')
 
   py_changed_file_paths = [
       f for f in output.splitlines() if f.endswith('.py') and