Improve handling of first CI run for new branch. (#29070)

(cherry picked from commit a1453a7917)

test/runner/lib/changes.py

@@ -74,8 +74,8 @@ class ShippableChanges(object):
                 self.paths = sorted(git.get_diff_names([last_successful_commit, self.commit]))
                 self.diff = git.get_diff([last_successful_commit, self.commit])
             else:
-                # tracked files (including unchanged)
-                self.paths = sorted(git.get_file_names(['--cached']))
+                # first run for branch
+                self.paths = None  # act as though change detection not enabled, do not filter targets
                 self.diff = []
 
     def get_merge_runs(self, project_id, branch):
@@ -117,6 +117,9 @@ class ShippableChanges(object):
                     if git.is_valid_ref(commit_sha):
                         last_successful_commit = commit_sha
 
+        if last_successful_commit is None:
+            display.warning('No successful commit found. All tests will be executed.')
+
         return last_successful_commit
 
 

test/runner/lib/executor.py

@@ -1120,6 +1120,9 @@ def detect_changes(args):
     else:
         return None  # change detection not enabled
 
+    if paths is None:
+        return None  # act as though change detection not enabled, do not filter targets
+
     display.info('Detected changes in %d file(s).' % len(paths))
 
     for path in paths:
@@ -1131,7 +1134,7 @@ def detect_changes(args):
 def detect_changes_shippable(args):
     """Initialize change detection on Shippable.
     :type args: TestConfig
-    :rtype: list[str]
+    :rtype: list[str] | None
     """
     git = Git(args)
     result = ShippableChanges(args, git)