Verify SHA in Shippable change detection.

(cherry picked from commit 5e9a2b8528)

test/runner/lib/changes.py

@@ -71,7 +71,7 @@ class ShippableChanges(object):
             self.paths = sorted(git.get_diff_names([self.branch]))
         else:
             merge_runs = self.get_merge_runs(self.project_id, self.branch)
-            last_successful_commit = self.get_last_successful_commit(merge_runs)
+            last_successful_commit = self.get_last_successful_commit(git, merge_runs)
 
             if last_successful_commit:
                 self.paths = sorted(git.get_diff_names([last_successful_commit, self.commit]))
@@ -96,8 +96,9 @@ class ShippableChanges(object):
         return response.json()
 
     @staticmethod
-    def get_last_successful_commit(merge_runs):
+    def get_last_successful_commit(git, merge_runs):
         """
+        :type git: Git
         :type merge_runs: dict | list[dict]
         :rtype: str
         """
@@ -114,6 +115,7 @@ class ShippableChanges(object):
             if commit_sha not in known_commits:
                 known_commits.add(commit_sha)
                 if merge_run['statusCode'] == 30:
+                    if git.is_valid_ref(commit_sha):
                         last_successful_commit = commit_sha
 
         return last_successful_commit

test/runner/lib/git.py

@@ -4,6 +4,7 @@ from __future__ import absolute_import, print_function
 
 from lib.util import (
     CommonConfig,
+    SubprocessError,
     run_command,
 )
 
@@ -55,6 +56,18 @@ class Git(object):
         cmd = ['merge-base', '--fork-point', branch]
         return self.run_git(cmd).strip()
 
+    def is_valid_ref(self, ref):
+        """
+        :type ref: str
+        :rtype: bool
+        """
+        cmd = ['show', ref]
+        try:
+            self.run_git(cmd)
+            return True
+        except SubprocessError:
+            return False
+
     def run_git_split(self, cmd, separator=None):
         """
         :type cmd: list[str]