Trigger build request using git try instead of try_job_http on bisect bots

Bisect builders fail to apply patches sent via try_job_http,
because of this we were not able to build revisions with DEPS patches.
Now we are using "git try" instead to trigger build request on bisect builders.

BUG=411418
NOTRY=true

Review URL: https://codereview.chromium.org/607803005

Cr-Commit-Position: refs/heads/master@{#297918}

tools/auto_bisect/bisect_perf_regression.py

@@ -176,10 +176,10 @@ HIGH_CONFIDENCE = 95
 # When a build requested is posted with a patch, bisect builders on try server,
 # once build is produced, it reads SHA value from this file and appends it
 # to build archive filename.
-DEPS_SHA_PATCH = """diff --git src/DEPS.sha src/DEPS.sha
+DEPS_SHA_PATCH = """diff --git DEPS.sha DEPS.sha
 new file mode 100644
 --- /dev/null
-+++ src/DEPS.sha
++++ DEPS.sha
 @@ -0,0 +1 @@
 +%(deps_sha)s
 """
@@ -250,6 +250,20 @@ O O | Visit http://www.chromium.org/developers/core-principles for Chrome's
  X  | policy on perf regressions. Contact chrome-perf-dashboard-team with any
 / \ | questions or suggestions about bisecting. THANK YOU."""
 
+# Git branch name used to run bisect try jobs.
+BISECT_TRYJOB_BRANCH = 'bisect-tryjob'
+# Git master branch name.
+BISECT_MASTER_BRANCH = 'master'
+# File to store 'git diff' content.
+BISECT_PATCH_FILE = 'deps_patch.txt'
+# SVN repo where the bisect try jobs are submitted.
+SVN_REPO_URL = 'svn://svn.chromium.org/chrome-try/try-perf'
+
+class RunGitError(Exception):
+
+  def __str__(self):
+    return '%s\nError executing git command.' % self.args[0]
+
 
 def _AddAdditionalDepotInfo(depot_info):
   """Adds additional depot info to the global depot variables."""
@@ -846,6 +860,95 @@ class DepotDirectoryRegistry(object):
     """
     os.chdir(self.GetDepotDir(depot_name))
 
+def _PrepareBisectBranch(parent_branch, new_branch):
+  """Creates a new branch to submit bisect try job.
+
+  Args:
+    parent_branch: Parent branch to be used to create new branch.
+    new_branch: New branch name.
+  """
+  current_branch, returncode = bisect_utils.RunGit(
+      ['rev-parse', '--abbrev-ref', 'HEAD'])
+  if returncode:
+    raise RunGitError('Must be in a git repository to send changes to trybots.')
+
+  current_branch = current_branch.strip()
+  # Make sure current branch is master.
+  if current_branch != parent_branch:
+    output, returncode = bisect_utils.RunGit(['checkout', '-f', parent_branch])
+    if returncode:
+      raise RunGitError('Failed to checkout branch: %s.' % output)
+
+  # Delete new branch if exists.
+  output, returncode = bisect_utils.RunGit(['branch', '--list' ])
+  if new_branch in output:
+    output, returncode = bisect_utils.RunGit(['branch', '-D', new_branch])
+    if returncode:
+      raise RunGitError('Deleting branch failed, %s', output)
+
+  # Check if the tree is dirty: make sure the index is up to date and then
+  # run diff-index.
+  bisect_utils.RunGit(['update-index', '--refresh', '-q'])
+  output, returncode = bisect_utils.RunGit(['diff-index', 'HEAD'])
+  if output:
+    raise RunGitError('Cannot send a try job with a dirty tree.')
+
+  # Create/check out the telemetry-tryjob branch, and edit the configs
+  # for the tryjob there.
+  output, returncode = bisect_utils.RunGit(['checkout', '-b', new_branch])
+  if returncode:
+    raise RunGitError('Failed to checkout branch: %s.' % output)
+
+  output, returncode = bisect_utils.RunGit(
+      ['branch', '--set-upstream-to', parent_branch])
+  if returncode:
+    raise RunGitError('Error in git branch --set-upstream-to')
+
+
+def _BuilderTryjob(git_revision, bot_name, bisect_job_name, patch=None):
+  """Attempts to run a tryjob from the current directory.
+
+  Args:
+    git_revision: A Git hash revision.
+    bot_name: Name of the bisect bot to be used for try job.
+    bisect_job_name: Bisect try job name.
+    patch: A DEPS patch (used while bisecting 3rd party repositories).
+  """
+  try:
+    # Temporary branch for running tryjob.
+    _PrepareBisectBranch(BISECT_MASTER_BRANCH, BISECT_TRYJOB_BRANCH)
+    patch_content = '/dev/null'
+    # Create a temporary patch file, if it fails raise an exception.
+    if patch:
+      WriteStringToFile(patch, BISECT_PATCH_FILE)
+      patch_content = BISECT_PATCH_FILE
+
+    try_cmd = ['try',
+               '-b', bot_name,
+               '-r', git_revision,
+               '-n', bisect_job_name,
+               '--svn_repo=%s' % SVN_REPO_URL,
+               '--diff=%s' % patch_content
+              ]
+    # Execute try job to build revision.
+    output, returncode = bisect_utils.RunGit(try_cmd)
+
+    if returncode:
+      raise RunGitError('Could not execute tryjob: %s.\n Error: %s' % (
+                         'git %s' % ' '.join(try_cmd), output))
+    print ('Try job successfully submitted.\n TryJob Details: %s\n%s' % (
+           'git %s' % ' '.join(try_cmd), output))
+  finally:
+    # Delete patch file if exists
+    try:
+      os.remove(BISECT_PATCH_FILE)
+    except OSError as e:
+      if e.errno != errno.ENOENT:
+        raise
+    # Checkout master branch and delete bisect-tryjob branch.
+    bisect_utils.RunGit(['checkout', '-f', BISECT_MASTER_BRANCH])
+    bisect_utils.RunGit(['branch', '-D', BISECT_TRYJOB_BRANCH])
+
 
 class BisectPerformanceMetrics(object):
   """This class contains functionality to perform a bisection of a range of
@@ -1182,7 +1285,7 @@ class BisectPerformanceMetrics(object):
         return FetchFromCloudStorage(gs_bucket, source_file, out_dir)
     return downloaded_archive
 
-  def DownloadCurrentBuild(self, revision, build_type='Release', patch=None):
+  def DownloadCurrentBuild(self, revision, depot, build_type='Release'):
     """Downloads the build archive for the given revision.
 
     Args:
@@ -1193,7 +1296,12 @@ class BisectPerformanceMetrics(object):
     Returns:
       True if download succeeds, otherwise False.
     """
+    patch = None
     patch_sha = None
+    if depot != 'chromium':
+      # Create a DEPS patch with new revision for dependency repository.
+      revision, patch = self.CreateDEPSPatch(depot, revision)
+
     if patch:
       # Get the SHA of the DEPS changes patch.
       patch_sha = GetSHA1HexDigest(patch)
@@ -1290,37 +1398,31 @@ class BisectPerformanceMetrics(object):
     if not fetch_build:
       return False
 
-    bot_name, build_timeout = GetBuilderNameAndBuildTime(
-       self.opts.target_platform, self.opts.target_arch)
-    builder_host = self.opts.builder_host
-    builder_port = self.opts.builder_port
     # Create a unique ID for each build request posted to try server builders.
     # This ID is added to "Reason" property of the build.
     build_request_id = GetSHA1HexDigest(
         '%s-%s-%s' % (git_revision, patch, time.time()))
 
-    # Creates a try job description.
-    # Always use Git hash to post build request since Commit positions are
-    # not supported by builders to build.
-    job_args = {
-        'revision': 'src@%s' % git_revision,
-        'bot': bot_name,
-        'name': build_request_id,
-    }
-    # Update patch information if supplied.
-    if patch:
-      job_args['patch'] = patch
-    # Posts job to build the revision on the server.
-    if request_build.PostTryJob(builder_host, builder_port, job_args):
+    # Reverts any changes to DEPS file.
+    self.source_control.CheckoutFileAtRevision(
+      bisect_utils.FILE_DEPS, git_revision, cwd=self.src_cwd)
+
+    bot_name, build_timeout = GetBuilderNameAndBuildTime(
+       self.opts.target_platform, self.opts.target_arch)
+    target_file = None
+    try:
+      # Execute try job request to build revision with patch.
+      _BuilderTryjob(git_revision, bot_name, build_request_id, patch)
       target_file, error_msg = _WaitUntilBuildIsReady(
-          fetch_build, bot_name, builder_host, builder_port, build_request_id,
-          build_timeout)
+          fetch_build, bot_name, self.opts.builder_host,
+          self.opts.builder_port, build_request_id, build_timeout)
       if not target_file:
         print '%s [revision: %s]' % (error_msg, git_revision)
-        return None
-      return target_file
-    print 'Failed to post build request for revision: [%s]' % git_revision
-    return None
+    except RunGitError as e:
+      print ('Failed to post builder try job for revision: [%s].\n'
+             'Error: %s' % (git_revision, e))
+
+    return target_file
 
   def IsDownloadable(self, depot):
     """Checks if build can be downloaded based on target platform and depot."""
@@ -1417,6 +1519,7 @@ class BisectPerformanceMetrics(object):
       print 'Something went wrong while updating DEPS file. [%s]' % e
     return False
 
+
   def CreateDEPSPatch(self, depot, revision):
     """Modifies DEPS and returns diff as text.
 
@@ -1444,8 +1547,8 @@ class BisectPerformanceMetrics(object):
         if self.UpdateDeps(revision, depot, deps_file_path):
           diff_command = [
               'diff',
-              '--src-prefix=src/',
-              '--dst-prefix=src/',
+              '--src-prefix=',
+              '--dst-prefix=',
               '--no-ext-diff',
                bisect_utils.FILE_DEPS,
           ]
@@ -1474,16 +1577,7 @@ class BisectPerformanceMetrics(object):
     # Fetch build archive for the given revision from the cloud storage when
     # the storage bucket is passed.
     if self.IsDownloadable(depot) and revision:
-      deps_patch = None
-      if depot != 'chromium':
-        # Create a DEPS patch with new revision for dependency repository.
-        revision, deps_patch = self.CreateDEPSPatch(depot, revision)
-      if self.DownloadCurrentBuild(revision, patch=deps_patch):
-        if deps_patch:
-          # Reverts the changes to DEPS file.
-          self.source_control.CheckoutFileAtRevision(
-              bisect_utils.FILE_DEPS, revision, cwd=self.src_cwd)
-        build_success = True
+      build_success = self.DownloadCurrentBuild(revision, depot)
     else:
       # These codes are executed when bisect bots builds binaries locally.
       build_success = self.builder.Build(depot, self.opts)
@@ -1779,17 +1873,7 @@ class BisectPerformanceMetrics(object):
     Returns:
       True if successful.
     """
-    if depot == 'chromium' or depot == 'android-chrome':
-      # Removes third_party/libjingle. At some point, libjingle was causing
-      # issues syncing when using the git workflow (crbug.com/266324).
-      os.chdir(self.src_cwd)
-      if not bisect_utils.RemoveThirdPartyDirectory('libjingle'):
-        return False
-      # Removes third_party/skia. At some point, skia was causing
-      # issues syncing when using the git workflow (crbug.com/377951).
-      if not bisect_utils.RemoveThirdPartyDirectory('skia'):
-        return False
-    elif depot == 'cros':
+    if depot == 'cros':
       return self.PerformCrosChrootCleanup()
     return True
 
@@ -2639,7 +2723,8 @@ class BisectPerformanceMetrics(object):
             current_data['depot'])
         if not cl_link:
           cl_link = current_id
-        commit_position = self.source_control.GetCommitPosition(current_id)
+        commit_position = self.source_control.GetCommitPosition(
+            current_id, self.depot_registry.GetDepotDir(current_data['depot']))
         commit_position = str(commit_position)
         if not commit_position:
           commit_position = ''

tools/auto_bisect/bisect_perf_regression_test.py

@@ -5,12 +5,18 @@
 import os
 import re
 import shutil
+import sys
 import unittest
 
+SRC = os.path.join(os.path.dirname(__file__), os.path.pardir, os.path.pardir)
+sys.path.append(os.path.join(SRC, 'third_party', 'pymock'))
+
 import bisect_perf_regression
 import bisect_results
+import mock
 import source_control as source_control_module
 
+
 def _GetBisectPerformanceMetricsInstance():
   """Returns an instance of the BisectPerformanceMetrics class."""
   options_dict = {
@@ -348,5 +354,156 @@ class DepotDirectoryRegistryTest(unittest.TestCase):
     self.assertEqual(self.cur_dir, '/mock/src/foo')
 
 
+# The tests below test private functions (W0212).
+# pylint: disable=W0212
+class GitTryJobTestCases(unittest.TestCase):
+  """Test case for bisect try job."""
+  def setUp(self):
+    bisect_utils_patcher = mock.patch('bisect_perf_regression.bisect_utils')
+    self.mock_bisect_utils = bisect_utils_patcher.start()
+    self.addCleanup(bisect_utils_patcher.stop)
+
+  def _SetupRunGitMock(self, git_cmds):
+    """Setup RunGit mock with expected output for given git command."""
+    def side_effect(git_cmd_args):
+      for val in git_cmds:
+        if set(val[0]) == set(git_cmd_args):
+          return val[1]
+    self.mock_bisect_utils.RunGit = mock.Mock(side_effect=side_effect)
+
+  def _AssertRunGitExceptions(self, git_cmds, func, *args):
+    """Setup RunGit mock and tests RunGitException.
+
+      Args:
+        git_cmds: List of tuples with git command and expected output.
+        func: Callback function to be executed.
+        args: List of arguments to be passed to the function.
+    """
+    self._SetupRunGitMock(git_cmds)
+    self.assertRaises(bisect_perf_regression.RunGitError,
+                      func,
+                      *args)
+
+  def testNotGitRepo(self):
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    cmds = [(['rev-parse', '--abbrev-ref', 'HEAD'], (None, 128))]
+    self._AssertRunGitExceptions(cmds,
+                                 bisect_perf_regression._PrepareBisectBranch,
+                                 parent_branch, new_branch)
+
+  def testFailedCheckoutMaster(self):
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    cmds = [
+        (['rev-parse', '--abbrev-ref', 'HEAD'], (new_branch, 0)),
+        (['checkout', '-f', parent_branch], ('Checkout Failed', 1))]
+    self._AssertRunGitExceptions(cmds,
+                                 bisect_perf_regression._PrepareBisectBranch,
+                                 parent_branch, new_branch)
+
+  def testDeleteBisectBranchIfExists(self):
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    cmds = [
+        (['rev-parse', '--abbrev-ref', 'HEAD'], (parent_branch, 0)),
+        (['branch', '--list' ], ('bisect-tryjob\n*master\nsomebranch', 0)),
+        (['branch', '-D', new_branch], ('Failed to delete branch', 128))
+        ]
+    self._AssertRunGitExceptions(cmds,
+                                 bisect_perf_regression._PrepareBisectBranch,
+                                 parent_branch, new_branch)
+
+  def testCreatNewBranchFails(self):
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    cmds = [
+        (['rev-parse', '--abbrev-ref', 'HEAD'], (parent_branch, 0)),
+        (['branch', '--list' ], ('bisect-tryjob\n*master\nsomebranch', 0)),
+        (['branch', '-D', new_branch], ('None', 0)),
+        (['update-index', '--refresh', '-q'], (None, 0)),
+        (['diff-index', 'HEAD'], (None, 0)),
+        (['checkout', '-b', new_branch], ('Failed to create branch', 128))
+        ]
+
+    self._AssertRunGitExceptions(cmds,
+                                 bisect_perf_regression._PrepareBisectBranch,
+                                 parent_branch, new_branch)
+
+  def testSetUpstreamToFails(self):
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    cmds = [
+        (['rev-parse', '--abbrev-ref', 'HEAD'], (parent_branch, 0)),
+        (['branch', '--list' ], ('bisect-tryjob\n*master\nsomebranch', 0)),
+        (['branch', '-D', new_branch], ('None', 0)),
+        (['update-index', '--refresh', '-q'], (None, 0)),
+        (['diff-index', 'HEAD'], (None, 0)),
+        (['checkout', '-b', new_branch], ('None', 0)),
+        (['branch', '--set-upstream-to', parent_branch],
+         ('Setuptream fails', 1))
+        ]
+    self._AssertRunGitExceptions(cmds,
+                                 bisect_perf_regression._PrepareBisectBranch,
+                                 parent_branch, new_branch)
+  def testBuilderTryJobForException(self):
+    git_revision = 'ac4a9f31fe2610bd146857bbd55d7a260003a888'
+    bot_name = 'linux_perf_bisect_builder'
+    bisect_job_name = 'testBisectJobname'
+    patch = None
+    patch_content = '/dev/null'
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    try_cmd = [
+        (['rev-parse', '--abbrev-ref', 'HEAD'], (parent_branch, 0)),
+        (['branch', '--list' ], ('bisect-tryjob\n*master\nsomebranch', 0)),
+        (['branch', '-D', new_branch], ('None', 0)),
+        (['update-index', '--refresh', '-q'], (None, 0)),
+        (['diff-index', 'HEAD'], (None, 0)),
+        (['checkout', '-b', new_branch], ('None', 0)),
+        (['branch', '--set-upstream-to', parent_branch],
+         ('Setuptream fails', 0)),
+        (['try',
+          '-b', bot_name,
+          '-r', git_revision,
+          '-n', bisect_job_name,
+          '--svn_repo=%s' % bisect_perf_regression.SVN_REPO_URL,
+          '--diff=%s' % patch_content
+          ], (None, 1))
+        ]
+    self._AssertRunGitExceptions(try_cmd,
+                                 bisect_perf_regression._BuilderTryjob,
+                                 git_revision, bot_name, bisect_job_name, patch)
+
+  def testBuilderTryJob(self):
+    git_revision = 'ac4a9f31fe2610bd146857bbd55d7a260003a888'
+    bot_name = 'linux_perf_bisect_builder'
+    bisect_job_name = 'testBisectJobname'
+    patch = None
+    patch_content = '/dev/null'
+    new_branch = bisect_perf_regression.BISECT_TRYJOB_BRANCH
+    parent_branch = bisect_perf_regression.BISECT_MASTER_BRANCH
+    try_cmd = [
+        (['rev-parse', '--abbrev-ref', 'HEAD'], (parent_branch, 0)),
+        (['branch', '--list' ], ('bisect-tryjob\n*master\nsomebranch', 0)),
+        (['branch', '-D', new_branch], ('None', 0)),
+        (['update-index', '--refresh', '-q'], (None, 0)),
+        (['diff-index', 'HEAD'], (None, 0)),
+        (['checkout', '-b', new_branch], ('None', 0)),
+        (['branch', '--set-upstream-to', parent_branch],
+         ('Setuptream fails', 0)),
+        (['try',
+          '-b', bot_name,
+          '-r', git_revision,
+          '-n', bisect_job_name,
+          '--svn_repo=%s' % bisect_perf_regression.SVN_REPO_URL,
+          '--diff=%s' % patch_content
+          ], (None, 0))
+        ]
+    self._SetupRunGitMock(try_cmd)
+    bisect_perf_regression._BuilderTryjob(
+        git_revision, bot_name, bisect_job_name, patch)
+
+
 if __name__ == '__main__':
   unittest.main()

tools/auto_bisect/bisect_utils.py

@@ -11,7 +11,6 @@ annotations for the Buildbot waterfall.
 import errno
 import imp
 import os
-import shutil
 import stat
 import subprocess
 import sys
@@ -83,6 +82,9 @@ REPO_PARAMS = [
     'https://git.chromium.org/external/repo.git'
 ]
 
+# Bisect working directory.
+BISECT_DIR = 'bisect'
+
 
 def OutputAnnotationStepStart(name):
   """Outputs annotation to signal the start of a step to a try bot.
@@ -162,12 +164,12 @@ def _CreateAndChangeToSourceDirectory(working_directory):
   cwd = os.getcwd()
   os.chdir(working_directory)
   try:
-    os.mkdir('bisect')
+    os.mkdir(BISECT_DIR)
   except OSError, e:
     if e.errno != errno.EEXIST:  # EEXIST indicates that it already exists.
       os.chdir(cwd)
       return False
-  os.chdir('bisect')
+  os.chdir(BISECT_DIR)
   return True
 
 
@@ -308,28 +310,6 @@ def OnAccessError(func, path, _):
     raise
 
 
-def RemoveThirdPartyDirectory(dir_name):
-  """Removes third_party directory from the source.
-
-  At some point, some of the third_parties were causing issues to changes in
-  the way they are synced. We remove such folder in order to avoid sync errors
-  while bisecting.
-
-  Returns:
-    True on success, otherwise False.
-  """
-  path_to_dir = os.path.join(os.getcwd(), 'third_party', dir_name)
-  try:
-    if os.path.exists(path_to_dir):
-      shutil.rmtree(path_to_dir, onerror=OnAccessError)
-  except OSError, e:
-    print 'Error #%d while running shutil.rmtree(%s): %s' % (
-        e.errno, path_to_dir, str(e))
-    if e.errno != errno.ENOENT:
-      return False
-  return True
-
-
 def _CleanupPreviousGitRuns():
   """Cleans up any leftover index.lock files after running git."""
   # If a previous run of git crashed, or bot was reset, etc., then we might
@@ -350,7 +330,8 @@ def RunGClientAndSync(cwd=None):
   Returns:
     The return code of the call.
   """
-  params = ['sync', '--verbose', '--nohooks', '--reset', '--force']
+  params = ['sync', '--verbose', '--nohooks', '--reset', '--force',
+            '--delete_unversioned_trees']
   return RunGClient(params, cwd=cwd)
 
 
@@ -368,35 +349,19 @@ def SetupGitDepot(opts, custom_deps):
     otherwise.
   """
   name = 'Setting up Bisection Depot'
+  try:
+    if opts.output_buildbot_annotations:
+      OutputAnnotationStepStart(name)
 
-  if opts.output_buildbot_annotations:
-    OutputAnnotationStepStart(name)
-
-  passed = False
-
-  if not RunGClientAndCreateConfig(opts, custom_deps):
-    passed_deps_check = True
-    if os.path.isfile(os.path.join('src', FILE_DEPS_GIT)):
-      cwd = os.getcwd()
-      os.chdir('src')
-      if passed_deps_check:
-        passed_deps_check = RemoveThirdPartyDirectory('libjingle')
-      if passed_deps_check:
-        passed_deps_check = RemoveThirdPartyDirectory('skia')
-      os.chdir(cwd)
-
-    if passed_deps_check:
-      _CleanupPreviousGitRuns()
-
-      RunGClient(['revert'])
-      if not RunGClientAndSync():
-        passed = True
+    if RunGClientAndCreateConfig(opts, custom_deps):
+      return False
 
-  if opts.output_buildbot_annotations:
-    print
-    OutputAnnotationStepClosed()
-
-  return passed
+    _CleanupPreviousGitRuns()
+    RunGClient(['revert'])
+    return not RunGClientAndSync()
+  finally:
+    if opts.output_buildbot_annotations:
+      OutputAnnotationStepClosed()
 
 
 def CheckIfBisectDepotExists(opts):
@@ -408,7 +373,7 @@ def CheckIfBisectDepotExists(opts):
   Returns:
     Returns True if it exists.
   """
-  path_to_dir = os.path.join(opts.working_directory, 'bisect', 'src')
+  path_to_dir = os.path.join(opts.working_directory, BISECT_DIR, 'src')
   return os.path.exists(path_to_dir)
 
 
@@ -451,6 +416,15 @@ def CreateBisectDirectoryAndSetupDepot(opts, custom_deps):
     opts: The options parsed from the command line through parse_args().
     custom_deps: A dictionary of additional dependencies to add to .gclient.
   """
+  if CheckIfBisectDepotExists(opts):
+    path_to_dir = os.path.join(os.path.abspath(opts.working_directory),
+                               BISECT_DIR, 'src')
+    (output, _) = RunGit(['rev-parse', '--is-inside-work-tree'],
+                         cwd=path_to_dir)
+    if output.strip() == 'true':
+      # Checks out the master branch, throws an exception if git command fails.
+      CheckRunGit(['checkout', '-f', 'master'], cwd=path_to_dir)
+
   if not _CreateAndChangeToSourceDirectory(opts.working_directory):
     raise RuntimeError('Could not create bisect directory.')