This build:

https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8931960173807897536

Ran on a bot that had rebooted after it's previous build (see  https://crbug.com/895955#c5 ).

However, The cleanup stage was unable to delete the chroot because of "device busy". That normally means the chroot is still mounted from a previous build.

  06:56:00: INFO: Deleting chroot.
  umount: /b/swarming/w/ir/cache/cbuild/repository/chroot: device is busy.
          (In some cases useful info about processes that use
           the device is found by lsof(8) or fuser(1))
  06:56:07: ERROR: <class 'chromite.lib.cros_build_lib.RunCommandError'>: return code: 1; command: sudo -n 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache' 'CROS_SUDO_KEEP_ALIVE=unknown' -- umount -d /b/swarming/w/ir/cache/cbuild/repository/chroot
  cmd=['sudo', '-n', 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache', 'CROS_SUDO_KEEP_ALIVE=unknown', '--', 'umount', '-d', '/b/swarming/w/ir/cache/cbuild/repository/chroot']
  Traceback (most recent call last):
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/parallel.py", line 441, in _Run
      self._task(*self._task_args, **self._task_kwargs)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/cbuildbot/stages/build_stages.py", line 97, in _DeleteChroot
      cros_sdk_lib.CleanupChrootMount(chroot, delete=True)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/timeout_util.py", line 191, in TimeoutWrapper
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/cros_sdk_lib.py", line 394, in CleanupChrootMount
      osutils.UmountTree(chroot)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/osutils.py", line 902, in UmountTree
      UmountDir(mount_pt, lazy=False, cleanup=False)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/osutils.py", line 864, in UmountDir
      runcmd(cmd, print_cmd=False)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/cros_build_lib.py", line 322, in SudoRunCommand
      return RunCommand(sudo_cmd, **kwargs)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/cros_build_lib.py", line 647, in RunCommand
      raise RunCommandError(msg, cmd_result)
  RunCommandError: return code: 1; command: sudo -n 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache' 'CROS_SUDO_KEEP_ALIVE=unknown' -- umount -d /b/swarming/w/ir/cache/cbuild/repository/chroot
  cmd=['sudo', '-n', 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache', 'CROS_SUDO_KEEP_ALIVE=unknown', '--', 'umount', '-d', '/b/swarming/w/ir/cache/cbuild/repository/chroot']
  
  06:56:07: INFO: Translating result <class 'chromite.lib.cros_build_lib.RunCommandError'>: return code: 1; command: sudo -n 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache' 'CROS_SUDO_KEEP_ALIVE=unknown' -- umount -d /b/swarming/w/ir/cache/cbuild/repository/chroot
  cmd=['sudo', '-n', 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache', 'CROS_SUDO_KEEP_ALIVE=unknown', '--', 'umount', '-d', '/b/swarming/w/ir/cache/cbuild/repository/chroot']
  Traceback (most recent call last):
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/parallel.py", line 441, in _Run
      self._task(*self._task_args, **self._task_kwargs)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/cbuildbot/stages/build_stages.py", line 97, in _DeleteChroot
      cros_sdk_lib.CleanupChrootMount(chroot, delete=True)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/timeout_util.py", line 191, in TimeoutWrapper
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/cros_sdk_lib.py", line 394, in CleanupChrootMount
      osutils.UmountTree(chroot)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/osutils.py", line 902, in UmountTree
      UmountDir(mount_pt, lazy=False, cleanup=False)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/osutils.py", line 864, in UmountDir
      runcmd(cmd, print_cmd=False)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/cros_build_lib.py", line 322, in SudoRunCommand
      return RunCommand(sudo_cmd, **kwargs)
    File "/b/swarming/w/ir/cache/cbuild/repository/chromite/lib/cros_build_lib.py", line 647, in RunCommand
      raise RunCommandError(msg, cmd_result)
  RunCommandError: return code: 1; command: sudo -n 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache' 'CROS_SUDO_KEEP_ALIVE=unknown' -- umount -d /b/swarming/w/ir/cache/cbuild/repository/chroot
  cmd=['sudo', '-n', 'CROS_CACHEDIR=/b/swarming/w/ir/cache/cbuild/repository/.cache', 'CROS_SUDO_KEEP_ALIVE=unknown', '--', 'umount', '-d', '/b/swarming/w/ir/cache/cbuild/repository/chroot']
   to fail.