Full error message:

mktemp: failed to create directory via template '/mnt/stateful_partition/unencrypted/preserve/cros-update/tmp.XXXXXXXXXX': Read-only file system

Happened here:

https://uberchromegw.corp.google.com/i/chromeos/builders/veyron_speedy-paladin/builds/4419

https://pantheon.corp.google.com/storage/browser/chromeos-autotest-results/101809607-chromeos-test/chromeos4-row4-rack11-host6/debug/

02/17 10:27:49.912 DEBUG|             suite:0854| Scheduling video_VideoSanity.h264.arc
02/17 10:27:50.273 DEBUG|             suite:1206| Adding job keyval for video_VideoSanity.h264.arc=101809607-chromeos-test

02/17 10:34:21.518 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/get_au_status?build_name=veyron_speedy-paladin/R58-9292.0.0-rc3&force_update=True&pid=24173&host_name=chromeos4-row4-rack11-host6&full_update=False"''
02/17 10:34:22.813 DEBUG|        dev_server:1853| Failed to trigger auto-update process on devserver
02/17 10:34:22.814 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/handler_cleanup?pid=24173&host_name=chromeos4-row4-rack11-host6"''
02/17 10:34:24.016 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/collect_cros_au_log?pid=24173&host_name=chromeos4-row4-rack11-host6"''
02/17 10:34:25.275 DEBUG|        dev_server:1685| Saving auto-update logs into /usr/local/autotest/results/hosts/chromeos4-row4-rack11-host6/2622218-provision/20171702102822/autoupdate_logs/CrOS_update_chromeos4-row4-rack11-host6_24173.log
02/17 10:34:25.276 DEBUG|        dev_server:1952| Exception raised on auto_update attempt #1:
 Traceback (most recent call last):
  File "/home/chromeos-test/chromiumos/src/platform/dev/cros_update.py", line 222, in TriggerAU
    self._RootfsUpdate(chromeos_AU)
  File "/home/chromeos-test/chromiumos/src/platform/dev/cros_update.py", line 151, in _RootfsUpdate
    cros_updater.UpdateRootfs()
  File "/home/chromeos-test/chromiumos/chromite/lib/auto_updater.py", line 501, in UpdateRootfs
    raise RootfsUpdateError(error_msg % e)
RootfsUpdateError: Failed to perform rootfs update: DevServerStartupError('Devserver portfile does not exist!',)
02/17 10:34:25.277 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/kill_au_proc?host_name=chromeos4-row4-rack11-host6"''
02/17 10:34:46.661 DEBUG|        dev_server:1970| AU failed, trying IP instead of hostname: 100.115.195.148
02/17 10:34:46.662 DEBUG|        dev_server:1915| Start CrOS auto-update for host chromeos4-row4-rack11-host6 at 2 time(s).
02/17 10:34:46.663 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/cros_au?build_name=veyron_speedy-paladin/R58-9292.0.0-rc3&async=True&force_update=True&host_name=100.115.195.148&full_update=False"''
02/17 10:34:47.942 INFO |        dev_server:1735| Received response from devserver for cros_au call: '[true, 9803]'
02/17 10:34:47.944 DEBUG|        dev_server:1850| start process 9803 for auto_update in devserver
02/17 10:34:47.944 DEBUG|        dev_server:1756| Check the progress for auto-update process 9803
02/17 10:34:47.945 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/get_au_status?build_name=veyron_speedy-paladin/R58-9292.0.0-rc3&force_update=True&pid=9803&host_name=100.115.195.148&full_update=False"''
02/17 10:34:49.204 DEBUG|        dev_server:1853| Failed to trigger auto-update process on devserver
02/17 10:34:49.207 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/handler_cleanup?pid=9803&host_name=100.115.195.148"''
02/17 10:34:50.465 DEBUG|        base_utils:0185| Running 'ssh 100.115.219.129 'curl "http://100.115.219.129:8082/collect_cros_au_log?pid=9803&host_name=100.115.195.148"''
02/17 10:34:51.588 DEBUG|        dev_server:1685| Saving auto-update logs into /usr/local/autotest/results/hosts/chromeos4-row4-rack11-host6/2622218-provision/20171702102822/autoupdate_logs/CrOS_update_100.115.195.148_9803.log
02/17 10:34:51.590 DEBUG|        dev_server:1952| Exception raised on auto_update attempt #2:
 Traceback (most recent call last):
  File "/home/chromeos-test/chromiumos/src/platform/dev/cros_update.py", line 179, in TriggerAU
    yes=True)
  File "/home/chromeos-test/chromiumos/chromite/lib/auto_updater.py", line 663, in __init__
    clobber_stateful=clobber_stateful, yes=yes)
  File "/home/chromeos-test/chromiumos/chromite/lib/auto_updater.py", line 202, in __init__
    self.device_dev_dir = os.path.join(self.device.work_dir, 'src')
  File "/home/chromeos-test/chromiumos/chromite/lib/remote_access.py", line 652, in work_dir
    capture_output=True).output.strip()
  File "/home/chromeos-test/chromiumos/chromite/lib/remote_access.py", line 849, in BaseRunCommand
    return self.GetAgent().RemoteSh(cmd, **kwargs)
  File "/home/chromeos-test/chromiumos/chromite/lib/remote_access.py", line 336, in RemoteSh
    return cros_build_lib.RunCommand(ssh_cmd, **kwargs)
  File "/home/chromeos-test/chromiumos/chromite/lib/cros_build_lib.py", line 625, in RunCommand
    raise RunCommandError(msg, cmd_result)
RunCommandError: return code: 1; command: ssh -p 22 '-oConnectionAttempts=4' '-oUserKnownHostsFile=/dev/null' '-oProtocol=2' '-oConnectTimeout=30' '-oServerAliveCountMax=3' '-oStrictHostKeyChecking=no' '-oServerAliveInterval=10' '-oNumberOfPasswordPrompts=0' '-oIdentitiesOnly=yes' -i /tmp/ssh-tmpFk6bGn/testing_rsa root@100.115.195.148 -- mkdir -p /mnt/stateful_partition/unencrypted/preserve/cros-update '&&' mktemp -d '--tmpdir=/mnt/stateful_partition/unencrypted/preserve/cros-update'
Warning: Permanently added '100.115.195.148' (RSA) to the list of known hosts.
mktemp: failed to create directory via template '/mnt/stateful_partition/unencrypted/preserve/cros-update/tmp.XXXXXXXXXX': Read-only file system