[Bug 1834875] Re: cloud-init growpart race with udev

[Ryan Harper]

Yes, this is my read on the issue as well.  The trigger is related to
the inotify watch that systemd-udevd puts on the disk.  Something that
might help that we could try per xnox's comment around use of flock.

if growpart were to flock /dev/sda (we need to sort out what flags are
needed to prevent udev probe and rule execution) prior to running
sgdisk, and not release this flock until after partx runs (to inform the
kernel of the update) and a udevadm settle (and possibly a trigger of a
CHANGE event).

This should prevent early reads of the modified but not update-to-date
partition data in the kernel.

Lastly, another area of exploration is:  why isn't a change event
emitted when partx runs to update the kernel with new partition data?

If there is one, and the rules then generate the correct symlink;
another approach for growpart is for it to block until the size of the
new partition is correct.  growpart calculates what the new size should
be; so it could poll for this value thus blocking cloud-init's size
check from running until growpart is confident that the kernel has
updated the correct value (and the subsequent rules have completed
execution)!

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https://bugs.launchpad.net/bugs/1834875

Title:
  cloud-init growpart race with udev

Status in cloud-init:
  Incomplete
Status in cloud-utils:
  New
Status in linux-azure package in Ubuntu:
  New
Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  On Azure, it happens regularly (20-30%), that cloud-init's growpart
  module fails to extend the partition to full size.

  Such as in this example:

  ========================================

  2019-06-28 12:24:18,666 - util.py[DEBUG]: Running command ['growpart', '--dry-run', '/dev/sda', '1'] with allowed return codes [0] (shell=False, capture=True)
  2019-06-28 12:24:19,157 - util.py[DEBUG]: Running command ['growpart', '/dev/sda', '1'] with allowed return codes [0] (shell=False, capture=True)
  2019-06-28 12:24:19,726 - util.py[DEBUG]: resize_devices took 1.075 seconds
  2019-06-28 12:24:19,726 - handlers.py[DEBUG]: finish: init-network/config-growpart: FAIL: running config-growpart with frequency always
  2019-06-28 12:24:19,727 - util.py[WARNING]: Running module growpart (<module 'cloudinit.config.cc_growpart' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_growpart.py'>) failed
  2019-06-28 12:24:19,727 - util.py[DEBUG]: Running module growpart (<module 'cloudinit.config.cc_growpart' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_growpart.py'>) failed
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 812, in _run_modules
      freq=freq)
    File "/usr/lib/python3/dist-packages/cloudinit/cloud.py", line 54, in run
      return self._runners.run(name, functor, args, freq, clear_on_fail)
    File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 187, in run
      results = functor(*args)
    File "/usr/lib/python3/dist-packages/cloudinit/config/cc_growpart.py", line 351, in handle
      func=resize_devices, args=(resizer, devices))
    File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 2521, in log_time
      ret = func(*args, **kwargs)
    File "/usr/lib/python3/dist-packages/cloudinit/config/cc_growpart.py", line 298, in resize_devices
      (old, new) = resizer.resize(disk, ptnum, blockdev)
    File "/usr/lib/python3/dist-packages/cloudinit/config/cc_growpart.py", line 159, in resize
      return (before, get_size(partdev))
    File "/usr/lib/python3/dist-packages/cloudinit/config/cc_growpart.py", line 198, in get_size
      fd = os.open(filename, os.O_RDONLY)
  FileNotFoundError: [Errno 2] No such file or directory: '/dev/disk/by-partuuid/a5f2b49f-abd6-427f-bbc4-ba5559235cf3'

  ========================================

  @rcj suggested this is a race with udev. This seems to only happen on
  Cosmic and later.

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