[Bug 2084032] Re: pc-kernel umount failure

[Olivier Gayot]

In all the reports I've seen so far, there's a cancellation during
`_get_systems` when we're in the middle of requesting info from snapd
though its API:

```python
return await self.app.snapdapi.v2.systems[label].GET()
```

However, cancelling on the client (i.e., subiquity) side does not
necessarily means the server (i.e., snapd will stop executing its query
handler). So we can't immediately expect snapd not to be using the
systems directory anymore. We'd need to wait until the request has been
responded.

```python
        self._examine_systems_task = SingleInstanceTask(self._examine_systems)
```
^ this is created with `cancel_restart=True` (the default) and is probably the reason for cancellation?

```python
        self.app.hub.subscribe(
            (InstallerChannels.CONFIGURED, "source"),
            self._examine_systems_task.start_sync,
        )
```

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https://bugs.launchpad.net/bugs/2084032

Title:
  pc-kernel umount failure

Status in subiquity:
  In Progress
Status in subiquity package in Ubuntu:
  Confirmed

Bug description:
  Doing some install tests with the Oracular October 8 ISO, I had an
  installer crash.

  I booted with a live system and proceed to wipe the internal SSD hard
  disk using `blkdiscard -f /dev/sda` (this disk is quite old, so it's
  recognized as a SATA one, not as a NVME). After that, I completely
  shut down the computer, and booted the Oracular Desktop LiveCD using
  an USB stick created with the October, 8 ISO. I chose "Spanish" and
  "Spanish keyboard", no `accessibility` options, connected to my WiFi
  (no Ethernet available in the laptop), clicked next, next, chose
  `extended selection` to install "all the stuff", clicked next, marked
  both `Install third party software for graphics and WiFi hardware` and
  `Download and install support for additional media formats`, and when
  I press the `Next` button, the installer crashed.

  I attach the contents of the `/var/log` folder.

  I also checked the first 400KBytes of the hard disk, and it's all
  zeros. No data there, as expected after a `blkdiscard`.

  The laptop is an Asus Zenbook UX21E from 2012, with 4GBytes of RAM,
  with an Intel Core i5-2467M CPU.

  The bug can be consistently reproduced after using `blkdiscard`. But
  if the disk has partitions, it doesn't seem to fail.

  I didn't test with other languages. Also, didn't test using `normal
  selection` or not marking `third party drivers` or `additional media
  format`. Only tested that specific combination.

  No FDE/TPM install is being done (the laptop is too old for that).

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