[Bug 2101904] [NEW] do-release-update fails to recognise there is sufficient space when given a different Dir::Cache location

[Tony Lewis]

Public bug reported:

I only have 8GB of storage on my Dell Wyse 3040 device that runs as a
gateway.  In order to upgrade releases from 22.04 to 24.04, I plugged in
a USB drive, formatted it for EXT4 and mounted it on `/mnt`.  I then set
apt to use that for the cache directory.

When I run `do-release-upgrade` it still report insufficient space:
```
Not enough free disk space

The upgrade has aborted. The upgrade needs a total of 1024 M free
space on disk '/mnt'. Please free at least an additional 1024 M of
disk space on '/mnt'.


Restoring original system state

Aborting
```

The USB drive is mounted:
```
# mount | grep mnt
/dev/sda1 on /mnt type ext4 (rw,relatime)
```
And it has capacity:
```# df -h /mnt
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       7.1G  223M  6.5G   4% /mnt
```

And apt is configured to use it:
```
# apt-config dump | grep -i cache
... (snip) ...
Dir::Cache "/mnt";
Dir::Cache::archives "archives/";
Dir::Cache::srcpkgcache "srcpkgcache.bin";
Dir::Cache::pkgcache "pkgcache.bin";
... (snip) ...
binary::apt::APT::Cache "";
binary::apt::APT::Cache::Show "";
binary::apt::APT::Cache::Show::Version "2";
binary::apt::APT::Cache::AllVersions "0";
binary::apt::APT::Cache::ShowVirtuals "1";
binary::apt::APT::Cache::Search "";
binary::apt::APT::Cache::Search::Version "2";
binary::apt::APT::Cache::ShowDependencyType "1";
binary::apt::APT::Cache::ShowVersion "1";
```
```

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2101904

Title:
  do-release-update fails to recognise there is sufficient space when
  given a different Dir::Cache location

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  I only have 8GB of storage on my Dell Wyse 3040 device that runs as a
  gateway.  In order to upgrade releases from 22.04 to 24.04, I plugged
  in a USB drive, formatted it for EXT4 and mounted it on `/mnt`.  I
  then set apt to use that for the cache directory.

  When I run `do-release-upgrade` it still report insufficient space:
  ```
  Not enough free disk space

  The upgrade has aborted. The upgrade needs a total of 1024 M free
  space on disk '/mnt'. Please free at least an additional 1024 M of
  disk space on '/mnt'.

  
  Restoring original system state

  Aborting
  ```

  The USB drive is mounted:
  ```
  # mount | grep mnt
  /dev/sda1 on /mnt type ext4 (rw,relatime)
  ```
  And it has capacity:
  ```# df -h /mnt
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/sda1       7.1G  223M  6.5G   4% /mnt
  ```

  And apt is configured to use it:
  ```
  # apt-config dump | grep -i cache
  ... (snip) ...
  Dir::Cache "/mnt";
  Dir::Cache::archives "archives/";
  Dir::Cache::srcpkgcache "srcpkgcache.bin";
  Dir::Cache::pkgcache "pkgcache.bin";
  ... (snip) ...
  binary::apt::APT::Cache "";
  binary::apt::APT::Cache::Show "";
  binary::apt::APT::Cache::Show::Version "2";
  binary::apt::APT::Cache::AllVersions "0";
  binary::apt::APT::Cache::ShowVirtuals "1";
  binary::apt::APT::Cache::Search "";
  binary::apt::APT::Cache::Search::Version "2";
  binary::apt::APT::Cache::ShowDependencyType "1";
  binary::apt::APT::Cache::ShowVersion "1";
  ```
  ```

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