[Bug 2130273] Re: do-release-upgrade deletes sources.list format entries during update to 25.10

[Nick Rosbrook]

This is basically a duplicate of bug 2110032 and bug 2086506.

The python-apt package cannot handle the [signed-by=] specifier in the
mozilla.list sources line. It considers this an "invalid" source, and
when ubuntu-release-upgrader is iterating over the configured sources,
it skips invalid ones. So, that's one problem.

The other problem is that despite not being able to migrate the file,
ubuntu-release-upgrader does not restore the original. That's a second
problem, and needs to be fixed in ubuntu-release-upgrader.


** Also affects: python-apt (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: python-apt (Ubuntu)
       Status: New => Confirmed

** Changed in: ubuntu-release-upgrader (Ubuntu)
       Status: Confirmed => Triaged

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https://bugs.launchpad.net/bugs/2130273

Title:
  do-release-upgrade deletes sources.list format entries during update
  to 25.10

Status in python-apt package in Ubuntu:
  Confirmed
Status in ubuntu-release-upgrader package in Ubuntu:
  Triaged

Bug description:
  To reproduce:
  1. On Ubuntu 25.04, add a repository in the old sources.list format (https://wiki.debian.org/SourcesList#sources.list_format). For example, I've added the Mozilla Firefox repo via following https://support.mozilla.org/en-US/kb/install-firefox-linux "For Debian Bookworm or older" instructions.
  2. Start upgrade process to 25.10 via do-release-upgrade. In the beginning of the process, the sources.list format repos will be moved into e.g. "mozilla.list.migrate", while the deb822 repos will be copied into e.g. "ubuntu.sources.distUpgrade" 
  3. Finish upgrade. All files with deb822 format will still be there, but mozilla.list will be gone entirely.

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