[Bug 2025965] Re: aptsources deb822 Section constructor fails on valid .sources file

[Julian Andres Klode]

** No longer affects: command-not-found (Ubuntu)

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Title:
  aptsources deb822 Section constructor fails on valid .sources file

Status in python-apt package in Ubuntu:
  New

Bug description:
  When attempting to parse (what I believe is) a perfectly valid
  deb822-formated `.sources` file, the library's `_deb822.Section`
  constructor parses my file incorrectly, resulting in an exception
  being thrown.

  At a low level, the error is `ValueError: Unable to parse section
  data` and is thrown in the `_deb822.Section` constructor, located at
  line 41 of `aptsources/_deb822.py`. The error was caused by the
  evaluation of `apt_pkg.TagSection(trimmed_section)` on that line with
  `trimmed_section` being an empty string.

  At a high level, based on a cursory debug, it seems the parser
  incorrectly considers the first three lines of my `.sources` file --
  which are, in fact, comments -- to be a stanza, and is trying to
  instantiate an object for them, despite them having no non-ignored
  content.

  Notably, -- supporting my conclusion -- this issue disappears entirely
  if I remove the first blank line in the file (located after those
  comments and before the first stanza).

  I am attaching my .sources file for reference.

  
  Notes:
  - FWIW, I don't use this library directly; the code at issue gets invoked by the `command-not-found` package/bash handler. (The warnings therefore show up every time I mistype a command.) In any case, I traced the issue down to this package.
  - My python3-apt package is version 2.5.3ubuntu1 from lunar/main.
  - This error began for me upon an upgrade from jammy to lunar. This sources file was unchanged during the upgrade (except for renaming `jammy` to `lunar`, and changing `Enabled` to `no` in the `partner` stanza).

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