Bug#1040299: apt changelog should display local changelogs if fetching fails

[Fabian Grünbichler <f.gruenbichler@proxmox.com>]

Package: apt
Version: 2.6.1
Severity: minor

Dear Maintainer,

with the recent change of long changelogs being truncated automatically, and
the full version of such truncated changelogs always being fetched by
`apt[-get] changelog`, there is no way anymore to use `apt changelog` to just
display "the changelog", if either

- the repository metadata doesn't contain a `Changelogs` reference (many 3rd
  party repositories, but also stable-security and stable-updates!)
- the "online" changelog does not exist (yet)

this is the case for security updates as well, e.g. today's ghostscript update
for bookworm (10.0.0-dfsg11+deb12u1):
- before the update, it fails because the changelog for that version is not
  found on metadata.ftp-masters.debian.org (ghostscript is also shipped by the
  "main" repository that has metadata.f-m.d.o as Changelogs host)
- after the update, it still fails for the same reason, even though at least
  the truncated changelog is available by virtue of the package being
  installed..

it would be nice if we could get back the old behaviour of "if fetching fails,
display local changelog contents" (possibly with a hint/warning about that fact).

it also seems to me like it is possible to confuse apt about where to fetch
changelogs from, as per the above example (security upgrade fetches changelog
from regular repo) - but since that requires control of the Release file, it's
probably not much of an issue in practice.

-- System Information:
Debian Release: 12.0
  APT prefers stable-security
  APT policy: (500, 'stable-security'), (500, 'stable-debug'), (500, 'stable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages apt depends on:
ii  adduser                 3.134
ii  debian-archive-keyring  2023.3
ii  gpgv                    2.2.40-1.1
ii  libapt-pkg6.0           2.6.1
ii  libc6                   2.36-9
ii  libgcc-s1               12.2.0-14
ii  libgnutls30             3.7.9-2
ii  libseccomp2             2.5.4-1+b3
ii  libstdc++6              12.2.0-14
ii  libsystemd0             252.6-1

Versions of packages apt recommends:
ii  ca-certificates  20230311

Versions of packages apt suggests:
pn  apt-doc         <none>
ii  aptitude        0.8.13-5
ii  dpkg-dev        1.21.22
ii  gnupg           2.2.40-1.1
pn  powermgmt-base  <none>

-- no debconf information