[Bug 2125648] Re: Apt output for installation problems is not helpful

[Julian Andres Klode]

The reasoning for not installing libgs10-common is so complex that we
had to actually try it and backtrack and lost information about it in
the process; but it should mean that libgs10-common is not installable
at the root, i.e. it's a broken package.

Unfortunately I'm unable to provide further information at this point.
If you can reproduce this locally you can see the reason by passing -o
Debug::APT::Solver=2 and reading paragraphs starting with "Branch
failed".

The plan here would be to record the reasoning trace for the failure
when backtracking rather than losing it, but we can't quite do that now
because I don't know how it aligns with the non-chronological
backtracking which we do need for more complex scenarios (not
encountered so far in the wild) to not hang.

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Title:
  Apt output for installation problems is not helpful

Status in apt package in Ubuntu:
  New

Bug description:
  For a Questing build
  https://launchpadlibrarian.net/820374022/buildlog_ubuntu-questing-
  riscv64.coinor-dylp_1.10.4-3ubuntu1_BUILDING.txt.gz

  I see this apt output:

  The following packages have unmet dependencies:
   sbuild-build-depends-main-dummy : Depends: debhelper-compat (= 12)
                                     Depends: doxygen-latex but it is not going to be installed
                                     Depends: graphviz but it is not going to be installed
                                     Depends: texlive-formats-extra but it is not going to be installed
  E: Unable to satisfy dependencies. Reached two conflicting decisions:
     1. libgs10-common:riscv64=10.05.0dfsg1-0ubuntu2 is not selected for install
     2. libgs10-common:riscv64=10.05.0dfsg1-0ubuntu2 is selected for install because:
        1. sbuild-build-depends-main-dummy:riscv64=0.invalid.0 is selected for install
        2. sbuild-build-depends-main-dummy:riscv64 Depends doxygen-latex
        3. doxygen-latex:riscv64 Depends ghostscript
        4. ghostscript:riscv64 Depends libgs10 (= 10.05.0dfsg1-0ubuntu2)
        5. libgs10:riscv64 Depends libgs10-common (= 10.05.0dfsg1-0ubuntu2)
  apt-get failed.
  E: Package installation failed

  For the user it remains completely in the dark why libgs10:riscv64 would not be installed.
  No conflicts leading to non-installation are mentioned.

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