How to recover from ill-advisedly installing gcc-13 on Ubuntu 22.04

I thought I’d try compiling Chorale with gcc-13, which is newer than the default gcc-12 version in Ubuntu 22.04 – compiler diversity drives out bugs, and all that. Easy peasy, there’s a “toolchain testing” repository that packages gcc-13 for 22.04. Adding it was as straightforward as:

 sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo apt install g++-13

and, with a couple of new warnings fixed, everything compiled with gcc-13 as fine as ever. So I installed gcc-13 on the CI host in the same way, and pushed the branch.

And all the builds failed.

It turns out that installing gcc-13 updates the libstdc++ headers – to ones which neither the default clang-14 nor clang-18 is able to compile. I thought about making clang use (LLVM’s) libc++ instead of (GCC’s, the default) libstdc++ – but you can’t have both libc++ and libstdc++ installed in parallel, because they disagree about who owns libunwind. Oddly, libgstreamer depends directly on libstd++’s libunwind, so installing libc++’s libunwind instead would uninstall GStreamer. That’s not really on.

So there was nothing else to do but uninstall gcc-13 again. But unwinding that installation process was not straightforward: as well as the gcc-13 and g++-13 packages themselves, which live happily alongside gcc-12 and g++-12, the upgrade also upgraded the single, system-wide copies of several foundational packages such as libgcc-s1. Undoing all those upgrades was necessary too: partly because Clang unconditionally uses the highest installed version number of libstdc++ to locate its headers, but also because having these packages stuck at 13.x would mean never getting Ubuntu security updates for them ever again (updates which would have 12.x version numbers).

Somewhat naively I imagined that just un-adding the repository might solve that:

sudo apt-add-repository -r ppa:ubuntu-toolchain-r/test

but even after apt update the hoped-for, “You’ve got all these packages installed that match none of my repositories, shall I replace them with pukka ones?” message abjectly failed to appear. Nor does apt install pkg or apt upgrade override a newer locally-installed package with an older one from a repository. (Which I grudgingly admit is usually the correct thing to do.) And completely uninstalling these foundational packages, even momentarily, seemed like a bad idea – especially as this is actually Kubuntu, and the entire desktop is in C++ and links against libstdc++.

I ended up manually searching for packages using dpkg -l – fortunately, all the packages installed alongside gcc-13 shared its exact version number (13.1.0), which I could just grep for.

Each one of those packages needed to be replaced with the corresponding Ubuntu 22.04 version, which is indicated by using Ubuntu release codenames – for 22.04, that’s “Jammy”:

sudo apt install libgcc-s1/jammy
... etc., for about 30 packages

But worse, some of these packages depended on each other, so ideally there’d be just one apt install command-line. I ended up doing this:

 sudo apt-add-repository -r ppa:ubuntu-toolchain-r/test
 sudo apt remove g++-13 gcc-13
 sudo apt install `dpkg -l | fgrep 13.1.0 | grep -v -- -13 | sed -E 's/[ \t]+/\t/g' | cut -f 2 | sed -e 's,$,/jammy,'`
 sudo apt remove cpp-13 gcc-13-base

The long shell substitution in the install line uses dpkg to list all packages, then filters on the version 13.1.0, then filters out packages with “-13” in the actual name (which won’t have 22.04 equivalents), then use sed and cut to extract just the package name from the verbose dpkg output, then add the trailing /jammy and pass all the results as arguments to a single apt install invocation.

The apt install command politely told me exactly what it was going to do – downgrade those packages, no more or less than that – so it wasn’t stressful to give it the Y to proceed.