Gonna try something like this:

AsyncPostBuildHookInfo info;
info.act = std::make_unique<Activity>(
    *logger,
    lvlTalkative,
    actPostBuildHook,
    fmt("running async-post-build-hook '%s'", worker.settings.asyncPostBuildHook),
    Logger::Fields{worker.store.printStorePath(drvPath)});
info.sink = std::make_unique<LogSink>(*info.act);
info.out = std::make_unique<Pipe>();
info.out->create();
info.out->writeSide.close();
info.handleOutput = std::thread([&](Descriptor readSide, Sink & sink) {
    drainFD(readSide, sink);
}, info.out->readSide.get(), *info.sink);

// PushActivity pact(info.act->id);

auto pid = runAsyncPostBuildHook(worker.settings, worker.store, *info.out, drvPath, outputPaths);

worker.asyncPostBuildHooks[pid] = std::move(info);

Gonna try something like this:

AsyncPostBuildHookInfo info;
info.act = std::make_unique<Activity>(
    *logger,
    lvlTalkative,
    actPostBuildHook,
    fmt("running async-post-build-hook '%s'", worker.settings.asyncPostBuildHook),
    Logger::Fields{worker.store.printStorePath(drvPath)});
info.sink = std::make_unique<LogSink>(*info.act);
info.out = std::make_unique<Pipe>();
info.out->create();
info.out->writeSide.close();
info.handleOutput = std::thread([](Descriptor readSide, Sink & sink) {
    drainFD(readSide, sink);
}, info.out->readSide.get(), *info.sink);

// PushActivity pact(info.act->id);

auto pid = runAsyncPostBuildHook(worker.settings, worker.store, *info.out, drvPath, outputPaths);

worker.asyncPostBuildHooks[pid] = std::move(info);

i have made an abomination: binfmt-misc support in nix https://github.com/dramforever/nix/tree/binfmt-misc
see also tracking stuff in https://github.com/dramforever/nix/pull/25
tl;dr nix --extra-platforms x86_64-linux --binfmt-misc 'x86_64-linux=:x86_64-linux:M:...' build ...
"just" requires 6.7+ kernel and user namespaces, and it works with diverted store and uid-range

featuring: double namespacing!

not yet even draft pr quality, docs are "i may be here to answer questions", but it's working well enough that i think it's worth having y'all take a look

i have made an abomination: binfmt-misc support in nix https://github.com/dramforever/nix/tree/binfmt-misc
see also tracking stuff in https://github.com/dramforever/nix/pull/25
tl;dr nix --extra-platforms x86_64-linux --binfmt-misc 'x86_64-linux=:x86_64-linux:M:...' build ...
"just" requires linux 6.7+ kernel and user namespaces, and it works with diverted store and uid-range

featuring: double namespacing!

not yet even draft pr quality, docs are "i may be here to answer questions", but it's working well enough that i think it's worth having y'all take a look