/* The currently active builder threads. FIXME: We could re-use
       these, but since they're fairly long-running, it's probably not
       worth it. */
    // std::vector<std::thread> builderThreads;

            bool supported = false;
            {
                auto machines_(machines.lock()); // FIXME: use shared_mutex
                for (auto & m : *machines_)
                    if (m->supportsStep(r)) { supported = true; break; }
            }
            if (!supported) {
                printMsg(lvlError, format("aborting unsupported build %1%") % build->id);
                buildStatus = bsUnsupported;
                buildStepStatus = bssUnsupported;
            }

            if (buildStatus == bsSuccess) {
                bool supported = false;
                {
                    auto machines_(machines.lock()); // FIXME: use shared_mutex
                    for (auto & m : *machines_)
                        if (m->supportsStep(r)) { supported = true; break; }
                }
                if (!supported) {
                    printMsg(lvlError, format("aborting unsupported build %1%") % build->id);
                    buildStatus = bsUnsupported;
                    buildStepStatus = bssUnsupported;
                }
            }