static Path gcRootsDir;

            Path outPath, drvPath;

            Path drvPath;

            xmlAttrs["drvPath"] = drv.queryDrvPath(state);

            xmlAttrs["drvPath"] = drvPath = drv.queryDrvPath(state);

            /* Register the derivation as a GC root.  !!! This
               registers roots for jobs that we may have already
               done. */
            Path root = gcRootsDir + "/" + baseNameOf(drvPath);
            if (!pathExists(root)) addPermRoot(drvPath, root, false);
            

        else if (arg == "--gc-roots-dir") {
            if (i == args.end()) throw UsageError("missing argument");
            gcRootsDir = *i++;
        }

    
    if (gcRootsDir == "") throw UsageError("--gc-roots-dir not specified");

    registerRoot getGCRootsDir

    registerRoot getGCRootsDir gcRootFor

    return "/nix/var/nix/gcroots/per-user/$ENV{LOGNAME}/hydra-roots";

    my $dir = "/nix/var/nix/gcroots/per-user/$ENV{LOGNAME}/hydra-roots";
    mkpath $dir if !-e $dir;
    return $dir;

sub registerRoot {

sub gcRootFor {

    return getGCRootsDir . basename $path;
}

    my $gcRootsDir = getGCRootsDir;

sub registerRoot {
    my ($path) = @_;
    
    my $link = gcRootFor $path;

    mkpath($gcRootsDir) if !-e $gcRootsDir;
    my $link = "$gcRootsDir/" . basename $path;
        

            or die "cannot create symlink in $gcRootsDir to $path";

            or die "cannot create GC root `$link' to `$path'";

    
    registerRoot $outPath;

            "--log-type flat --print-build-trace --realise $drvPath 2>&1";

            "--log-type flat --print-build-trace --realise $drvPath " .
            "--add-root " . gcRootFor $outPath . " 2>&1";

        
        # !!! this should really by done by nix-instantiate to prevent a GC race.
        registerRoot $drvPath;

        "hydra_eval_jobs", $nixExprPath, inputsToArgs($inputInfo));

        "hydra_eval_jobs", $nixExprPath, "--gc-roots-dir", getGCRootsDir,
        inputsToArgs($inputInfo));

    print STDERR "$stderr";

# For scheduled builds, we register the derivation and the output as a GC root.

# For scheduled builds, we register the derivation as a GC root.

        registerRoot $build->outpath;

        registerRoot $build->outpath if -e $build->outpath;