ExecResult r = exec(cmd);
        if (r.exitCode != 0) {
            throw new AssertionError(String.join(" ", cmd) + " failed: " + r);
        }
        return r.stdout;
    }

    private static final class ExecResult {
        final int exitCode;
        final String stdout;
        final String stderr;

        ExecResult(int exitCode, String stdout, String stderr) {