Enable HTTP/2 support

The binary cache store can now use HTTP/2 to do lookups. This is much
more efficient than HTTP/1.1 due to multiplexing: we can issue many
requests in parallel over a single TCP connection. Thus it's no longer
necessary to use a bunch of concurrent TCP connections (25 by
default).

For example, downloading 802 .narinfo files from
https://cache.nixos.org/, using a single TCP connection, takes 11.8s
with HTTP/1.1, but only 0.61s with HTTP/2.

This did require a fairly substantial rewrite of the Downloader class
to use the curl multi interface, because otherwise curl wouldn't be
able to do multiplexing for us. As a bonus, we get connection reuse
even with HTTP/1.1. All downloads are now handled by a single worker
thread. Clients call Downloader::enqueueDownload() to tell the worker
thread to start the download, getting a std::future to the result.

src/libexpr/common-opts.cc

@@ -55,7 +55,7 @@ bool parseSearchPathArg(Strings::iterator & i,
Path lookupFileArg(EvalState & state, string s)
{
    if (isUri(s))
        return makeDownloader()->downloadCached(state.store, s, true);
        return getDownloader()->downloadCached(state.store, s, true);
    else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
        Path p = s.substr(1, s.size() - 2);
        return state.findFile(p);

src/libexpr/parser.y

@@ -662,7 +662,7 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
                // FIXME: support specifying revision/branch
                res = { true, exportGit(store, elem.second, "master") };
            else
                res = { true, makeDownloader()->downloadCached(store, elem.second, true) };
                res = { true, getDownloader()->downloadCached(store, elem.second, true) };
        } catch (DownloadError & e) {
            printMsg(lvlError, format("warning: Nix search path entry ‘%1%’ cannot be downloaded, ignoring") % elem.second);
            res = { false, "" };

src/libexpr/primops.cc

@@ -1769,7 +1769,7 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
    if (state.restricted && !expectedHash)
        throw Error(format("‘%1%’ is not allowed in restricted mode") % who);

    Path res = makeDownloader()->downloadCached(state.store, url, unpack, name, expectedHash);
    Path res = getDownloader()->downloadCached(state.store, url, unpack, name, expectedHash);
    mkString(v, res, PathSet({res}));
}

src/libstore/builtins.cc

@@ -17,13 +17,15 @@ void builtinFetchurl(const BasicDerivation & drv)
    auto fetch = [&](const string & url) {
        /* No need to do TLS verification, because we check the hash of
           the result anyway. */
        DownloadOptions options;
        options.verifyTLS = false;
        DownloadRequest request(url);
        request.verifyTLS = false;

        /* Show a progress indicator, even though stderr is not a tty. */
        options.showProgress = DownloadOptions::yes;
        request.showProgress = DownloadRequest::yes;

        auto data = makeDownloader()->download(url, options);
        /* Note: have to use a fresh downloader here because we're in
           a forked process. */
        auto data = makeDownloader()->download(request);
        assert(data.data);

        return data.data;