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.

1 files changed, 6 insertions, 4 deletions

diff --git a/src/libstore/builtins.cc b/src/libstore/builtins.cc
index d3194a9057..a30f30906f 100644
--- a/src/libstore/builtins.cc
+++ b/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;