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, 4 insertions, 4 deletions

diff --git a/src/nix-channel/nix-channel.cc b/src/nix-channel/nix-channel.cc
index 0f7858aa53..5b4c218199 100755
--- a/src/nix-channel/nix-channel.cc
+++ b/src/nix-channel/nix-channel.cc
@@ -85,7 +85,7 @@ static void update(const StringSet & channelNames)
         // got redirected in the process, so that we can grab the various parts of a nix channel
         // definition from a consistent location if the redirect changes mid-download.
         auto effectiveUrl = string{};
-        auto dl = makeDownloader();
+        auto dl = getDownloader();
         auto filename = dl->downloadCached(store, url, false, "", Hash(), &effectiveUrl);
         url = chomp(std::move(effectiveUrl));
 
@@ -114,10 +114,10 @@ static void update(const StringSet & channelNames)
         if (!unpacked) {
             // The URL doesn't unpack directly, so let's try treating it like a full channel folder with files in it
             // Check if the channel advertises a binary cache.
-            DownloadOptions opts;
-            opts.showProgress = DownloadOptions::no;
+            DownloadRequest request(url + "/binary-cache-url");
+            request.showProgress = DownloadRequest::no;
             try {
-                auto dlRes = dl->download(url + "/binary-cache-url", opts);
+                auto dlRes = dl->download(request);
                 extraAttrs = "binaryCacheURL = \"" + *dlRes.data + "\";";
             } catch (DownloadError & e) {
             }