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, 1 insertions, 1 deletions

diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index 00f5ae28d1dc..2bf2b2e5c3b1 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -158,7 +158,7 @@ int main(int argc, char * * argv)
             auto actualUri = resolveMirrorUri(state, uri);
 
             /* Download the file. */
-            auto result = makeDownloader()->download(actualUri, DownloadOptions());
+            auto result = getDownloader()->download(DownloadRequest(actualUri));
 
             AutoDelete tmpDir(createTempDir(), true);
             Path tmpFile = (Path) tmpDir + "/tmp";