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, 28 insertions, 7 deletions

diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index 011b85f47..6b90ff202 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -4,24 +4,30 @@
 #include "hash.hh"
 
 #include <string>
+#include <future>
 
 namespace nix {
 
-struct DownloadOptions
+struct DownloadRequest
 {
+    std::string uri;
     std::string expectedETag;
     bool verifyTLS = true;
     enum { yes, no, automatic } showProgress = yes;
     bool head = false;
     size_t tries = 1;
-    unsigned int baseRetryTimeMs = 100;
+    unsigned int baseRetryTimeMs = 250;
+
+    DownloadRequest(const std::string & uri) : uri(uri) { }
 };
 
 struct DownloadResult
 {
+    enum Status { Success, NotFound, Forbidden, Misc, Transient };
+    Status status;
     bool cached;
-    string etag;
-    string effectiveUrl;
+    std::string etag;
+    std::string effectiveUrl;
     std::shared_ptr<std::string> data;
 };
 
@@ -29,14 +35,29 @@ class Store;
 
 struct Downloader
 {
-    virtual DownloadResult download(string url, const DownloadOptions & options) = 0;
+    /* Enqueue a download request, returning a future to the result of
+       the download. The future may throw a DownloadError
+       exception. */
+    virtual std::future<DownloadResult> enqueueDownload(const DownloadRequest & request) = 0;
+
+    /* Synchronously download a file. */
+    DownloadResult download(const DownloadRequest & request);
 
-    Path downloadCached(ref<Store> store, const string & url, bool unpack, string name = "",
-        const Hash & expectedHash = Hash(), string * effectiveUrl = nullptr);
+    /* Check if the specified file is already in ~/.cache/nix/tarballs
+       and is more recent than ‘tarball-ttl’ seconds. Otherwise,
+       use the recorded ETag to verify if the server has a more
+       recent version, and if so, download it to the Nix store. */
+    Path downloadCached(ref<Store> store, const string & uri, bool unpack, string name = "",
+        const Hash & expectedHash = Hash(), string * effectiveUri = nullptr);
 
     enum Error { NotFound, Forbidden, Misc, Transient };
 };
 
+/* Return a shared Downloader object. Using this object is preferred
+   because it enables connection reuse and HTTP/2 multiplexing. */
+ref<Downloader> getDownloader();
+
+/* Return a new Downloader object. */
 ref<Downloader> makeDownloader();
 
 class DownloadError : public Error