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Enable HTTP/2 support

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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.
This commit is contained in:
Eelco Dolstra 2016-09-14 16:00:40 +02:00
parent a75d11a7e6
commit 90ad02bf62
9 changed files with 433 additions and 210 deletions
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2
src/libexpr/primops.cc
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@ -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}));
}