we no longer use thread pools for querying missing derivations. this
binds queryMissing to a single thread for now, but query performance
is still greatly improved. we may want to optimize the store code in
the near future too though since queryMissing is now fully cpu bound
Change-Id: I08a9c8cc199963ef5981572ca4a32d90dbdec028
The code used to query substituters for derivations has been rewritten slightly
to take advantage of our asynchronous runtime. Such queries run for every build
that could download from substituters and processes every derivation that isn't
yet present on the local system. Previously Lix would use http-connections to
limit query concurrency, even for modern caches that support HTTP/2 and have no
limit on how many queries can be run concurrently on one single connection. Lix
no longer does this, resulting in approximately 60% reduction in query time for
medium-sized closures (e.g. NixOS system closures) during testing, although the
exact number depends greatly on local network latency and generally improves as
latency increases. Unlike previously setting http-connections to 1 or other
low values no longer brings a massive penalty in query performance if the cache
in use by the querying system supports HTTP/2 (as e.g. cache.nixos.org does).
eldritch horrors
877b0d7121