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.

It was failing on some platforms.

http://hydra.nixos.org/build/39538866

GCC 4.9 doesn't like reassigning a std::stringstream.

http://hydra.nixos.org/build/40371644

That's just silly. Hopefully this also fixes the Debian build failure:

http://hydra.nixos.org/build/40371644

This largely reverts c68e5913. Running
builds as root breaks "cp -p", since when running as root, "cp -p"
assumes that it can succesfully chown() files. But that's not actually
the case since the user namespace doesn't provide a complete uid
mapping. So it barfs with a fatal error message ("cp: failed to
preserve ownership for 'foo': Invalid argument").

BASH_ENV causes all non-interactive shells called via eg. /etc/bashrc to
remove the rc-file before the main shell gets to run it. Completion
scripts will often do this. Fixes #976.

Adapted from and fixes #1034.

Implement resolve-system-dependencies in C++

Nix requires either $HOME or $XDG_CACHE_HOME to be set.

This fixes an assertion failure in "assert(goal);" in
Worker::waitForInput() after a substitution goal is cancelled by the
termination of another goal. The problem was the line

  //worker.childTerminated(shared_from_this()); // FIXME

in the SubstitutionGoal destructor. This was disabled because
shared_from_this() obviously doesn't work from a destructor. So we now
use a real pointer for object identity.

These don't support regex_replace either.

This reverts commit f32c6ed8. It
breaks the RPM builds because Nixpkgs's rpmBuild function only copies
in the .tar.gz file.

These don't support regex_replace.

http://hydra.nixos.org/build/39363999
http://hydra.nixos.org/build/39363981

The implementation of "partition" in Nixpkgs is O(n^2) (because of the
use of ++), and for some reason was causing stack overflows in
multi-threaded evaluation (not sure why).

This reduces "nix-env -qa --drv-path" runtime by 0.197s and memory
usage by 298 MiB (in non-Boehm mode).

Also, factor out JSON generation from value-to-json.{cc,hh}, and
support producing indented JSON.

Normally it's impossible to take a reference to the function passed to
callFunction, so some callers (e.g. ExprApp::eval) allocate that value
on the stack. For functors, a reference to the functor itself may be
kept, so we need to have it on the heap.

Fixes #1045

The inner lambda was returning a SQLite-internal char * rather than a
std::string, leading to Hydra errors liks

  Caught exception in Hydra::Controller::Root->narinfo "path ‘ø˜£â€™ is not in the Nix store at /nix/store/6mvvyb8fgwj23miyal5mdr8ik4ixk15w-hydra-0.1.1234.abcdef/libexec/hydra/lib/Hydra/Controller/Root.pm line 352."

That is, unless --file is specified, the Nix search path is
synthesized into an attribute set. Thus you can say

  $ nix build nixpkgs.hello

assuming $NIX_PATH contains an entry of the form "nixpkgs=...". This
is more verbose than

  $ nix build hello

but is less ambiguous.

For example, you can now say:

  configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";

The strings returned by the ‘placeholder’ builtin are replaced at
build time by the actual store paths corresponding to the specified
outputs.

Previously, you had to work around the inability to self-reference by doing stuff like:

  preConfigure = ''
    configureFlags+=" --prefix $out --includedir=$dev"
  '';

or rely on ad-hoc variable interpolation semantics in Autoconf or Make
(e.g. --prefix=\$(out)), which doesn't always work.

Allow contexted strings in replaceStrings