We can now write

  throw Error("file '%s' not found", path);

instead of

  throw Error(format("file '%s' not found") % path);

and similarly

  printError("file '%s' not found", path);

instead of

  printMsg(lvlError, format("file '%s' not found") % path);

Document the { __toString } interface

Add a new option to disable documentation generation at configure time

We were passing "p=$PATH" rather than "p=$PATH;", resulting in some
invalid shell code.

Also, construct a separate environment for the child rather than
overwriting the parent's.

Otherwise the shell and its children will be bound to one CPU core...

The fact that queryPathInfo() is synchronous meant that we needed a
thread for every concurrent binary cache lookup, even though they end
up being handled by the same download thread. Requiring hundreds of
threads is not a good idea. So now there is an asynchronous version of
queryPathInfo() that takes a callback function to process the
result. Similarly, enqueueDownload() now takes a callback rather than
returning a future.

Thus, a command like

  nix path-info --store https://cache.nixos.org/ -r /nix/store/slljrzwmpygy1daay14kjszsr9xix063-nixos-16.09beta231.dccf8c5

that returns 4941 paths now takes 1.87s using only 2 threads (the main
thread and the downloader thread). (This is with a prewarmed
CloudFront.)

Having the logger function potentially throw exceptions is
Heisenbuggy.

It's a slight misnomer now because it actually limits *all* downloads,
not just binary cache lookups.

Also add a "enable-http2" option to allow disabling use of HTTP/2
(enabled by default).

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