This allows overriding the name component of the resulting Nix store
path, which is necessary if the base name of the URI contains
"illegal" characters.

This is in particular useful for fetchFromGitHub et al., ensuring that
the store path produced by nix-prefetch-url corresponds to what those
functions expect.

This allows nix-prefetch-url to prefetch the output of fetchzip and
its wrappers (like fetchFromGitHub). For example:

  $ nix-prefetch-url --unpack https://github.com/NixOS/patchelf/archive/0.8.tar.gz

or from a Nix expression:

  $ nix-prefetch-url -A nix-repl.src

In the latter case, --unpack can be omitted because nix-repl.src is a
fetchFromGitHub derivation and thus has "outputHashMode" set to
"recursive".

For example,

  $ nix-prefetch-url -A hello.src

will prefetch the file specified by the fetchurl call in the attribute
‘hello.src’ from the Nix expression in the current directory. This
differs from ‘nix-build -A hello.src’ in that it doesn't verify the
hash.

You can also specify a path to the Nix expression:

  $ nix-prefetch-url ~/Dev/nixpkgs -A hello.src

List elements (typically used in ‘patches’ attributes) also work:

  $ nix-prefetch-url -A portmidi.patches.0

It was strange to show "upgrading" when the version was getting lower.
This is left on "upgrading" when the versions are the same,
as I can't see any better wording.

Until now, if one explicitly installed a low-priority version,
nix-env --upgrade would downgrade it by default and even with --leq.
Let's never accept an upgrade with version not matching the upgradeType.
Additionally, let's never decrease the priority of an installed package;
you can use --install to force that.

Also refactor to use variable bestVersion instead of bestName,
as only version was used from it.

Fixes https://github.com/NixOS/nixpkgs/issues/9504.

Note that this means we may have a non-functional /bin/sh in the
chroot while rebuilding Bash or one of its dependencies. Ideally those
packages don't rely on /bin/sh though.

Fixes #616.

Fixes #609.

* src/libstore/build.cc (CHROOT_ENABLED): Remove HAVE_UNSHARE.

* If the path ends with a slash, drop it.
* If the remaining path doesn’t contain slashes, just return it.

Fixes #574.

This breaks the build on 32-bit systems.

http://hydra.nixos.org/build/24373658

Turns out that "nix-build -vvv" with chroots enabled has been broken
for some time, because some debug message got interpreted as an error.

This can be used to implement functions like ‘imap’ (or for that
matter, ‘map’) without the quadratic complexity incurred by calling
‘++’ repeatedly.

This is a generalisation of replaceChars in Nixpkgs.

This fixes the quadratic behaviour of concatStrings/concatStringsSep
in Nixpkgs.

This reduces the number of Bindings allocations by about 10%.

Pointed out by @cstrahan, thanks!

The value pointers of lists with 1 or 2 elements are now stored in the
list value itself. In particular, this makes the "concatMap (x: if
cond then [(f x)] else [])" idiom cheaper.

These are used thousands of times during NixOS evaluation, so it's
useful to speed them up.

Such as whether Nix is built with signed binary cache support, and the
location of the configuration file.

Turns out getgrouplist() is not POSIX.

http://hydra.nixos.org/build/23881243

This ensures that 1) the derivation doesn't change when Nix changes;
2) the derivation closure doesn't contain Nix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.

In particular, hydra-queue-runner can now distinguish between remote
build / substitution / already-valid. For instance, if a path already
existed on the remote side, we don't want to store a log file.

And make exportPath() less spammy by default.