Release 1.2 (2012-12-06)

This release has the following improvements and changes:

Nix has a new binary substituter mechanism: the binary cache. A binary cache contains pre-built binaries of Nix packages. Whenever Nix wants to build a missing Nix store path, it will check a set of binary caches to see if any of them has a pre-built binary of that path. The configuration setting binary-caches contains a list of URLs of binary caches. For instance, doing
```
$ nix-env -i thunderbird --option binary-caches http://cache.nixos.org
```
will install Thunderbird and its dependencies, using the available pre-built binaries in http://cache.nixos.org. The main advantage over the old “manifest”-based method of getting pre-built binaries is that you don’t have to worry about your manifest being in sync with the Nix expressions you’re installing from; i.e., you don’t need to run nix-pull to update your manifest. It’s also more scalable because you don’t need to redownload a giant manifest file every time.

A Nix channel can provide a binary cache URL that will be used automatically if you subscribe to that channel. If you use the Nixpkgs or NixOS channels (http://nixos.org/channels) you automatically get the cache http://cache.nixos.org.

Binary caches are created using nix-push. For details on the operation and format of binary caches, see the nix-push manpage. More details are provided in this nix-dev posting.
Multiple output support should now be usable. A derivation can declare that it wants to produce multiple store paths by saying something like
```
outputs = [ "lib" "headers" "doc" ];
```
This will cause Nix to pass the intended store path of each output to the builder through the environment variables lib, headers and doc. Other packages can refer to a specific output by referring to pkg.output, e.g.
```
buildInputs = [ pkg.lib pkg.headers ];
```
If you install a package with multiple outputs using nix-env, each output path will be symlinked into the user environment.
Dashes are now valid as part of identifiers and attribute names.
The new operation nix-store --repair-path allows corrupted or missing store paths to be repaired by redownloading them. nix-store --verify --check-contents --repair will scan and repair all paths in the Nix store. Similarly, nix-env, nix-build, nix-instantiate and nix-store --realise have a --repair flag to detect and fix bad paths by rebuilding or redownloading them.
Nix no longer sets the immutable bit on files in the Nix store. Instead, the recommended way to guard the Nix store against accidental modification on Linux is to make it a read-only bind mount, like this:
```
$ mount --bind /nix/store /nix/store
$ mount -o remount,ro,bind /nix/store
```
Nix will automatically make /nix/store writable as needed (using a private mount namespace) to allow modifications.
Store optimisation (replacing identical files in the store with hard links) can now be done automatically every time a path is added to the store. This is enabled by setting the configuration option auto-optimise-store to true (disabled by default).
Nix now supports xz compression for NARs in addition to bzip2. It compresses about 30% better on typical archives and decompresses about twice as fast.
Basic Nix expression evaluation profiling: setting the environment variable NIX_COUNT_CALLS to 1 will cause Nix to print how many times each primop or function was executed.
New primops: concatLists, elem, elemAt and filter.
The command nix-copy-closure has a new flag --use-substitutes (-s) to download missing paths on the target machine using the substitute mechanism.
The command nix-worker has been renamed to nix-daemon. Support for running the Nix worker in “slave” mode has been removed.
The --help flag of every Nix command now invokes man.
Chroot builds are now supported on systemd machines.

This release has contributions from Eelco Dolstra, Florian Friesdorf, Mats Erik Andersson and Shea Levy.