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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="ssec-relnotes-1.2">
<title>Release 1.2 (December 6, 2012)</title>
<para>This release has the following improvements and changes:</para>
<itemizedlist>
<listitem>
<para>Nix has a new binary substituter mechanism: the
<emphasis>binary cache</emphasis>. 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 <option>binary-caches</option> contains a
list of URLs of binary caches. For instance, doing
<screen>
$ nix-env -i thunderbird --option binary-caches http://cache.nixos.org
</screen>
will install Thunderbird and its dependencies, using the available
pre-built binaries in <uri>http://cache.nixos.org</uri>.
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 <command>nix-pull</command> to
update your manifest. It’s also more scalable because you don’t
need to redownload a giant manifest file every time.
</para>
<para>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
(<uri>http://nixos.org/channels</uri>) you automatically get the
cache <uri>http://cache.nixos.org</uri>.</para>
<para>Binary caches are created using <command>nix-push</command>.
For details on the operation and format of binary caches, see the
<command>nix-push</command> manpage. More details are provided in
<link xlink:href="http://lists.science.uu.nl/pipermail/nix-dev/2012-September/009826.html">this
nix-dev posting</link>.</para>
</listitem>
<listitem>
<para>Multiple output support should now be usable. A derivation
can declare that it wants to produce multiple store paths by
saying something like
<programlisting>
outputs = [ "lib" "headers" "doc" ];
</programlisting>
This will cause Nix to pass the intended store path of each output
to the builder through the environment variables
<literal>lib</literal>, <literal>headers</literal> and
<literal>doc</literal>. Other packages can refer to a specific
output by referring to
<literal><replaceable>pkg</replaceable>.<replaceable>output</replaceable></literal>,
e.g.
<programlisting>
buildInputs = [ pkg.lib pkg.headers ];
</programlisting>
If you install a package with multiple outputs using
<command>nix-env</command>, each output path will be symlinked
into the user environment.</para>
</listitem>
<listitem>
<para>Dashes are now valid as part of identifiers and attribute
names.</para>
</listitem>
<listitem>
<para>The new operation <command>nix-store --repair-path</command>
allows corrupted or missing store paths to be repaired by
redownloading them. <command>nix-store --verify --check-contents
--repair</command> will scan and repair all paths in the Nix
store. Similarly, <command>nix-env</command>,
<command>nix-build</command>, <command>nix-instantiate</command>
and <command>nix-store --realise</command> have a
<option>--repair</option> flag to detect and fix bad paths by
rebuilding or redownloading them.</para>
</listitem>
<listitem>
<para>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:
<screen>
$ mount --bind /nix/store /nix/store
$ mount -o remount,ro,bind /nix/store
</screen>
Nix will automatically make <filename>/nix/store</filename>
writable as needed (using a private mount namespace) to allow
modifications.</para>
</listitem>
<listitem>
<para>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 <literal>auto-optimise-store</literal> to
<literal>true</literal> (disabled by default).</para>
</listitem>
<listitem>
<para>Nix now supports <command>xz</command> compression for NARs
in addition to <command>bzip2</command>. It compresses about 30%
better on typical archives and decompresses about twice as
fast.</para>
</listitem>
<listitem>
<para>Basic Nix expression evaluation profiling: setting the
environment variable <envar>NIX_COUNT_CALLS</envar> to
<literal>1</literal> will cause Nix to print how many times each
primop or function was executed.</para>
</listitem>
<listitem>
<para>New primops: <varname>concatLists</varname>,
<varname>elem</varname>, <varname>elemAt</varname> and
<varname>filter</varname>.</para>
</listitem>
<listitem>
<para>The command <command>nix-copy-closure</command> has a new
flag <option>--use-substitutes</option> (<option>-s</option>) to
download missing paths on the target machine using the substitute
mechanism.</para>
</listitem>
<listitem>
<para>The command <command>nix-worker</command> has been renamed
to <command>nix-daemon</command>. Support for running the Nix
worker in “slave” mode has been removed.</para>
</listitem>
<listitem>
<para>The <option>--help</option> flag of every Nix command now
invokes <command>man</command>.</para>
</listitem>
<listitem>
<para>Chroot builds are now supported on systemd machines.</para>
</listitem>
</itemizedlist>
<para>This release has contributions from Eelco Dolstra, Florian
Friesdorf, Mats Erik Andersson and Shea Levy.</para>
</chapter>
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