Nix/doc/manual/release-notes/rl-2.0.xml

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      xml:id="ssec-relnotes-2.0">

<title>Release 2.0 (2018-02-??)</title>

<para>The following incompatible changes have been made:</para>

<itemizedlist>

  <listitem>
    <para>The manifest-based substituter mechanism
    (<command>download-using-manifests</command>) has been <link
    xlink:href="https://github.com/NixOS/nix/commit/867967265b80946dfe1db72d40324b4f9af988ed">removed</link>. It
    has been superseded by the binary cache substituter mechanism
    since several years. As a result, the following programs have been
    removed:

    <itemizedlist>
      <listitem><para><command>nix-pull</command></para></listitem>
      <listitem><para><command>nix-generate-patches</command></para></listitem>
      <listitem><para><command>bsdiff</command></para></listitem>
      <listitem><para><command>bspatch</command></para></listitem>
    </itemizedlist>
    </para>
  </listitem>

  <listitem>
    <para>The “copy from other stores” substituter mechanism
    (<command>copy-from-other-stores</command> and the
    <envar>NIX_OTHER_STORES</envar> environment variable) has been
    removed. It was primarily used by the NixOS installer to copy
    available paths from the installation medium. The replacement is
    to use a chroot store as a substituter
    (e.g. <literal>--substituters /mnt</literal>), or to build into a
    chroot store (e.g. <literal>--store /mnt --substituter /</literal>).</para>
  </listitem>

  <listitem>
    <para>The command <command>nix-push</command> has been removed as
    part of the effort to eliminate Nix's dependency on Perl. You can
    use <command>nix copy</command> instead, e.g. <literal>nix copy
    --to /tmp/my-binary-cache <replaceable>paths…</replaceable></literal></para>
  </listitem>

  <listitem>
    <para>The “nested” log output feature (<option>--log-type
    pretty</option>) has been removed. As a result,
    <command>nix-log2xml</command> was also removed.</para>
  </listitem>

  <listitem>
    <para>OpenSSL-based signing has been <link
    xlink:href="https://github.com/NixOS/nix/commit/f435f8247553656774dd1b2c88e9de5d59cab203">removed</link>. This
    feature was never well-supported. A better alternative is provided
    by the <option>secret-key-files</option> and
    <option>trusted-public-keys</option> options.</para>
  </listitem>

  <listitem>
    <para>Failed build caching has been <link
    xlink:href="https://github.com/NixOS/nix/commit/8cffec84859cec8b610a2a22ab0c4d462a9351ff">removed</link>. This
    feature was introduced to support the Hydra continuous build
    system, but Hydra no longer uses it.</para>
  </listitem>

  <listitem>
    <para><filename>nix-mode.el</filename> has been removed from
    Nix. It is now <link
    xlink:href="https://github.com/NixOS/nix-mode">a separate
    repository</link> and can be installed through the MELPA package
    repository.</para>
  </listitem>

</itemizedlist>

<para>This release has the following new features:</para>

<itemizedlist>

  <listitem>
    <para>It introduces a new command named <command>nix</command>,
    which is intended to eventually replace all
    <command>nix-*</command> commands with a more consistent and
    better designed user interface. It currently provides replacements
    for some (but not all) of the functionality provided by
    <command>nix-store</command>, <command>nix-build</command>,
    <command>nix-shell -p</command>, <command>nix-env -qa</command>,
    <command>nix-instantiate --eval</command>,
    <command>nix-push</command> and
    <command>nix-copy-closure</command>. It has the following major
    features:</para>

    <itemizedlist>

      <listitem>
        <para>Unlike the legacy commands, it has a consistent way to
        refer to packages and package-like argumements (like store
        paths). For example, the following commands all copy the GNU
        Hello package to a remote machine:

        <screen>nix copy --to ssh://machine nixpkgs.hello</screen>
        <screen>nix copy --to ssh://machine /nix/store/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9-hello-2.10</screen>
        <screen>nix copy --to ssh://machine '(with import &lt;nixpkgs> {}; hello)'</screen>

        By contrast, <command>nix-copy-closure</command> only accepted
        store paths as arguments.</para>
      </listitem>

      <listitem>
        <para>It is self-documenting: <option>--help</option> shows
        all available command-line arguments. If
        <option>--help</option> is given after a subcommand, it shows
        examples for that subcommand. <command>nix
        --help-config</command> shows all configuration
        options.</para>
      </listitem>

      <listitem>
        <para>It is much less verbose. By default, it displays a
        single-line progress indicator that shows how many packages
        are left to be built or downloaded, and (if there are running
        builds) the most recent line of builder output. If a build
        fails, it shows the last few lines of builder output. The full
        build log can be retrieved using <command>nix
        log</command>.</para>
      </listitem>

      <listitem>
        <para>It <link
        xlink:href="https://github.com/NixOS/nix/commit/b8283773bd64d7da6859ed520ee19867742a03ba">provides</link>
        all <filename>nix.conf</filename> configuration options as
        command line flags. For example, instead of <literal>--option
        http-connections 100</literal> you can write
        <literal>--http-connections 100</literal>. Boolean options can
        be written as
        <literal>--<replaceable>foo</replaceable></literal> or
        <literal>--no-<replaceable>foo</replaceable></literal>
        (e.g. <option>--no-auto-optimise-store</option>).</para>
      </listitem>

      <listitem>
        <para>Many subcommands have a <option>--json</option> flag to
        write results to stdout in JSON format.</para>
      </listitem>

    </itemizedlist>

    <warning><para>Please note that the <command>nix</command> command
    is a work in progress and the interface is subject to
    change.</para></warning>

    <para>It provides the following high-level (“porcelain”)
    subcommands:</para>

    <itemizedlist>

      <listitem>
        <para><command>nix build</command> is a replacement for
        <command>nix-build</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix run</command> executes a command in an
        environment in which the specified packages are available. It
        is (roughly) a replacement for <command>nix-shell
        -p</command>. Unlike that command, it does not execute the
        command in a shell, and has a flag (<command>-c</command>)
        that specifies the unquoted command line to be
        executed.</para>

        <para>It is particularly useful in conjunction with chroot
        stores, allowing Linux users who do not have permission to
        install Nix in <command>/nix/store</command> to still use
        binary substitutes that assume
        <command>/nix/store</command>. For example,

        <screen>nix run --store ~/my-nix nixpkgs.hello -c hello --greeting 'Hi everybody!'</screen>

        downloads (or if not substitutes are available, builds) the
        GNU Hello package into
        <filename>~/my-nix/nix/store</filename>, then runs
        <command>hello</command> in a mount namespace where
        <filename>~/my-nix/nix/store</filename> is mounted onto
        <command>/nix/store</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix search</command> replaces <command>nix-env
        -qa</command>. It searches the available packages for
        occurences of a search string in the attribute name, package
        name or description. Unlike <command>nix-env -qa</command>, it
        has a cache to speed up subsequent searches.</para>
      </listitem>

      <listitem>
        <para><command>nix copy</command> copies paths between
        arbitrary Nix stores, generalising
        <command>nix-copy-closure</command> and
        <command>nix-push</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix repl</command> replaces the external
        program <command>nix-repl</command>. It provides an
        interactive environment for evaluating and building Nix
        expressions. Note that it uses <literal>linenoise-ng</literal>
        instead of GNU Readline.</para>
      </listitem>

      <listitem>
        <para><command>nix upgrade-nix</command> upgrades Nix to the
        latest stable version. This requires that Nix is installed in
        a profile. (Thus it won’t work on NixOS, or if it’s installed
        outside of the Nix store.)</para>
      </listitem>

      <listitem>
        <para><command>nix verify</command> checks whether store paths
        are unmodified and/or “trusted” (see below). It replaces
        <command>nix-store --verify</command> and <command>nix-store
        --verify-path</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix log</command> shows the build log of a
        package or path. If the build log is not available locally, it
        will try to obtain it from the configured substituters (such
        as <uri>cache.nixos.org</uri>, which now provides build
        logs).</para>
      </listitem>

      <listitem>
        <para><command>nix edit</command> opens the source code of a
        package in your editor.</para>
      </listitem>

      <listitem>
        <para><command>nix eval</command> replaces
        <command>nix-instantiate --eval</command>.</para>
      </listitem>

      <listitem>
        <para><command
        xlink:href="https://github.com/NixOS/nix/commit/d41c5eb13f4f3a37d80dbc6d3888644170c3b44a">nix
        why-depends</command> shows why one store path has another in
        its closure. This is primarily useful to finding the causes of
        closure bloat. For example,

        <screen>nix why-depends nixpkgs.vlc nixpkgs.libdrm.dev</screen>

        shows a chain of files and fragments of file contents that
        cause the VLC package to have the “dev” output of
        <literal>libdrm</literal> in its closure — an undesirable
        situation.</para>
      </listitem>

      <listitem>
        <para><command>nix path-info</command> shows information about
        store paths, replacing <command>nix-store -q</command>. A
        useful feature is the option <option>--closure-size</option>
        (<option>-S</option>). For example, the following command show
        the closure sizes of every path in the current NixOS system
        closure, sorted by size:

        <screen>nix path-info -rS /run/current-system | sort -nk2</screen>

        </para>
      </listitem>

      <listitem>
        <para><command>nix optimise-store</command> replaces
        <command>nix-store --optimise</command>. The main difference
        is that it has a progress indicator.</para>
      </listitem>

    </itemizedlist>

    <para>A number of low-level (“plumbing”) commands are also
    available:</para>

    <itemizedlist>

      <listitem>
        <para><command>nix ls-store</command> and <command>nix
        ls-nar</command> list the contents of a store path or NAR
        file. The former is primarily useful in conjunction with
        remote stores, e.g.

        <screen>nix ls-store --store https://cache.nixos.org/ -lR /nix/store/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9-hello-2.10</screen>

        lists the contents of path in a binary cache.</para>
      </listitem>

      <listitem>
        <para><command>nix cat-store</command> and <command>nix
        cat-nar</command> allow extracting a file from a store path or
        NAR file.</para>
      </listitem>

      <listitem>
        <para><command>nix dump-path</command> writes the contents of
        a store path to stdout in NAR format. This replaces
        <command>nix-store --dump</command>.</para>
      </listitem>

      <listitem>
        <para><command
        xlink:href="https://github.com/NixOS/nix/commit/e8d6ee7c1b90a2fe6d824f1a875acc56799ae6e2">nix
        show-derivation</command> displays a store derivation in JSON
        format. This is an alternative to
        <command>pp-aterm</command>.</para>
      </listitem>

      <listitem>
        <para><command
        xlink:href="https://github.com/NixOS/nix/commit/970366266b8df712f5f9cedb45af183ef5a8357f">nix
        add-to-store</command> replaces <command>nix-store
        --add</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix sign-paths</command> signs store
        paths. (TODO: add examples)</para>
      </listitem>

      <listitem>
        <para><command>nix copy-sigs</command> copies signatures from
        one store to another. (TODO: add examples and
        tests)</para>
      </listitem>

      <listitem>
        <para><command>nix show-config</command> shows all
        configuration options and their current values.</para>
      </listitem>

    </itemizedlist>

  </listitem>

  <listitem>
    <para>The store abstraction that Nix has had for a long time to
    support store access via the Nix daemon has been extended
    significantly. In particular, substituters (which used to be
    external programs such as
    <command>download-from-binary-cache</command>) are now subclasses
    of the abstract <classname>Store</classname> class. This allows
    many Nix commands to operate on such store types. For example,
    <command>nix path-info</command> shows information about paths in
    your local Nix store, while <command>nix path-info --store
    https://cache.nixos.org/</command> shows information about paths
    in the specified binary cache. Similarly,
    <command>nix-copy-closure</command>, <command>nix-push</command>
    and substitution are all instances of the general notion of
    copying paths between different kinds of Nix stores.</para>

    <para>Stores are specified using an URI-like syntax,
    e.g. <uri>https://cache.nixos.org/</uri> or
    <uri>ssh://machine</uri>. The following store types are supported:

    <itemizedlist>

      <listitem>

        <para><classname>LocalStore</classname> (stori URI
        <literal>local</literal> or an absolute path) and the misnamed
        <classname>RemoteStore</classname> (<literal>daemon</literal>)
        provide access to a local Nix store, the latter via the Nix
        daemon. You can use <literal>auto</literal> or the empty
        string to auto-select a local or daemon store depending on
        whether you have write permission to the Nix store. It is no
        longer necessary to set the <envar>NIX_REMOTE</envar>
        environment variable to use the Nix daemon.</para>

        <para>As noted above, <classname>LocalStore</classname> now
        supports chroot builds, allowing the “physical” location of
        the Nix store
        (e.g. <filename>/home/alice/nix/store</filename>) to differ
        from its “logical” location (typically
        <filename>/nix/store</filename>). This allows non-root users
        to use Nix while still getting the benefits from prebuilt
        binaries from <uri>cache.nixos.org</uri>.</para>

      </listitem>

      <listitem>

        <para><classname>BinaryCacheStore</classname> is the abstract
        superclass of all binary cache stores. It supports writing
        build logs and NAR content listings in JSON format.</para>

      </listitem>

      <listitem>

        <para><classname>HttpBinaryCacheStore</classname>
        (<literal>http://</literal>, <literal>https://</literal>)
        supports binary caches via HTTP or HTTPS. If the server
        supports <literal>PUT</literal> requests, it supports
        uploading store paths via commands such as <command>nix
        copy</command>.</para>

      </listitem>

      <listitem>

        <para><classname>LocalBinaryCacheStore</classname>
        (<literal>file://</literal>) supports binary caches in the
        local filesystem.</para>

      </listitem>

      <listitem>

        <para><classname>S3BinaryCacheStore</classname>
        (<literal>s3://</literal>) supports binary caches stored in
        Amazon S3, if enabled at compile time.</para>

      </listitem>

      <listitem>

        <para><classname>LegacySSHStore</classname> (<literal>ssh://</literal>)
        is used to implement remote builds and
        <command>nix-copy-closure</command>.</para>

      </listitem>

      <listitem>

        <para><classname>SSHStore</classname>
        (<literal>ssh-ng://</literal>) supports arbitrary Nix
        operations on a remote machine via the same protocol used by
        <command>nix-daemon</command>.</para>

      </listitem>

    </itemizedlist>

    </para>

  </listitem>

  <listitem>

    <para>Security has been improved in various ways:

    <itemizedlist>

      <listitem>
        <para>Nix now stores signatures for local store
        paths. When paths are copied between stores (e.g., copied from
        a binary cache to a local store), signatures are
        propagated.</para>

        <para>Locally-built paths are signed automatically using the
        secret keys specified by the <option>secret-key-files</option>
        store option. Secret/public key pairs can be generated using
        <command>nix-store
        --generate-binary-cache-key</command>. (TODO: rename)</para>

        <para>In addition, locally-built store paths are marked as
        “ultimately trusted”, but this bit is not propagated when
        paths are copied between stores.</para>
      </listitem>

      <listitem>
        <para>Content-addressable store paths no longer require
        signatures — they can be imported into a store by unprivileged
        users even if they lack signatures.</para>
      </listitem>

      <listitem>
        <para>The command <command>nix verify</command> checks whether
        the specified paths are trusted, i.e., have a certain number
        of trusted signatures, are ultimately trusted, or are
        content-addressed.</para>
      </listitem>

      <listitem>
        <para>Substitutions from binary caches <link
        xlink:href="https://github.com/NixOS/nix/commit/ecbc3fedd3d5bdc5a0e1a0a51b29062f2874ac8b">now</link>
        require signatures by default. This was already the case on
        NixOS.</para>
      </listitem>

      <listitem>
        <para>In Linux sandbox builds, we <link
        xlink:href="https://github.com/NixOS/nix/commit/eba840c8a13b465ace90172ff76a0db2899ab11b">now</link>
        use <filename>/build</filename> instead of
        <filename>/tmp</filename> as the temporary build
        directory. This fixes potential security problems when a build
        accidentally stores its <envar>TMPDIR</envar> in some
        security-sensitive place, such as an RPATH.</para>
      </listitem>

    </itemizedlist>

    </para>

  </listitem>

  <listitem>
    <para><emphasis>Pure evaluation mode</emphasis>. This is a variant
    of the existing restricted evaluation mode. In pure mode, the Nix
    evaluator forbids access to anything that could cause different
    evaluations of the same command line arguments to produce a
    different result. This includes builtin functions such as
    <function>builtins.getEnv</function>, but more importantly,
    <emphasis>all</emphasis> filesystem or network access unless a
    content hash or commit hash is specified. For example, calls to
    <function>builtins.fetchGit</function> are only allowed if a
    <varname>rev</varname> attribute is specified.</para>

    <para>The goal of this feature is to enable true reproducibility
    and traceability of builds (including NixOS system configurations)
    at the evaluation level. For example, in the future,
    <command>nixos-rebuild</command> might build configurations from a
    Nix expression in a Git repository in pure mode. That expression
    might fetch other repositories such as Nixpkgs via
    <function>builtins.fetchGit</function>. The commit hash of the
    top-level repository then uniquely identifies a running system,
    and, in conjunction with that repository, allows it to be
    reproduced or modified.</para>

  </listitem>

  <listitem>
    <para>There are several new features to support binary
    reproducibility (i.e. to help ensure that multiple builds of the
    same derivation produce exactly the same output). When
    <option>enforce-determinism</option> is set to
    <literal>false</literal>, it’s <link
    xlink:href="https://github.com/NixOS/nix/commit/8bdf83f936adae6f2c907a6d2541e80d4120f051">no
    longer</link> a fatal error if build rounds produce different
    output. Also, a hook named <option>diff-hook</option> is <link
    xlink:href="https://github.com/NixOS/nix/commit/9a313469a4bdea2d1e8df24d16289dc2a172a169w">provided</link>
    to allow you to run tools such as <command>diffoscope</command>
    when build rounds produce different output.</para>
  </listitem>

  <listitem>
    <para>Configuring remote builds is a lot easier now. Provided you
    are not using the Nix daemon, you can now just specify a remote
    build machine on the command line, e.g. <literal>--option builders
    'ssh://my-mac x86_64-darwin'</literal>. The environment variable
    <envar>NIX_BUILD_HOOK</envar> has been removed and is no longer
    needed. The environment variable <envar>NIX_REMOTE_SYSTEMS</envar>
    is still supported for compatibility, but it is also possible to
    specify builders in <command>nix.conf</command> by setting the
    option <literal>builders =
    @<replaceable>path</replaceable></literal>.</para>
  </listitem>

  <listitem>
    <para>If a fixed-output derivation produces a result with an
    incorrect hash, the output path is moved to the location
    corresponding to the actual hash and registered as valid. Thus, a
    subsequent build of the fixed-output derivation with the correct
    hash is unnecessary.</para>
  </listitem>

  <listitem>
    <para><command>nix-shell</command> <link
    xlink:href="https://github.com/NixOS/nix/commit/ea59f39326c8e9dc42dfed4bcbf597fbce58797c">now</link>
    sets the <varname>IN_NIX_SHELL</varname> environment variable
    during evaluation and in the shell itself. This can be used to
    perform different actions depending on whether you’re in a Nix
    shell or in a regular build. Nixpkgs provides
    <varname>lib.inNixShell</varname> to check this variable during
    evaluation.</para>
  </listitem>

  <listitem>
    <para><envar>NIX_PATH</envar> is now lazy, so URIs in the path are
    only downloaded if they are needed for evaluation.</para>
  </listitem>

  <listitem>
    <para>You can now use
    <uri>channel:<replaceable>channel-name</replaceable></uri> as a
    short-hand for
    <uri>https://nixos.org/channels/<replaceable>channel-name</replaceable>/nixexprs.tar.xz</uri>. For
    example, <literal>nix-build channel:nixos-15.09 -A hello</literal>
    will build the GNU Hello package from the
    <literal>nixos-15.09</literal> channel. In the future, this may
    use Git to fetch updates more efficiently.</para>
  </listitem>

  <listitem>
    <para>When <option>--no-build-output</option> is given, the last
    10 lines of the build log will be shown if a build
    fails.</para>
  </listitem>

  <listitem>
    <para>Networking has been improved:

    <itemizedlist>

      <listitem>
        <para>HTTP/2 is now supported. This makes binary cache lookups
        <link
        xlink:href="https://github.com/NixOS/nix/commit/90ad02bf626b885a5dd8967894e2eafc953bdf92">much
        more efficient</link>.</para>
      </listitem>

      <listitem>
        <para>We now retry downloads on many HTTP errors, making
        binary caches substituters more resilient to temporary
        failures.</para>
      </listitem>

      <listitem>
        <para>HTTP credentials can now be configured via the standard
        <filename>netrc</filename> mechanism.</para>
      </listitem>

      <listitem>
        <para>If S3 support is enabled at compile time,
        <uri>s3://</uri> URIs are <link
        xlink:href="https://github.com/NixOS/nix/commit/9ff9c3f2f80ba4108e9c945bbfda2c64735f987b">supported</link>
        in all places where Nix allows URIs.</para>
      </listitem>

      <listitem>
        <para>Brotli compression is now supported. In particular,
        <uri>cache.nixos.org</uri> build logs are now compressed using
        Brotli.</para>
      </listitem>

    </itemizedlist>

    </para>

  </listitem>

  <listitem>
    <para><command>nix-env</command> <link
    xlink:href="https://github.com/NixOS/nix/commit/b0cb11722626e906a73f10dd9a0c9eea29faf43a">now</link>
    ignores packages with bad derivation names (in particular those
    starting with a digit or containing a dot).</para>
  </listitem>

  <listitem>
    <para>Many configuration options have been renamed, either because
    they were unnecessarily verbose
    (e.g. <option>build-use-sandbox</option> is now just
    <option>sandbox</option>) or to reflect generalised behaviour
    (e.g. <option>binary-caches</option> is now
    <option>substituters</option> because it allows arbitrary store
    URIs). The old names are still supported for compatibility.</para>
  </listitem>

  <listitem>
    <para>The <option>max-jobs</option> option can <link
    xlink:href="https://github.com/NixOS/nix/commit/7251d048fa812d2551b7003bc9f13a8f5d4c95a5">now</link>
    be set to <literal>auto</literal> to use the number of CPUs in the
    system.</para>
  </listitem>

  <listitem>
    <para>Hashes can <link
    xlink:href="https://github.com/NixOS/nix/commit/c0015e87af70f539f24d2aa2bc224a9d8b84276b">now</link>
    be specified in base-64 format, in addition to base-16 and the
    non-standard base-32.</para>
  </listitem>

  <listitem>
    <para><command>nix-shell</command> now uses
    <varname>bashInteractive</varname> from Nixpkgs, rather than the
    <command>bash</command> command that happens to be in the caller’s
    <envar>PATH</envar>. This is especially important on macOS where
    the <command>bash</command> provided by the system is seriously
    outdated and cannot execute <literal>stdenv</literal>’s setup
    script.</para>
  </listitem>

  <listitem>
    <para>Nix can now automatically trigger a garbage collection if
    free disk space drops below a certain level during a build. This
    is configured using the <option>min-free</option> and
    <option>max-free</option> options.</para>
  </listitem>

  <listitem>
    <para><command>nix-store -q --roots</command> and
    <command>nix-store --gc --print-roots</command> now show temporary
    and in-memory roots.</para>
  </listitem>

  <listitem>
    <para>
      Nix can now be extended with plugins. See the documentation of
      the <option>plugin-files</option> option for more details.
    </para>
  </listitem>

</itemizedlist>

<para>The Nix language has the following new features:

<itemizedlist>

  <listitem>
    <para>It supports floating point numbers. They are based on the
    C++ <literal>float</literal> type and are supported by the
    existing numerical operators. Export and import to and from JSON
    and XML works, too.</para>
  </listitem>

  <listitem>
    <para>Derivation attributes can now reference the outputs of the
    derivation using the <function>placeholder</function> builtin
    function. For example, the attribute

<programlisting>
configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";
</programlisting>

    will cause the <envar>configureFlags</envar> environment variable
    to contain the actual store paths corresponding to the
    <literal>out</literal> and <literal>dev</literal> outputs. TODO:
    add docs.</para>
  </listitem>

</itemizedlist>

</para>

<para>The following builtin functions are new or extended:

<itemizedlist>

  <listitem>
    <para><function
    xlink:href="https://github.com/NixOS/nix/commit/38539b943a060d9cdfc24d6e5d997c0885b8aa2f">builtins.fetchGit</function>
    allows Git repositories to be fetched at evaluation time. Thus it
    differs from the <function>fetchgit</function> function in
    Nixpkgs, which fetches at build time and cannot be used to fetch
    Nix expressions during evaluation. A typical use case is to import
    external NixOS modules from your configuration, e.g.

    <programlisting>imports = [ (builtins.fetchGit https://github.com/edolstra/dwarffs + "/module.nix") ];</programlisting>

    </para>
  </listitem>

  <listitem>
    <para>Similarly, <function>builtins.fetchMercurial</function>
    allows you to fetch Mercurial repositories.</para>
  </listitem>

  <listitem>
    <para><function>builtins.path</function> generalises
    <function>builtins.filterSource</function> and path literals
    (e.g. <literal>./foo</literal>). It allows specifying a store path
    name that differs from the source path name
    (e.g. <literal>builtins.path { path = ./foo; name = "bar";
    }</literal>) and also supports filtering out unwanted
    files.</para>
  </listitem>

  <listitem>
    <para><function>builtins.fetchurl</function> and
    <function>builtins.fetchTarball</function> now support
    <varname>sha256</varname> and <varname>name</varname>
    attributes.</para>
  </listitem>

  <listitem>
    <para><function
    xlink:href="https://github.com/NixOS/nix/commit/b8867a0239b1930a16f9ef3f7f3e864b01416dff">builtins.split</function>
    splits a string using a POSIX extended regular expression as the
    separator.</para>
  </listitem>

  <listitem>
    <para><function
    xlink:href="https://github.com/NixOS/nix/commit/26d92017d3b36cff940dcb7d1611c42232edb81a">builtins.partition</function>
    partitions the elements of a list into two lists, depending on a
    Boolean predicate.</para>
  </listitem>

  <listitem>
    <para><literal>&lt;nix/fetchurl.nix&gt;</literal> now uses the
    content-addressable tarball cache at
    <uri>http://tarballs.nixos.org/</uri>, just like
    <function>fetchurl</function> in
    Nixpkgs. (f2682e6e18a76ecbfb8a12c17e3a0ca15c084197)</para>
  </listitem>

  <listitem>
    <para>In restricted and pure evaluation mode, builtin functions
    that download from the network (such as
    <function>fetchGit</function>) are permitted to fetch underneath a
    list of URI prefixes specified in the option
    <option>allowed-uris</option>.</para>
  </listitem>

</itemizedlist>

</para>

<para>The Nix build environment has the following changes:

<itemizedlist>

  <listitem>
    <para>Values such as Booleans, integers, (nested) lists and
    attribute sets can <link
    xlink:href="https://github.com/NixOS/nix/commit/6de33a9c675b187437a2e1abbcb290981a89ecb1">now</link>
    be passed to builders in a non-lossy way. If the special attribute
    <varname>__structuredAttrs</varname> is set to
    <literal>true</literal>, the other derivation attributes are
    serialised in JSON format and made available to the builder via
    the file <envar>.attrs.json</envar> in the builder’s temporary
    directory. This obviates the need for
    <varname>passAsFile</varname> since JSON files have no size
    restrictions, unlike process environments.</para>

    <para><link
    xlink:href="https://github.com/NixOS/nix/commit/2d5b1b24bf70a498e4c0b378704cfdb6471cc699">As
    a convenience to Bash builders</link>, Nix writes a script named
    <envar>.attrs.sh</envar> to the builder’s directory that
    initialises shell variables corresponding to all attributes that
    are representable in Bash. This includes non-nested (associative)
    arrays. For example, the attribute <literal>hardening.format =
    true</literal> ends up as the Bash associative array element
    <literal>${hardening[format]}</literal>.</para>
  </listitem>

  <listitem>
    <para>Builders can <link
    xlink:href="https://github.com/NixOS/nix/commit/88e6bb76de5564b3217be9688677d1c89101b2a3">now</link>
    communicate what build phase they are in by writing messages to
    the file descriptor specified in <envar>NIX_LOG_FD</envar>. The
    current phase is shown by the <command>nix</command> progress
    indicator.
    </para>
  </listitem>

  <listitem>
    <para>In Linux sandbox builds, we <link
    xlink:href="https://github.com/NixOS/nix/commit/a2d92bb20e82a0957067ede60e91fab256948b41">now</link>
    provide a default <filename>/bin/sh</filename> (namely
    <filename>ash</filename> from BusyBox).</para>
  </listitem>

  <listitem>
    <para>In structured attribute mode,
    <varname>exportReferencesGraph</varname> <link
    xlink:href="https://github.com/NixOS/nix/commit/c2b0d8749f7e77afc1c4b3e8dd36b7ee9720af4a">exports</link>
    extended information about closures in JSON format. In particular,
    it includes the sizes and hashes of paths. This is primarily
    useful for NixOS image builders.</para>
  </listitem>

  <listitem>
    <para>Builds are <link
    xlink:href="https://github.com/NixOS/nix/commit/21948deed99a3295e4d5666e027a6ca42dc00b40">now</link>
    killed as soon as Nix receives EOF on the builder’s stdout or
    stderr. This fixes a bug that allowed builds to hang Nix
    indefinitely, regardless of
    timeouts.</para>
  </listitem>

  <listitem>
    <para>The <option>sandbox-paths</option> configuration
    option can now specify optional paths by appending a
    <literal>?</literal>, e.g. <literal>/dev/nvidiactl?</literal> will
    bind-mount <varname>/dev/nvidiactl</varname> only if it
    exists.</para>
  </listitem>

  <listitem>
    <para>On Linux, builds are now executed in a user
    namespace with uid 1000 and gid 100.</para>
  </listitem>

</itemizedlist>

</para>

<para>A number of significant internal changes were made:

<itemizedlist>

  <listitem>
    <para>Nix no longer depends on Perl and all Perl components have
    been rewritten in C++ or removed. The Perl bindings that used to
    be part of Nix have been moved to a separate package,
    <literal>nix-perl</literal>.</para>
  </listitem>

  <listitem>
    <para>All <classname>Store</classname> classes are now
    thread-safe. <classname>RemoteStore</classname> supports multiple
    concurrent connections to the daemon. This is primarily useful in
    multi-threaded programs such as
    <command>hydra-queue-runner</command>.</para>
  </listitem>

</itemizedlist>

</para>

<para>This release has contributions from TBD.</para>

</section>