<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-package-notes">
 <title>Package Notes</title>
 <para>
  This chapter contains information about how to use and maintain the Nix expressions for a number of specific packages, such as the Linux kernel or X.org.
 </para>
<!--============================================================-->
 <section xml:id="sec-linux-kernel">
  <title>Linux kernel</title>

  <para>
   The Nix expressions to build the Linux kernel are in <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/kernel"><filename>pkgs/os-specific/linux/kernel</filename></link>.
  </para>

  <para>
   The function that builds the kernel has an argument <varname>kernelPatches</varname> which should be a list of <literal>{name, patch, extraConfig}</literal> attribute sets, where <varname>name</varname> is the name of the patch (which is included in the kernel’s <varname>meta.description</varname> attribute), <varname>patch</varname> is the patch itself (possibly compressed), and <varname>extraConfig</varname> (optional) is a string specifying extra options to be concatenated to the kernel configuration file (<filename>.config</filename>).
  </para>

  <para>
   The kernel derivation exports an attribute <varname>features</varname> specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour. For instance, if the kernel has the <varname>iwlwifi</varname> feature (i.e. has built-in support for Intel wireless chipsets), then NixOS doesn’t have to build the external <varname>iwlwifi</varname> package:
<programlisting>
modulesTree = [kernel]
  ++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
  ++ ...;
</programlisting>
  </para>

  <para>
   How to add a new (major) version of the Linux kernel to Nixpkgs:
   <orderedlist>
    <listitem>
     <para>
      Copy the old Nix expression (e.g. <filename>linux-2.6.21.nix</filename>) to the new one (e.g. <filename>linux-2.6.22.nix</filename>) and update it.
     </para>
    </listitem>
    <listitem>
     <para>
      Add the new kernel to <filename>all-packages.nix</filename> (e.g., create an attribute <varname>kernel_2_6_22</varname>).
     </para>
    </listitem>
    <listitem>
     <para>
      Now we’re going to update the kernel configuration. First unpack the kernel. Then for each supported platform (<literal>i686</literal>, <literal>x86_64</literal>, <literal>uml</literal>) do the following:
      <orderedlist>
       <listitem>
        <para>
         Make an copy from the old config (e.g. <filename>config-2.6.21-i686-smp</filename>) to the new one (e.g. <filename>config-2.6.22-i686-smp</filename>).
        </para>
       </listitem>
       <listitem>
        <para>
         Copy the config file for this platform (e.g. <filename>config-2.6.22-i686-smp</filename>) to <filename>.config</filename> in the kernel source tree.
        </para>
       </listitem>
       <listitem>
        <para>
         Run <literal>make oldconfig ARCH=<replaceable>{i386,x86_64,um}</replaceable></literal> and answer all questions. (For the uml configuration, also add <literal>SHELL=bash</literal>.) Make sure to keep the configuration consistent between platforms (i.e. don’t enable some feature on <literal>i686</literal> and disable it on <literal>x86_64</literal>).
        </para>
       </listitem>
       <listitem>
        <para>
         If needed you can also run <literal>make menuconfig</literal>:
<screen>
<prompt>$ </prompt>nix-env -i ncurses
<prompt>$ </prompt>export NIX_CFLAGS_LINK=-lncurses
<prompt>$ </prompt>make menuconfig ARCH=<replaceable>arch</replaceable></screen>
        </para>
       </listitem>
       <listitem>
        <para>
         Copy <filename>.config</filename> over the new config file (e.g. <filename>config-2.6.22-i686-smp</filename>).
        </para>
       </listitem>
      </orderedlist>
     </para>
    </listitem>
    <listitem>
     <para>
      Test building the kernel: <literal>nix-build -A kernel_2_6_22</literal>. If it compiles, ship it! For extra credit, try booting NixOS with it.
     </para>
    </listitem>
    <listitem>
     <para>
      It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the <varname>linuxPackagesFor</varname> function in <filename>all-packages.nix</filename> (such as the NVIDIA drivers, AUFS, etc.). If the updated packages aren’t backwards compatible with older kernels, you may need to keep the older versions around.
     </para>
    </listitem>
   </orderedlist>
  </para>
 </section>
<!--============================================================-->
 <section xml:id="sec-xorg">
  <title>X.org</title>

  <para>
   The Nix expressions for the X.org packages reside in <filename>pkgs/servers/x11/xorg/default.nix</filename>. This file is automatically generated from lists of tarballs in an X.org release. As such it should not be modified directly; rather, you should modify the lists, the generator script or the file <filename>pkgs/servers/x11/xorg/overrides.nix</filename>, in which you can override or add to the derivations produced by the generator.
  </para>

  <para>
   The generator is invoked as follows:
<screen>
<prompt>$ </prompt>cd pkgs/servers/x11/xorg
<prompt>$ </prompt>cat tarballs-7.5.list extra.list old.list \
  | perl ./generate-expr-from-tarballs.pl
</screen>
   For each of the tarballs in the <filename>.list</filename> files, the script downloads it, unpacks it, and searches its <filename>configure.ac</filename> and <filename>*.pc.in</filename> files for dependencies. This information is used to generate <filename>default.nix</filename>. The generator caches downloaded tarballs between runs. Pay close attention to the <literal>NOT FOUND: <replaceable>name</replaceable></literal> messages at the end of the run, since they may indicate missing dependencies. (Some might be optional dependencies, however.)
  </para>

  <para>
   A file like <filename>tarballs-7.5.list</filename> contains all tarballs in a X.org release. It can be generated like this:
<screen>
<prompt>$ </prompt>export i="mirror://xorg/X11R7.4/src/everything/"
<prompt>$ </prompt>cat $(PRINT_PATH=1 nix-prefetch-url $i | tail -n 1) \
  | perl -e 'while (&lt;>) { if (/(href|HREF)="([^"]*.bz2)"/) { print "$ENV{'i'}$2\n"; }; }' \
  | sort > tarballs-7.4.list
</screen>
   <filename>extra.list</filename> contains libraries that aren’t part of X.org proper, but are closely related to it, such as <literal>libxcb</literal>. <filename>old.list</filename> contains some packages that were removed from X.org, but are still needed by some people or by other packages (such as <varname>imake</varname>).
  </para>

  <para>
   If the expression for a package requires derivation attributes that the generator cannot figure out automatically (say, <varname>patches</varname> or a <varname>postInstall</varname> hook), you should modify <filename>pkgs/servers/x11/xorg/overrides.nix</filename>.
  </para>
 </section>
<!--============================================================-->
<!--
<section xml:id="sec-package-notes-gnome">
  <title>Gnome</title>
  <para>* Expression is auto-generated</para>
  <para>* How to update</para>
</section>
-->
<!--============================================================-->
<!--
<section xml:id="sec-package-notes-gcc">
  <title>GCC</title>
  <para>…</para>
</section>
-->
<!--============================================================-->
 <section xml:id="sec-eclipse">
  <title>Eclipse</title>

  <para>
   The Nix expressions related to the Eclipse platform and IDE are in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/eclipse"><filename>pkgs/applications/editors/eclipse</filename></link>.
  </para>

  <para>
   Nixpkgs provides a number of packages that will install Eclipse in its various forms. These range from the bare-bones Eclipse Platform to the more fully featured Eclipse SDK or Scala-IDE packages and multiple version are often available. It is possible to list available Eclipse packages by issuing the command:
<screen>
<prompt>$ </prompt>nix-env -f '&lt;nixpkgs&gt;' -qaP -A eclipses --description
</screen>
   Once an Eclipse variant is installed it can be run using the <command>eclipse</command> command, as expected. From within Eclipse it is then possible to install plugins in the usual manner by either manually specifying an Eclipse update site or by installing the Marketplace Client plugin and using it to discover and install other plugins. This installation method provides an Eclipse installation that closely resemble a manually installed Eclipse.
  </para>

  <para>
   If you prefer to install plugins in a more declarative manner then Nixpkgs also offer a number of Eclipse plugins that can be installed in an <emphasis>Eclipse environment</emphasis>. This type of environment is created using the function <varname>eclipseWithPlugins</varname> found inside the <varname>nixpkgs.eclipses</varname> attribute set. This function takes as argument <literal>{ eclipse, plugins ? [], jvmArgs ? [] }</literal> where <varname>eclipse</varname> is a one of the Eclipse packages described above, <varname>plugins</varname> is a list of plugin derivations, and <varname>jvmArgs</varname> is a list of arguments given to the JVM running the Eclipse. For example, say you wish to install the latest Eclipse Platform with the popular Eclipse Color Theme plugin and also allow Eclipse to use more RAM. You could then add
<screen>
packageOverrides = pkgs: {
  myEclipse = with pkgs.eclipses; eclipseWithPlugins {
    eclipse = eclipse-platform;
    jvmArgs = [ "-Xmx2048m" ];
    plugins = [ plugins.color-theme ];
  };
}
</screen>
   to your Nixpkgs configuration (<filename>~/.config/nixpkgs/config.nix</filename>) and install it by running <command>nix-env -f '&lt;nixpkgs&gt;' -iA myEclipse</command> and afterward run Eclipse as usual. It is possible to find out which plugins are available for installation using <varname>eclipseWithPlugins</varname> by running
<screen>
<prompt>$ </prompt>nix-env -f '&lt;nixpkgs&gt;' -qaP -A eclipses.plugins --description
</screen>
  </para>

  <para>
   If there is a need to install plugins that are not available in Nixpkgs then it may be possible to define these plugins outside Nixpkgs using the <varname>buildEclipseUpdateSite</varname> and <varname>buildEclipsePlugin</varname> functions found in the <varname>nixpkgs.eclipses.plugins</varname> attribute set. Use the <varname>buildEclipseUpdateSite</varname> function to install a plugin distributed as an Eclipse update site. This function takes <literal>{ name, src }</literal> as argument where <literal>src</literal> indicates the Eclipse update site archive. All Eclipse features and plugins within the downloaded update site will be installed. When an update site archive is not available then the <varname>buildEclipsePlugin</varname> function can be used to install a plugin that consists of a pair of feature and plugin JARs. This function takes an argument <literal>{ name, srcFeature, srcPlugin }</literal> where <literal>srcFeature</literal> and <literal>srcPlugin</literal> are the feature and plugin JARs, respectively.
  </para>

  <para>
   Expanding the previous example with two plugins using the above functions we have
<screen>
packageOverrides = pkgs: {
  myEclipse = with pkgs.eclipses; eclipseWithPlugins {
    eclipse = eclipse-platform;
    jvmArgs = [ "-Xmx2048m" ];
    plugins = [
      plugins.color-theme
      (plugins.buildEclipsePlugin {
        name = "myplugin1-1.0";
        srcFeature = fetchurl {
          url = "http://…/features/myplugin1.jar";
          sha256 = "123…";
        };
        srcPlugin = fetchurl {
          url = "http://…/plugins/myplugin1.jar";
          sha256 = "123…";
        };
      });
      (plugins.buildEclipseUpdateSite {
        name = "myplugin2-1.0";
        src = fetchurl {
          stripRoot = false;
          url = "http://…/myplugin2.zip";
          sha256 = "123…";
        };
      });
    ];
  };
}
</screen>
  </para>
 </section>
 <section xml:id="sec-elm">
  <title>Elm</title>

  <para>
   To start a development environment do <command>nix-shell -p elmPackages.elm elmPackages.elm-format</command>
  </para>

  <para>
   To update Elm compiler, see <filename>nixpkgs/pkgs/development/compilers/elm/README.md</filename>.
  </para>

  <para>
   To package Elm applications, <link xlink:href="https://github.com/hercules-ci/elm2nix#elm2nix">read about elm2nix</link>.
  </para>
 </section>
 <section xml:id="sec-kakoune">
  <title>Kakoune</title>

  <para>
   Kakoune can be built to autoload plugins:
<programlisting>(kakoune.override {
  configure = {
    plugins = with pkgs.kakounePlugins; [ parinfer-rust ];
  };
})</programlisting>
  </para>
 </section>
 <section xml:id="sec-shell-helpers">
  <title>Interactive shell helpers</title>

  <para>
   Some packages provide the shell integration to be more useful. But unlike other systems, nix doesn't have a standard share directory location. This is why a bunch <command>PACKAGE-share</command> scripts are shipped that print the location of the corresponding shared folder. Current list of such packages is as following:
   <itemizedlist>
    <listitem>
     <para>
      <literal>autojump</literal>: <command>autojump-share</command>
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>fzf</literal>: <command>fzf-share</command>
     </para>
    </listitem>
   </itemizedlist>
   E.g. <literal>autojump</literal> can then used in the .bashrc like this:
<screen>
  source "$(autojump-share)/autojump.bash"
</screen>
  </para>
 </section>
 <section xml:id="sec-weechat">
  <title>Weechat</title>

  <para>
   Weechat can be configured to include your choice of plugins, reducing its closure size from the default configuration which includes all available plugins. To make use of this functionality, install an expression that overrides its configuration such as
<programlisting>weechat.override {configure = {availablePlugins, ...}: {
    plugins = with availablePlugins; [ python perl ];
  }
}</programlisting>
   If the <literal>configure</literal> function returns an attrset without the <literal>plugins</literal> attribute, <literal>availablePlugins</literal> will be used automatically.
  </para>

  <para>
   The plugins currently available are <literal>python</literal>, <literal>perl</literal>, <literal>ruby</literal>, <literal>guile</literal>, <literal>tcl</literal> and <literal>lua</literal>.
  </para>

  <para>
   The python and perl plugins allows the addition of extra libraries. For instance, the <literal>inotify.py</literal> script in weechat-scripts requires D-Bus or libnotify, and the <literal>fish.py</literal> script requires pycrypto. To use these scripts, use the plugin's <literal>withPackages</literal> attribute:
<programlisting>weechat.override { configure = {availablePlugins, ...}: {
    plugins = with availablePlugins; [
            (python.withPackages (ps: with ps; [ pycrypto python-dbus ]))
        ];
    };
}
</programlisting>
  </para>

  <para>
   In order to also keep all default plugins installed, it is possible to use the following method:
<programlisting>weechat.override { configure = { availablePlugins, ... }: {
  plugins = builtins.attrValues (availablePlugins // {
    python = availablePlugins.python.withPackages (ps: with ps; [ pycrypto python-dbus ]);
  });
}; }
</programlisting>
  </para>

  <para>
   WeeChat allows to set defaults on startup using the <literal>--run-command</literal>. The <literal>configure</literal> method can be used to pass commands to the program:
<programlisting>weechat.override {
  configure = { availablePlugins, ... }: {
    init = ''
      /set foo bar
      /server add freenode chat.freenode.org
    '';
  };
}</programlisting>
   Further values can be added to the list of commands when running <literal>weechat --run-command "your-commands"</literal>.
  </para>

  <para>
   Additionally it's possible to specify scripts to be loaded when starting <literal>weechat</literal>. These will be loaded before the commands from <literal>init</literal>:
<programlisting>weechat.override {
  configure = { availablePlugins, ... }: {
    scripts = with pkgs.weechatScripts; [
      weechat-xmpp weechat-matrix-bridge wee-slack
    ];
    init = ''
      /set plugins.var.python.jabber.key "val"
    '':
  };
}</programlisting>
  </para>

  <para>
   In <literal>nixpkgs</literal> there's a subpackage which contains derivations for WeeChat scripts. Such derivations expect a <literal>passthru.scripts</literal> attribute which contains a list of all scripts inside the store path. Furthermore all scripts have to live in <literal>$out/share</literal>. An exemplary derivation looks like this:
<programlisting>{ stdenv, fetchurl }:

stdenv.mkDerivation {
  name = "exemplary-weechat-script";
  src = fetchurl {
    url = "https://scripts.tld/your-scripts.tar.gz";
    sha256 = "...";
  };
  passthru.scripts = [ "foo.py" "bar.lua" ];
  installPhase = ''
    mkdir $out/share
    cp foo.py $out/share
    cp bar.lua $out/share
  '';
}</programlisting>
  </para>
 </section>
 <section xml:id="sec-ibus-typing-booster">
  <title>ibus-engines.typing-booster</title>

  <para>
   This package is an ibus-based completion method to speed up typing.
  </para>

  <section xml:id="sec-ibus-typing-booster-activate">
   <title>Activating the engine</title>

   <para>
    IBus needs to be configured accordingly to activate <literal>typing-booster</literal>. The configuration depends on the desktop manager in use. For detailed instructions, please refer to the <link xlink:href="https://mike-fabian.github.io/ibus-typing-booster/documentation.html">upstream docs</link>.
   </para>

   <para>
    On NixOS you need to explicitly enable <literal>ibus</literal> with given engines before customizing your desktop to use <literal>typing-booster</literal>. This can be achieved using the <literal>ibus</literal> module:
<programlisting>{ pkgs, ... }: {
  i18n.inputMethod = {
    enabled = "ibus";
    ibus.engines = with pkgs.ibus-engines; [ typing-booster ];
  };
}</programlisting>
   </para>
  </section>

  <section xml:id="sec-ibus-typing-booster-customize-hunspell">
   <title>Using custom hunspell dictionaries</title>

   <para>
    The IBus engine is based on <literal>hunspell</literal> to support completion in many languages. By default the dictionaries <literal>de-de</literal>, <literal>en-us</literal>, <literal>fr-moderne</literal> <literal>es-es</literal>, <literal>it-it</literal>, <literal>sv-se</literal> and <literal>sv-fi</literal> are in use. To add another dictionary, the package can be overridden like this:
<programlisting>ibus-engines.typing-booster.override {
  langs = [ "de-at" "en-gb" ];
}</programlisting>
   </para>

   <para>
    <emphasis>Note: each language passed to <literal>langs</literal> must be an attribute name in <literal>pkgs.hunspellDicts</literal>.</emphasis>
   </para>
  </section>

  <section xml:id="sec-ibus-typing-booster-emoji-picker">
   <title>Built-in emoji picker</title>

   <para>
    The <literal>ibus-engines.typing-booster</literal> package contains a program named <literal>emoji-picker</literal>. To display all emojis correctly, a special font such as <literal>noto-fonts-emoji</literal> is needed:
   </para>

   <para>
    On NixOS it can be installed using the following expression:
<programlisting>{ pkgs, ... }: {
  fonts.fonts = with pkgs; [ noto-fonts-emoji ];
}</programlisting>
   </para>
  </section>
 </section>
 <section xml:id="sec-nginx">
  <title>Nginx</title>

  <para>
   <link xlink:href="https://nginx.org/">Nginx</link> is a reverse proxy and lightweight webserver.
  </para>

  <section xml:id="sec-nginx-etag">
   <title>ETags on static files served from the Nix store</title>

   <para>
    HTTP has a couple different mechanisms for caching to prevent clients from having to download the same content repeatedly if a resource has not changed since the last time it was requested. When nginx is used as a server for static files, it implements the caching mechanism based on the <link xlink:href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified"><literal>Last-Modified</literal></link> response header automatically; unfortunately, it works by using filesystem timestamps to determine the value of the <literal>Last-Modified</literal> header. This doesn't give the desired behavior when the file is in the Nix store, because all file timestamps are set to 0 (for reasons related to build reproducibility).
   </para>

   <para>
    Fortunately, HTTP supports an alternative (and more effective) caching mechanism: the <link xlink:href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag"><literal>ETag</literal></link> response header. The value of the <literal>ETag</literal> header specifies some identifier for the particular content that the server is sending (e.g. a hash). When a client makes a second request for the same resource, it sends that value back in an <literal>If-None-Match</literal> header. If the ETag value is unchanged, then the server does not need to resend the content.
   </para>

   <para>
    As of NixOS 19.09, the nginx package in Nixpkgs is patched such that when nginx serves a file out of <filename>/nix/store</filename>, the hash in the store path is used as the <literal>ETag</literal> header in the HTTP response, thus providing proper caching functionality. This happens automatically; you do not need to do modify any configuration to get this behavior.
   </para>
  </section>
 </section>
</chapter>
