nixpkgs/doc/package-notes.xml

<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>
$ nix-env -i ncurses
$ export NIX_CFLAGS_LINK=-lncurses
$ 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>
$ cd pkgs/servers/x11/xorg
$ 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>
$ export i="mirror://xorg/X11R7.4/src/everything/"
$ 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>
$ 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>
$ 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-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-steam">
  <title>Steam</title>

  <section xml:id="sec-steam-nix">
   <title>Steam in Nix</title>

   <para>
    Steam is distributed as a <filename>.deb</filename> file, for now only as
    an i686 package (the amd64 package only has documentation). When unpacked,
    it has a script called <filename>steam</filename> that in ubuntu (their
    target distro) would go to <filename>/usr/bin </filename>. When run for the
    first time, this script copies some files to the user's home, which include
    another script that is the ultimate responsible for launching the steam
    binary, which is also in $HOME.
   </para>

   <para>
    Nix problems and constraints:
    <itemizedlist>
     <listitem>
      <para>
       We don't have <filename>/bin/bash</filename> and many scripts point
       there. Similarly for <filename>/usr/bin/python</filename> .
      </para>
     </listitem>
     <listitem>
      <para>
       We don't have the dynamic loader in <filename>/lib </filename>.
      </para>
     </listitem>
     <listitem>
      <para>
       The <filename>steam.sh</filename> script in $HOME can not be patched, as
       it is checked and rewritten by steam.
      </para>
     </listitem>
     <listitem>
      <para>
       The steam binary cannot be patched, it's also checked.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The current approach to deploy Steam in NixOS is composing a FHS-compatible
    chroot environment, as documented
    <link xlink:href="http://sandervanderburg.blogspot.nl/2013/09/composing-fhs-compatible-chroot.html">here</link>.
    This allows us to have binaries in the expected paths without disrupting
    the system, and to avoid patching them to work in a non FHS environment.
   </para>
  </section>

  <section xml:id="sec-steam-play">
   <title>How to play</title>

   <para>
    For 64-bit systems it's important to have
<programlisting>hardware.opengl.driSupport32Bit = true;</programlisting>
    in your <filename>/etc/nixos/configuration.nix</filename>. You'll also need
<programlisting>hardware.pulseaudio.support32Bit = true;</programlisting>
    if you are using PulseAudio - this will enable 32bit ALSA apps integration.
    To use the Steam controller or other Steam supported controllers such as
    the DualShock 4 or Nintendo Switch Pro, you need to add
<programlisting>hardware.steam-hardware.enable = true;</programlisting>
    to your configuration.
   </para>
  </section>

  <section xml:id="sec-steam-troub">
   <title>Troubleshooting</title>

   <para>
    <variablelist>
     <varlistentry>
      <term>
       Steam fails to start. What do I do?
      </term>
      <listitem>
       <para>
        Try to run
<programlisting>strace steam</programlisting>
        to see what is causing steam to fail.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>
       Using the FOSS Radeon or nouveau (nvidia) drivers
      </term>
      <listitem>
       <itemizedlist>
        <listitem>
         <para>
          The <literal>newStdcpp</literal> parameter was removed since NixOS
          17.09 and should not be needed anymore.
         </para>
        </listitem>
        <listitem>
         <para>
          Steam ships statically linked with a version of libcrypto that
          conflics with the one dynamically loaded by radeonsi_dri.so. If you
          get the error
<programlisting>steam.sh: line 713: 7842 Segmentation fault (core dumped)</programlisting>
          have a look at
          <link xlink:href="https://github.com/NixOS/nixpkgs/pull/20269">this
          pull request</link>.
         </para>
        </listitem>
       </itemizedlist>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>
       Java
      </term>
      <listitem>
       <orderedlist>
        <listitem>
         <para>
          There is no java in steam chrootenv by default. If you get a message
          like
<programlisting>/home/foo/.local/share/Steam/SteamApps/common/towns/towns.sh: line 1: java: command not found</programlisting>
          You need to add
<programlisting> steam.override { withJava = true; };</programlisting>
          to your configuration.
         </para>
        </listitem>
       </orderedlist>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </section>

  <section xml:id="sec-steam-run">
   <title>steam-run</title>

   <para>
    The FHS-compatible chroot used for steam can also be used to run other
    linux games that expect a FHS environment. To do it, add
<programlisting>pkgs.(steam.override {
          nativeOnly = true;
          newStdcpp = true;
        }).run</programlisting>
    to your configuration, rebuild, and run the game with
<programlisting>steam-run ./foo</programlisting>
   </para>
  </section>
 </section>
 <section xml:id="sec-emacs">
  <title>Emacs</title>

  <section xml:id="sec-emacs-config">
   <title>Configuring Emacs</title>

   <para>
    The Emacs package comes with some extra helpers to make it easier to
    configure. <varname>emacsWithPackages</varname> allows you to manage
    packages from ELPA. This means that you will not have to install that
    packages from within Emacs. For instance, if you wanted to use
    <literal>company</literal>, <literal>counsel</literal>,
    <literal>flycheck</literal>, <literal>ivy</literal>,
    <literal>magit</literal>, <literal>projectile</literal>, and
    <literal>use-package</literal> you could use this as a
    <filename>~/.config/nixpkgs/config.nix</filename> override:
   </para>

<screen>
{
  packageOverrides = pkgs: with pkgs; {
    myEmacs = emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [
      company
      counsel
      flycheck
      ivy
      magit
      projectile
      use-package
    ]));
  }
}
</screen>

   <para>
    You can install it like any other packages via <command>nix-env -iA
    myEmacs</command>. However, this will only install those packages. It will
    not <literal>configure</literal> them for us. To do this, we need to
    provide a configuration file. Luckily, it is possible to do this from
    within Nix! By modifying the above example, we can make Emacs load a custom
    config file. The key is to create a package that provide a
    <filename>default.el</filename> file in
    <filename>/share/emacs/site-start/</filename>. Emacs knows to load this
    file automatically when it starts.
   </para>

<screen>
{
  packageOverrides = pkgs: with pkgs; rec {
    myEmacsConfig = writeText "default.el" ''
;; initialize package

(require 'package)
(package-initialize 'noactivate)
(eval-when-compile
  (require 'use-package))

;; load some packages

(use-package company
  :bind ("&lt;C-tab&gt;" . company-complete)
  :diminish company-mode
  :commands (company-mode global-company-mode)
  :defer 1
  :config
  (global-company-mode))

(use-package counsel
  :commands (counsel-descbinds)
  :bind (([remap execute-extended-command] . counsel-M-x)
         ("C-x C-f" . counsel-find-file)
         ("C-c g" . counsel-git)
         ("C-c j" . counsel-git-grep)
         ("C-c k" . counsel-ag)
         ("C-x l" . counsel-locate)
         ("M-y" . counsel-yank-pop)))

(use-package flycheck
  :defer 2
  :config (global-flycheck-mode))

(use-package ivy
  :defer 1
  :bind (("C-c C-r" . ivy-resume)
         ("C-x C-b" . ivy-switch-buffer)
         :map ivy-minibuffer-map
         ("C-j" . ivy-call))
  :diminish ivy-mode
  :commands ivy-mode
  :config
  (ivy-mode 1))

(use-package magit
  :defer
  :if (executable-find "git")
  :bind (("C-x g" . magit-status)
         ("C-x G" . magit-dispatch-popup))
  :init
  (setq magit-completing-read-function 'ivy-completing-read))

(use-package projectile
  :commands projectile-mode
  :bind-keymap ("C-c p" . projectile-command-map)
  :defer 5
  :config
  (projectile-global-mode))
    '';
    myEmacs = emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [
      (runCommand "default.el" {} ''
mkdir -p $out/share/emacs/site-lisp
cp ${myEmacsConfig} $out/share/emacs/site-lisp/default.el
'')
      company
      counsel
      flycheck
      ivy
      magit
      projectile
      use-package
    ]));
  };
}
</screen>

   <para>
    This provides a fairly full Emacs start file. It will load in addition to
    the user's presonal config. You can always disable it by passing
    <command>-q</command> to the Emacs command.
   </para>

   <para>
    Sometimes <varname>emacsWithPackages</varname> is not enough, as this
    package set has some priorities imposed on packages (with the lowest
    priority assigned to Melpa Unstable, and the highest for packages manually
    defined in <filename>pkgs/top-level/emacs-packages.nix</filename>). But you
    can't control this priorities when some package is installed as a
    dependency. You can override it on per-package-basis, providing all the
    required dependencies manually - but it's tedious and there is always a
    possibility that an unwanted dependency will sneak in through some other
    package. To completely override such a package you can use
    <varname>overrideScope'</varname>.
   </para>

<screen>
overrides = self: super: rec {
  haskell-mode = self.melpaPackages.haskell-mode;
  ...
};
((emacsPackagesNgGen emacs).overrideScope' overrides).emacsWithPackages (p: with p; [
  # here both these package will use haskell-mode of our own choice
  ghc-mod
  dante
])
</screen>
  </section>
 </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-citrix">
  <title>Citrix Receiver</title>

  <para>
   The <link xlink:href="https://www.citrix.com/products/receiver/">Citrix
   Receiver</link> is a remote desktop viewer which provides access to
   <link xlink:href="https://www.citrix.com/products/xenapp-xendesktop/">XenDesktop</link>
   installations.
  </para>

  <section xml:id="sec-citrix-base">
   <title>Basic usage</title>

   <para>
    The tarball archive needs to be downloaded manually as the licenses
    agreements of the vendor need to be accepted first. This is available at
    the
    <link xlink:href="https://www.citrix.com/downloads/citrix-receiver/">download
    page at citrix.com</link>. Then run <literal>nix-prefetch-url
    file://$PWD/linuxx64-$version.tar.gz</literal>. With the archive available
    in the store the package can be built and installed with Nix.
   </para>

   <para>
    <emphasis>Note: it's recommended to install <literal>Citrix
    Receiver</literal> using <literal>nix-env -i</literal> or globally to
    ensure that the <literal>.desktop</literal> files are installed properly
    into <literal>$XDG_CONFIG_DIRS</literal>. Otherwise it won't be possible to
    open <literal>.ica</literal> files automatically from the browser to start
    a Citrix connection.</emphasis>
   </para>
  </section>

  <section xml:id="sec-citrix-custom-certs">
   <title>Custom certificates</title>

   <para>
    The <literal>Citrix Receiver</literal> in <literal>nixpkgs</literal> trusts
    several certificates
    <link xlink:href="https://curl.haxx.se/docs/caextract.html">from the
    Mozilla database</link> by default. However several companies using Citrix
    might require their own corporate certificate. On distros with imperative
    packaging these certs can be stored easily in
    <link xlink:href="https://developer-docs.citrix.com/projects/receiver-for-linux-command-reference/en/13.7/"><literal>$ICAROOT</literal></link>,
    however this directory is a store path in <literal>nixpkgs</literal>. In
    order to work around this issue the package provides a simple mechanism to
    add custom certificates without rebuilding the entire package using
    <literal>symlinkJoin</literal>:
<programlisting>
<![CDATA[with import <nixpkgs> { config.allowUnfree = true; };
let extraCerts = [ ./custom-cert-1.pem ./custom-cert-2.pem /* ... */ ]; in
citrix_receiver.override {
  inherit extraCerts;
}]]>
</programlisting>
   </para>
  </section>
 </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>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="dlib">
  <title>DLib</title>

  <para>
    <link xlink:href="http://dlib.net/">DLib</link> is a modern, C++-based toolkit which
    provides several machine learning algorithms.
  </para>

  <section xml:id="compiling-without-avx-support">
   <title>Compiling without AVX support</title>

   <para>
     Especially older CPUs don't support
     <link xlink:href="https://en.wikipedia.org/wiki/Advanced_Vector_Extensions">AVX</link>
     (<abbrev>Advanced Vector Extensions</abbrev>) instructions that are used by DLib to
     optimize their algorithms.
   </para>

   <para>
    On the affected hardware errors like <literal>Illegal instruction</literal> will occur.
    In those cases AVX support needs to be disabled:
<programlisting>self: super: {
  dlib = super.dlib.override { avxSupport = false; };
}</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>