mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-12-25 05:26:47 +02:00
289 lines
10 KiB
XML
289 lines
10 KiB
XML
<section 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="sec-advanced-attributes">
|
||
|
||
<title>Advanced Attributes</title>
|
||
|
||
<para>Derivations can declare some infrequently used optional
|
||
attributes.</para>
|
||
|
||
<variablelist>
|
||
|
||
<varlistentry><term><varname>allowedReferences</varname></term>
|
||
|
||
<listitem><para>The optional attribute
|
||
<varname>allowedReferences</varname> specifies a list of legal
|
||
references (dependencies) of the output of the builder. For
|
||
example,
|
||
|
||
<programlisting>
|
||
allowedReferences = [];
|
||
</programlisting>
|
||
|
||
enforces that the output of a derivation cannot have any runtime
|
||
dependencies on its inputs. To allow an output to have a runtime
|
||
dependency on itself, use <literal>"out"</literal> as a list item.
|
||
This is used in NixOS to check that generated files such as
|
||
initial ramdisks for booting Linux don’t have accidental
|
||
dependencies on other paths in the Nix store.</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
|
||
<varlistentry><term><varname>allowedRequisites</varname></term>
|
||
|
||
<listitem><para>This attribute is similar to
|
||
<varname>allowedReferences</varname>, but it specifies the legal
|
||
requisites of the whole closure, so all the dependencies
|
||
recursively. For example,
|
||
|
||
<programlisting>
|
||
allowedRequisites = [ foobar ];
|
||
</programlisting>
|
||
|
||
enforces that the output of a derivation cannot have any other
|
||
runtime dependency than <varname>foobar</varname>, and in addition
|
||
it enforces that <varname>foobar</varname> itself doesn't
|
||
introduce any other dependency itself.</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
|
||
<varlistentry><term><varname>exportReferencesGraph</varname></term>
|
||
|
||
<listitem><para>This attribute allows builders access to the
|
||
references graph of their inputs. The attribute is a list of
|
||
inputs in the Nix store whose references graph the builder needs
|
||
to know. The value of this attribute should be a list of pairs
|
||
<literal>[ <replaceable>name1</replaceable>
|
||
<replaceable>path1</replaceable> <replaceable>name2</replaceable>
|
||
<replaceable>path2</replaceable> <replaceable>...</replaceable>
|
||
]</literal>. The references graph of each
|
||
<replaceable>pathN</replaceable> will be stored in a text file
|
||
<replaceable>nameN</replaceable> in the temporary build directory.
|
||
The text files have the format used by <command>nix-store
|
||
--register-validity</command> (with the deriver fields left
|
||
empty). For example, when the following derivation is built:
|
||
|
||
<programlisting>
|
||
derivation {
|
||
...
|
||
exportReferencesGraph = [ "libfoo-graph" libfoo ];
|
||
};
|
||
</programlisting>
|
||
|
||
the references graph of <literal>libfoo</literal> is placed in the
|
||
file <filename>libfoo-graph</filename> in the temporary build
|
||
directory.</para>
|
||
|
||
<para><varname>exportReferencesGraph</varname> is useful for
|
||
builders that want to do something with the closure of a store
|
||
path. Examples include the builders in NixOS that generate the
|
||
initial ramdisk for booting Linux (a <command>cpio</command>
|
||
archive containing the closure of the boot script) and the
|
||
ISO-9660 image for the installation CD (which is populated with a
|
||
Nix store containing the closure of a bootable NixOS
|
||
configuration).</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
|
||
<varlistentry><term><varname>impureEnvVars</varname></term>
|
||
|
||
<listitem><para>This attribute allows you to specify a list of
|
||
environment variables that should be passed from the environment
|
||
of the calling user to the builder. Usually, the environment is
|
||
cleared completely when the builder is executed, but with this
|
||
attribute you can allow specific environment variables to be
|
||
passed unmodified. For example, <function>fetchurl</function> in
|
||
Nixpkgs has the line
|
||
|
||
<programlisting>
|
||
impureEnvVars = [ "http_proxy" "https_proxy" <replaceable>...</replaceable> ];
|
||
</programlisting>
|
||
|
||
to make it use the proxy server configuration specified by the
|
||
user in the environment variables <envar>http_proxy</envar> and
|
||
friends.</para>
|
||
|
||
<para>This attribute is only allowed in <link
|
||
linkend="fixed-output-drvs">fixed-output derivations</link>, where
|
||
impurities such as these are okay since (the hash of) the output
|
||
is known in advance. It is ignored for all other
|
||
derivations.</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
|
||
<varlistentry xml:id="fixed-output-drvs">
|
||
<term><varname>outputHash</varname></term>
|
||
<term><varname>outputHashAlgo</varname></term>
|
||
<term><varname>outputHashMode</varname></term>
|
||
|
||
<listitem><para>These attributes declare that the derivation is a
|
||
so-called <emphasis>fixed-output derivation</emphasis>, which
|
||
means that a cryptographic hash of the output is already known in
|
||
advance. When the build of a fixed-output derivation finishes,
|
||
Nix computes the cryptographic hash of the output and compares it
|
||
to the hash declared with these attributes. If there is a
|
||
mismatch, the build fails.</para>
|
||
|
||
<para>The rationale for fixed-output derivations is derivations
|
||
such as those produced by the <function>fetchurl</function>
|
||
function. This function downloads a file from a given URL. To
|
||
ensure that the downloaded file has not been modified, the caller
|
||
must also specify a cryptographic hash of the file. For example,
|
||
|
||
<programlisting>
|
||
fetchurl {
|
||
url = http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz;
|
||
md5 = "70c9ccf9fac07f762c24f2df2290784d";
|
||
}
|
||
</programlisting>
|
||
|
||
It sometimes happens that the URL of the file changes, e.g.,
|
||
because servers are reorganised or no longer available. We then
|
||
must update the call to <function>fetchurl</function>, e.g.,
|
||
|
||
<programlisting>
|
||
fetchurl {
|
||
url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
|
||
md5 = "70c9ccf9fac07f762c24f2df2290784d";
|
||
}
|
||
</programlisting>
|
||
|
||
If a <function>fetchurl</function> derivation was treated like a
|
||
normal derivation, the output paths of the derivation and
|
||
<emphasis>all derivations depending on it</emphasis> would change.
|
||
For instance, if we were to change the URL of the Glibc source
|
||
distribution in Nixpkgs (a package on which almost all other
|
||
packages depend) massive rebuilds would be needed. This is
|
||
unfortunate for a change which we know cannot have a real effect
|
||
as it propagates upwards through the dependency graph.</para>
|
||
|
||
<para>For fixed-output derivations, on the other hand, the name of
|
||
the output path only depends on the <varname>outputHash*</varname>
|
||
and <varname>name</varname> attributes, while all other attributes
|
||
are ignored for the purpose of computing the output path. (The
|
||
<varname>name</varname> attribute is included because it is part
|
||
of the path.)</para>
|
||
|
||
<para>As an example, here is the (simplified) Nix expression for
|
||
<varname>fetchurl</varname>:
|
||
|
||
<programlisting>
|
||
{ stdenv, curl }: # The <command>curl</command> program is used for downloading.
|
||
|
||
{ url, md5 }:
|
||
|
||
stdenv.mkDerivation {
|
||
name = baseNameOf (toString url);
|
||
builder = ./builder.sh;
|
||
buildInputs = [ curl ];
|
||
|
||
# This is a fixed-output derivation; the output must be a regular
|
||
# file with MD5 hash <varname>md5</varname>.
|
||
outputHashMode = "flat";
|
||
outputHashAlgo = "md5";
|
||
outputHash = md5;
|
||
|
||
inherit url;
|
||
}
|
||
</programlisting>
|
||
|
||
</para>
|
||
|
||
<para>The <varname>outputHashAlgo</varname> attribute specifies
|
||
the hash algorithm used to compute the hash. It can currently be
|
||
<literal>"md5"</literal>, <literal>"sha1"</literal> or
|
||
<literal>"sha256"</literal>.</para>
|
||
|
||
<para>The <varname>outputHashMode</varname> attribute determines
|
||
how the hash is computed. It must be one of the following two
|
||
values:
|
||
|
||
<variablelist>
|
||
|
||
<varlistentry><term><literal>"flat"</literal></term>
|
||
|
||
<listitem><para>The output must be a non-executable regular
|
||
file. If it isn’t, the build fails. The hash is simply
|
||
computed over the contents of that file (so it’s equal to what
|
||
Unix commands like <command>md5sum</command> or
|
||
<command>sha1sum</command> produce).</para>
|
||
|
||
<para>This is the default.</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
<varlistentry><term><literal>"recursive"</literal></term>
|
||
|
||
<listitem><para>The hash is computed over the NAR archive dump
|
||
of the output (i.e., the result of <link
|
||
linkend="refsec-nix-store-dump"><command>nix-store
|
||
--dump</command></link>). In this case, the output can be
|
||
anything, including a directory tree.</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
</variablelist>
|
||
|
||
</para>
|
||
|
||
<para>The <varname>outputHash</varname> attribute, finally, must
|
||
be a string containing the hash in either hexadecimal or base-32
|
||
notation. (See the <link
|
||
linkend="sec-nix-hash"><command>nix-hash</command> command</link>
|
||
for information about converting to and from base-32
|
||
notation.)</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
|
||
<varlistentry><term><varname>passAsFile</varname></term>
|
||
|
||
<listitem><para>A list of names of attributes that should be
|
||
passed via files rather than environment variables. For example,
|
||
if you have
|
||
|
||
<programlisting>
|
||
passAsFile = ["big"];
|
||
big = "a very long string";
|
||
</programlisting>
|
||
|
||
then when the builder runs, the environment variable
|
||
<envar>bigPath</envar> will contain the absolute path to a
|
||
temporary file containing <literal>a very long
|
||
string</literal>. That is, for any attribute
|
||
<replaceable>x</replaceable> listed in
|
||
<varname>passAsFile</varname>, Nix will pass an environment
|
||
variable <envar><replaceable>x</replaceable>Path</envar> holding
|
||
the path of the file containing the value of attribute
|
||
<replaceable>x</replaceable>. This is useful when you need to pass
|
||
large strings to a builder, since most operating systems impose a
|
||
limit on the size of the environment (typically, a few hundred
|
||
kilobyte).</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
|
||
<varlistentry><term><varname>preferLocalBuild</varname></term>
|
||
|
||
<listitem><para>If this attribute is set to
|
||
<literal>true</literal>, it has two effects. First, the
|
||
derivation will always be built, not substituted, even if a
|
||
substitute is available. Second, if <link
|
||
linkend="chap-distributed-builds">distributed building is
|
||
enabled</link>, then, if possible, the derivaton will be built
|
||
locally instead of forwarded to a remote machine. This is
|
||
appropriate for trivial builders where the cost of doing a
|
||
download or remote build would exceed the cost of building
|
||
locally.</para></listitem>
|
||
|
||
</varlistentry>
|
||
|
||
</variablelist>
|
||
|
||
</section>
|