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diff --git a/doc/manual/expressions/advanced-attributes.xml b/doc/manual/expressions/advanced-attributes.xml new file mode 100644 index 000000000..f8b84b98c --- /dev/null +++ b/doc/manual/expressions/advanced-attributes.xml @@ -0,0 +1,262 @@ +<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> +allowedReferences = [ 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 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>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><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> |