Add the pre-build hook.

This hook can be used to set system specific per-derivation build
settings that don't fit into the derivation model and are too complex or
volatile to be hard-coded into nix. Currently, the pre-build hook can
only add chroot dirs/files.

The specific use case for this is systems where the operating system ABI
is more complex than just the kernel-supported system calls. For
example, on OS X there is a set of system-provided frameworks that can
reliably be accessed by any program linked to them, no matter the
version the program is running on. Unfortunately, those frameworks do
not necessarily live in the same locations on each version of OS X, nor
do their dependencies, and thus nix needs to know the specific version
of OS X currently running in order to make those frameworks available.
The pre-build hook is a perfect mechanism for doing just that.

1 files changed, 33 insertions, 0 deletions

diff --git a/doc/manual/command-ref/conf-file.xml b/doc/manual/command-ref/conf-file.xml
index 89b8aac7834f..329d2e485e07 100644
--- a/doc/manual/command-ref/conf-file.xml
+++ b/doc/manual/command-ref/conf-file.xml
@@ -562,6 +562,39 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
   </varlistentry>
 
 
+  <varlistentry xml:id="conf-pre-build-hook"><term><literal>pre-build-hook</literal></term>
+
+    <listitem>
+
+      <para>If set, the path to a program that can set extra
+      derivation-specific settings for this system. This is used for settings
+      that can't be captured by the derivation model itself and are too
+      variable between different versions of the same system to be hard-coded
+      into nix.</para>
+
+      <para>The hook listens on <literal>stdin</literal> for a derivation path.
+      It can then send a series of commands to modify various settings, followed
+      by an empty line to indicate completion. The currently recognized commands
+      are:</para>
+
+      <variablelist>
+        <varlistentry xml:id="extra-chroot-dirs"><term><literal>extra-chroot-dirs</literal></term>
+
+          <listitem>
+
+            <para>Pass a list of files and directories to be included in the
+            chroot for this build. One entry per line, terminated by an empty
+            line.</para>
+
+          </listitem>
+
+        </varlistentry>
+      </variablelist>
+    </listitem>
+
+  </varlistentry>
+
+
 </variablelist>
 
 </para>