lgi: Fix cairo bindings search path

Since commit e44038bccab0cae, cairo-1.0.typelib contains an absolute
path to cairo in the nix store so that no $LD_LIBRARY_PATH hacks are
needed. However, this did not yet work for lgi, because lgi does
dlopen("libcairo.so.2") without a full path, too.

To make this work, this commit ensures that lgi first uses
gobject-introspection to load libcairo. This uses the full path provided
by the typelib. Afterwards, dlopen("libcairo.so.2") does not hit the
filesystem anymore since the library is already loaded.

This commit adds a patch that reorders some code in lgi's cairo
initialisation. Previously, this started with core.module('cairo', 2),
which is where the dlopen happens. Now, this code is moved down and
instead core.gi.cairo.resolve is used to load the definitions of some
enums first. This part of the code goes through gobject-introspection
and causes libcairo to be loaded.

Signed-off-by: Uli Schlachter <psychon@znc.in>

pkgs/top-level/lgi-find-cairo-through-typelib.patch

@@ -0,0 +1,47 @@
+Reorder some code so that the cairo-gobject library is used before this tries to
+dlopen("libcairo.so.2"). Since cairo-gobject depends (RT_DEPEND) on cairo, this
+causes cairo to be used and so the missing search path for the dlopen() call is
+not a problem.
+
+diff --git a/lgi/override/cairo.lua b/lgi/override/cairo.lua
+index ca8193f..019239b 100644
+--- a/lgi/override/cairo.lua
++++ b/lgi/override/cairo.lua
+@@ -20,18 +20,8 @@ local record = require 'lgi.record'
+ local enum = require 'lgi.enum'
+ local ti = ffi.types
+ 
+-cairo._module = core.module('cairo', 2)
+ local module_gobject = core.gi.cairo.resolve
+ 
+--- Versioning support.
+-function cairo.version_encode(major, minor, micro)
+-   return 10000 * major + 100 * minor + micro
+-end
+-cairo.version = core.callable.new {
+-   addr = cairo._module.cairo_version, ret = ti.int } ()
+-cairo.version_string = core.callable.new {
+-   addr = cairo._module.cairo_version_string, ret = ti.utf8 } ()
+-
+ -- Load some constants.
+ cairo._constant = {
+    MIME_TYPE_JP2 = 'image/jp2',
+@@ -58,6 +48,18 @@ for _, name in pairs {
+    end
+ end
+ 
++-- Load libcairo.so directly; this has to happen after the typelib was used
++cairo._module = core.module('cairo', 2)
++
++-- Versioning support.
++function cairo.version_encode(major, minor, micro)
++   return 10000 * major + 100 * minor + micro
++end
++cairo.version = core.callable.new {
++   addr = cairo._module.cairo_version, ret = ti.int } ()
++cairo.version_string = core.callable.new {
++   addr = cairo._module.cairo_version_string, ret = ti.utf8 } ()
++
+ -- Load definitions of all boxed records.
+ cairo._struct = cairo._struct or {}
+ for index, struct in pairs {

pkgs/top-level/lua-packages.nix

@@ -671,6 +671,8 @@ let
       sed -i "s|/usr/local|$out|" lgi/Makefile
     '';
 
+    patches = [ ./lgi-find-cairo-through-typelib.patch ];
+
     meta = with stdenv.lib; {
       description = "GObject-introspection based dynamic Lua binding to GObject based libraries";
       homepage    = https://github.com/pavouk/lgi;