libglycin: Introduce setup hook for adding loader paths to wrapper

This setup hook discovers glycin loaders in the inputs list it is placed into, and adds them to `gappsWrapperArgs`. This should simplify glycin-dependent programs’ expressions.

Just add `libglycin.setupHook` (for Rust programs) or `libglycin` (for other languages) to `buildInputs`. The latter will also pull in the setup hook.

Note, the setup hook needs to go to the `buildInputs` since we cannot have a different offset when used as standalone hook instead of propagated from `libglycin`. We chose `hostOffset` to make it work with the proper placement of `libglycin` in `buildInputs`.

Co-authored-by: Seth Flynn <getchoo@tuta.io>
Co-authored-by: Jan Tojnar <jtojnar@gmail.com>

doc/hooks/libglycin.section.md

@@ -2,7 +2,7 @@
 
 [Glycin](https://gitlab.gnome.org/GNOME/glycin) is a library for sandboxed and extendable image loading.
 
-For most applications using it, individual image formats are loaded through binaries provided by `glycin-loaders`. The paths of these loaders must be injected into the environment, e.g. using [`wrapGAppsHook`](#ssec-gnome-hooks).
+[]{#libglycin-setup-hook} For most applications using it, individual image formats are loaded through binaries provided by `glycin-loaders`. The paths of these loaders must be injected into the environment, e.g. using [`wrapGAppsHook`](#ssec-gnome-hooks). `libglycin.setupHook` will do that.
 
 []{#libglycin-patch-vendor-hook} Additionally, for Rust projects `glycin` Rust crate itself requires a patch to become self-contained. `libglycin.patchVendorHook` will do that. This is not needed for projects using the ELF library from `libglycin` package.
 
@@ -27,11 +27,10 @@ rustPlatform.buildRustPackage {
     libglycin.patchVendorHook
   ];
 
-  preFixup = ''
-    gappsWrapperArgs+=(
-      --prefix XDG_DATA_DIRS : "${glycin-loaders}/share"
-    )
-  '';
+  buildInputs = [
+    libglycin.setupHook
+    glycin-loaders
+  ];
 
   # ...
 }
@@ -42,3 +41,7 @@ rustPlatform.buildRustPackage {
 ### `glycinCargoDepsPath` {#glycin-cargo-deps-path}
 
 Path to a directory containing the `glycin` crate to patch. Defaults to the crate directory created by `cargoSetupHook`, or `./vendor/`.
+
+### `dontWrapGlycinLoaders` {#glycin-dont-wrap}
+
+Disable adding the Glycin loaders path `XDG_DATA_DIRS` with `wrapGAppsHook`.

doc/languages-frameworks/gnome.section.md

@@ -129,6 +129,8 @@ The hooks do the following:
 
 - []{#ssec-gnome-hooks-gst-grl-plugins} Setup hooks of `gst_all_1.gstreamer` and `grilo` will populate the `GST_PLUGIN_SYSTEM_PATH_1_0` and `GRL_PLUGIN_PATH` variables, respectively, which will then be added to the wrapper by `wrapGApps*` hook.
 
+- []{#ssec-gnome-hooks-libglycin} `libglycin`'s [setup hook](#libglycin-setup-hook) will populate `XDG_DATA_DIRS` with the path to the loaders.
+
 You can also pass additional arguments to `makeWrapper` using `gappsWrapperArgs` in `preFixup` hook:
 
 ```nix

doc/redirects.json

@@ -2471,6 +2471,9 @@
   "libglycin-hooks": [
     "index.html#libglycin-hooks"
   ],
+  "libglycin-setup-hook": [
+    "index.html#libglycin-setup-hook"
+  ],
   "libglycin-patch-vendor-hook": [
     "index.html#libglycin-patch-vendor-hook"
   ],
@@ -2483,6 +2486,9 @@
   "glycin-cargo-deps-path": [
     "index.html#glycin-cargo-deps-path"
   ],
+  "glycin-dont-wrap": [
+    "index.html#glycin-dont-wrap"
+  ],
   "ghc": [
     "index.html#ghc"
   ],
@@ -3173,6 +3179,9 @@
   "ssec-gnome-hooks-gst-grl-plugins": [
     "index.html#ssec-gnome-hooks-gst-grl-plugins"
   ],
+  "ssec-gnome-hooks-libglycin": [
+    "index.html#ssec-gnome-hooks-libglycin"
+  ],
   "ssec-gnome-updating": [
     "index.html#ssec-gnome-updating"
   ],

pkgs/by-name/li/libglycin/package.nix

@@ -38,6 +38,8 @@ stdenv.mkDerivation (finalAttrs: {
     "devdoc"
   ];
 
+  setupHook = ./path-hook.sh;
+
   src = fetchFromGitLab {
     domain = "gitlab.gnome.org";
     owner = "GNOME";

pkgs/by-name/li/libglycin/path-hook.sh

@@ -0,0 +1,18 @@
+find_glycin_loader_paths() {
+  if [ -d "$1/share/glycin-loaders" ]; then
+    addToSearchPath NIX_GLYCIN_LOADER_PATHS $1/share
+  fi
+}
+
+addEnvHooks "$hostOffset" find_glycin_loader_paths
+
+glycinLoadersWrapperArgsHook() {
+  echo "executing glycinLoadersWrapperArgsHook"
+  if [[ -n "$NIX_GLYCIN_LOADER_PATHS" ]]; then
+    gappsWrapperArgs+=(--prefix XDG_DATA_DIRS : "$NIX_GLYCIN_LOADER_PATHS")
+  fi
+}
+
+if [[ -z ${dontWrapGlycinLoaders:-} ]]; then
+  preFixupPhases+=(glycinLoadersWrapperArgsHook)
+fi