nix: build PHP package using C4

Instead of sleeping one night, I wrote https://github.com/fossar/composition-c4.

Combined with napalm, this will allow us to fully build selfoss using Nix.

flake.nix

@@ -2,6 +2,9 @@
   description = "selfoss feed reader and aggregator";
 
   inputs = {
+    # Tool for downloading Composer dependencies using Nix.
+    c4.url = "github:fossar/composition-c4";
+
     # Shim to make flake.nix work with stable Nix.
     flake-compat = {
       url = "github:edolstra/flake-compat";
@@ -17,7 +20,7 @@
     phps.url = "github:fossar/nix-phps";
   };
 
-  outputs = { self, flake-compat, nixpkgs, phps, utils }:
+  outputs = { self, c4, flake-compat, nixpkgs, phps, utils }:
     let
       # Configure the development shell here (e.g. for CI).
 
@@ -28,7 +31,13 @@
       utils.lib.eachDefaultSystem (system:
         let
           # Get Nixpkgs packages for current platform.
-          pkgs = nixpkgs.legacyPackages.${system};
+          pkgs = import nixpkgs {
+            inherit system;
+            overlays = [
+              # Include c4 tool.
+              c4.overlay
+            ];
+          };
 
           # Create a PHP package from the selected PHP package, with some extra extensions enabled.
           php = phps.packages.${system}.${matrix.phpPackage}.withExtensions ({ enabled, all }: with all; enabled ++ [
@@ -69,6 +78,12 @@
             # node-gyp wants some locales, let’s make them available through an environment variable.
             LOCALE_ARCHIVE = "${pkgs.glibcLocales}/lib/locale/locale-archive";
           };
+
+          packages = {
+            selfoss = pkgs.callPackage ./utils/selfoss.nix {
+              src = self.outPath;
+            };
+          };
         }
       );
 }

utils/selfoss.nix

@@ -0,0 +1,32 @@
+{
+  stdenv,
+  lib,
+  src,
+  php,
+  c4,
+}:
+
+stdenv.mkDerivation rec {
+  pname = "selfoss";
+  version = (lib.importJSON "${src}/package.json").ver;
+
+  inherit src;
+
+  composerDeps = c4.fetchComposerDeps {
+    inherit src;
+  };
+
+  nativeBuildInputs = [
+    php.packages.composer
+    c4.composerSetupHook
+  ];
+
+  installPhase = ''
+    runHook preInstall
+
+    composer install
+    cp -r . $out
+
+    runHook postInstall
+  '';
+}