Add nix-all-output-paths

This implements the `fallback-paths.nix` format in NixOS

flake.nix

@@ -223,6 +223,10 @@
           default = self.packages.${system}.nix-ng;
           nix-internal-api-docs = nixpkgsFor.${system}.native.nixComponents.nix-internal-api-docs;
           nix-external-api-docs = nixpkgsFor.${system}.native.nixComponents.nix-external-api-docs;
+          nix-all-output-paths = nixpkgsFor.${system}.native.callPackage ./packaging/nix-all-output-paths.nix {
+            packages =
+              lib.mapAttrs (system: packages: packages.nix) self.packages;
+          };
         }
         # We need to flatten recursive attribute sets of derivations to pass `flake check`.
         // flatMapAttrs

packaging/nix-all-output-paths.nix

@@ -0,0 +1,48 @@
+{ lib, formats, nix, packages, stdenv, ... }:
+
+let
+  randomPick = packages."x86_64-linux";
+in
+stdenv.mkDerivation {
+  version = randomPick.version;
+  pname = "nix-all-output-paths";
+  passAsFile = [ "content" ];
+  nativeBuildInputs = [ nix ];
+  json = (formats.json {}).generate "all-output-paths.json" (lib.mapAttrs (name: pkg: "${lib.getBin pkg}") packages);
+  nix = ''
+    {
+      ${
+        lib.concatStringsSep "\n  " (
+          lib.mapAttrsToList
+            (name: pkg: "${lib.strings.escapeNixIdentifier name} = ${lib.strings.escapeNixString "${lib.getBin pkg}"};")
+            packages
+        )
+      }
+    }
+  '';
+  buildCommand = ''
+    mkdir -p $out
+    cp $json $out/all-output-paths.json
+    echo "$nix" > $out/all-output-paths.nix
+
+    mkdir -p $out/nix-support
+    {
+      echo "file all-output-paths $out/all-output-paths.json"
+      echo "file all-output-paths-nix-deprecated $out/all-output-paths.nix"
+    } >> $out/nix-support/hydra-build-products
+
+    r=$(nix-instantiate --store dummy:// --expr --eval --json \
+        'builtins.fromJSON (builtins.readFile '$out'/all-output-paths.json) == import '$out'/all-output-paths.nix')
+    if ! [[ $r == "true" ]]; then
+      echo "The generated Nix file does not match the JSON file"
+      echo "Nix file: $out/all-output-paths.nix"
+      cat -n $out/all-output-paths.nix
+      echo "JSON file: $out/all-output-paths.json"
+      cat -n $out/all-output-paths.json
+      exit 1
+    fi
+  '';
+  meta = {
+    description = "A lookup file that gives a store path containing the Nix package manager for all supported system types";
+  };
+}