dotnetCorePackages.fetchNupkg: add override mechanism

pkgs/build-support/dotnet/fetch-nupkg/default.nix

@@ -6,6 +6,8 @@
   unzip,
   patchNupkgs,
   nugetPackageHook,
+  callPackage,
+  overrides ? callPackage ./overrides.nix { },
 }:
 {
   pname,
@@ -15,61 +17,64 @@
   url ? "https://www.nuget.org/api/v2/package/${pname}/${version}",
   installable ? false,
 }:
-stdenvNoCC.mkDerivation rec {
-  inherit pname version;
+let
+  package = stdenvNoCC.mkDerivation rec {
+    inherit pname version;
 
-  src = fetchurl {
-    name = "${pname}.${version}.nupkg";
-    # There is no need to verify whether both sha256 and hash are
-    # valid here, because nuget-to-nix does not generate a deps.nix
-    # containing both.
-    inherit
-      url
-      sha256
-      hash
-      version
-      ;
+    src = fetchurl {
+      name = "${pname}.${version}.nupkg";
+      # There is no need to verify whether both sha256 and hash are
+      # valid here, because nuget-to-nix does not generate a deps.nix
+      # containing both.
+      inherit
+        url
+        sha256
+        hash
+        version
+        ;
+    };
+
+    nativeBuildInputs = [
+      unzip
+      patchNupkgs
+      nugetPackageHook
+    ];
+
+    unpackPhase = ''
+      runHook preUnpack
+
+      unzip -nqd source $src
+      chmod -R +rw source
+      cd source
+
+      runHook postUnpack
+    '';
+
+    prePatch = ''
+      shopt -s nullglob
+      local dir
+      for dir in tools runtimes/*/native; do
+        [[ ! -d "$dir" ]] || chmod -R +x "$dir"
+      done
+      rm -rf .signature.p7s
+    '';
+
+    installPhase = ''
+      runHook preInstall
+
+      dir=$out/share/nuget/packages/${lib.toLower pname}/${lib.toLower version}
+      mkdir -p $dir
+      cp -r . $dir
+      echo {} > "$dir"/.nupkg.metadata
+
+      runHook postInstall
+    '';
+
+    preFixup = ''
+      patch-nupkgs $out/share/nuget/packages
+    '';
+
+    createInstallableNugetSource = installable;
   };
-
-  nativeBuildInputs = [
-    unzip
-    patchNupkgs
-    nugetPackageHook
-  ];
-
-  unpackPhase = ''
-    runHook preUnpack
-
-    unzip -nqd source $src
-    chmod -R +rw source
-    cd source
-
-    runHook postUnpack
-  '';
-
-  prePatch = ''
-    shopt -s nullglob
-    local dir
-    for dir in tools runtimes/*/native; do
-      [[ ! -d "$dir" ]] || chmod -R +x "$dir"
-    done
-    rm -rf .signature.p7s
-  '';
-
-  installPhase = ''
-    runHook preInstall
-
-    dir=$out/share/nuget/packages/${lib.toLower pname}/${lib.toLower version}
-    mkdir -p $dir
-    cp -r . $dir
-    echo {} > "$dir"/.nupkg.metadata
-
-    runHook postInstall
-  '';
-
-  preFixup = ''
-    patch-nupkgs $out/share/nuget/packages
-  '';
-
-  createInstallableNugetSource = installable;
-}
+in
+overrides.${pname} or lib.id package

pkgs/build-support/dotnet/fetch-nupkg/overrides.nix

@@ -0,0 +1,9 @@
+{ autoPatchelfHook }:
+{
+  # e.g.
+  # "Package.Id" =
+  #   package:
+  #   package.overrideAttrs (old: {
+  #     buildInputs = old.buildInputs or [ ] ++ [ hello ];
+  #   });
+}