nixpkgs/pkgs/build-support/fetchdebianpatch/default.nix

{ lib, fetchpatch }:

lib.makeOverridable (
  { pname, version, debianRevision ? null, area ? "main",
    patch, name ? patch, hash }:
  let
    inherit (lib.strings) hasPrefix substring;
    prefix =
      substring 0 (if hasPrefix "lib" pname then 4 else 1) pname;
    versionString =
      if debianRevision == null then version
      else "${version}-${debianRevision}";
  in fetchpatch {
    inherit name hash;
    url =
      "https://sources.debian.org/data/${area}/${prefix}/"
      + "${pname}/${versionString}/debian/patches/${patch}";
  }
)