core/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}";
  })