core/pkgs/build-support/fetchurl/boot.nix

let mirrors = import ./mirrors.nix;

in { system }:

{ url ? builtins.head urls, urls ? [ ], sha256 ? "", hash ? ""
, name ? baseNameOf (toString url) }:

# assert exactly one hash is set
assert hash != "" || sha256 != "";
assert hash != "" -> sha256 == "";

import <nix/fetchurl.nix> {
  inherit system hash sha256 name;

  url =
    # Handle mirror:// URIs. Since <nix/fetchurl.nix> currently
    # supports only one URI, use the first listed mirror.
    let m = builtins.match "mirror://([a-z]+)/(.*)" url;
    in if m == null then
      url
    else
      builtins.head (mirrors.${builtins.elemAt m 0}) + (builtins.elemAt m 1);
}