{ lib ? rosSelf.lib or self.lib
, self ? null
, rosSelf ? null }:
{
  mkOverlay = overlays: let
    s = lib.composeManyExtensions overlays s {};
  in s;

  # Create a tarball of a package source. If the source is already an archive,
  # it will be unpacked and repacked as a tarball.
  tarSource = {
    compress ? false,
    hook ? "",
    hash ? null
  }: src: self.runCommand ("${src.name}.tar" + lib.optionalString compress ".gz") ({
    inherit src;
  } // lib.optionalAttrs (hash != null) {
    outputHashMode = "flat";
    outputHash = hash;
  }) ''
    unpackPhase
    pushd "$sourceRoot"
    ${hook}
    popd
    tar --sort=name \
      --format=gnu \
      --owner=0 --group=0 --numeric-owner \
      --mtime="@$SOURCE_DATE_EPOCH" \
      ${lib.optionalString compress "-z"} \
      -cf "$out" "$sourceRoot"
  '';

  patchVendorUrl = pkg: {
    url, hash ? "", sha256 ? "",
    originalUrl ? url,
    file ? "CMakeLists.txt"
  }: pkg.overrideAttrs ({
    postPatch ? "", ...
  }: {
    postPatch = ''
      substituteInPlace ${lib.escapeShellArg file} \
        --replace-fail ${lib.escapeShellArg originalUrl} ${lib.escapeShellArg (self.fetchurl { inherit url hash sha256; })}
    '' + postPatch;
  });

  patchExternalProjectGit = pkg: {
    url,
    rev,
    originalRev ? rev,
    originalUrl ? url,
    revVariable ? "",
    file ? "CMakeLists.txt",
    fetchgitArgs ? {}
  }: pkg.overrideAttrs ({
    postPatch ? "", ...
  }: {
    postPatch = let
      script = ''
        $0 ~ "GIT_REPOSITORY[[:blank:]]+" originalUrl \
          { print "URL \"" path "\""; foundUrl=1; next } \
          { print }
        $0 ~ "GIT_TAG[[:blank:]]+" originalRev { print; foundRev=1 }
        $0 ~ "set\\(" revVariable "[[:blank:]]+\"?" originalRev "\"?\\)" { print; foundRev=1 }
        END {
          if (!foundUrl) print "patchExternalProjectGit: did not find URL: " originalUrl > "/dev/stderr"
          if (!foundRev) print "patchExternalProjectGit: did not find revision: " originalRev > "/dev/stderr"
          exit !(foundUrl && foundRev)
        }
      '';
    in ''
      awk -i inplace \
        -v originalUrl=${lib.escapeShellArg originalUrl} \
        -v originalRev=${lib.escapeShellArg originalRev} \
        -v revVariable=${lib.escapeShellArg revVariable} \
        -v path=${lib.escapeShellArg (self.fetchgit ({ inherit url rev; } // fetchgitArgs))} \
        ${lib.escapeShellArg script} \
        ${lib.escapeShellArg file}
    '' + postPatch;
  });

  # Patch a vendored download that uses ament_vendor() with a Git repo as the
  # source.
  patchAmentVendorGit = pkg: {
    url,
    originalUrl ? url,
    rev, # Must correspond to the VCS_VERSION argument
    file ? "CMakeLists.txt",
    fetchgitArgs ? {},
    tarSourceArgs ? {}
  }: pkg.overrideAttrs ({
    nativeBuildInputs ? [],
    postPatch ? "", ...
  }: let
    # ament_vendor doesn't allow patches for path inputs, so we have to pack it
    # into a tar first. Additionally, vcstool only accepts tarballs with the
    # version number as the root directory name.
    vendor = lib.tarSource tarSourceArgs (self.fetchgit (fetchgitArgs // {
      name = rev;
      inherit url rev;
    }));
  in {
    # CMake ExternalProject patches are applied with git apply
    nativeBuildInputs = nativeBuildInputs ++ [ self.git ];
    postPatch = ''
      sed -i '\|VCS_URL\s*${originalUrl}|c\
        VCS_URL "file://${vendor}"\
        VCS_TYPE tar' \
        ${lib.escapeShellArg file}
    '' + postPatch;
  });

  # patchAmentVendorGit specialized for gz-*-vendor packages. In
  # addition to patching ament_vendor() calls, it adds a check to
  # CMakeLists.txt to detect upstream updates of the vendored package
  # version.
  patchGzAmentVendorGit = pkg: {
    version,
    hash,
    tarSourceArgs ? {}
  }: let
    stem = lib.strings.removeSuffix "-vendor"
      (lib.strings.removePrefix "ros-${pkg.rosDistro}-" pkg.pname); # e.g. gz-cmake
    majorNum = lib.versions.major version;
    patchedPkg = lib.patchAmentVendorGit pkg {
      url = "https://github.com/gazebosim/${stem}.git";
      originalUrl = "https://github.com/gazebosim/\${GITHUB_NAME}.git";
      rev = "${stem}${majorNum}_${version}"; # e.g. "gz-cmake3_3.5.3"
      fetchgitArgs.hash = hash;
      inherit tarSourceArgs;
    };
  in patchedPkg.overrideAttrs ({
    pname, postPatch ? "", ...
  }: {
    postPatch = postPatch + ''
      # Use standard installation paths rather than /opt
      substituteInPlace CMakeLists.txt \
        --replace-fail 'opt/''${PROJECT_NAME}/extra_cmake' 'share/extra_cmake'
      substituteInPlace *-extras.cmake.in \
        --replace-fail 'opt/@PROJECT_NAME@/extra_cmake' 'share/extra_cmake'

      cat >> CMakeLists.txt <<'EOF'
      if(NOT ''${LIB_VER} VERSION_EQUAL "${version}")
        message(FATAL_ERROR "Mismatch in ${pname} version (Nix: ${version}, upstream: ''${LIB_VER}). Fix this in overrides.nix.")
      endif()
      EOF
    '';
  });

  # Many ROS packages claim to have a dependency on Boost signals when they
  # really don't or they actually depend on signals2. Boost 1.69 removed
  # signals causing these packages to fail to build.
  patchBoostSignals = pkg: pkg.overrideAttrs ({
    postPatch ? "", ...
  }: {
    postPatch = ''
      sed -i '/find_package(Boost [^)]*/s/signals//g' CMakeLists.txt
    '' + postPatch;
  });
}