Applying a patch from GitHub to a formula when the source archive uses different paths?

[Kreeblah]

I'm looking to create a pull request for sdl2_mixer.rb to fix libsdl-org/SDL_mixer@6160668 since there's a use-after-free bug in the latest released version that's been patched in the tree upstream.

However, pulling the patch directly from GitHub in the formula fails, since the source archive at https://github.com/libsdl-org/SDL_mixer/archive/refs/tags/release-2.0.4.tar.gz has the source file in a different path. Instead of being at /src/codecs/music_fluidsynth.c like it is in the GitHub repository, it's located in the source archive at /music_fluidsynth.c. What would be the best way of getting this patch applied to the builds here?

Here's what I have for changes so far, in case it helps:

class Sdl2Mixer < Formula
  desc "Sample multi-channel audio mixer library"
  homepage "https://www.libsdl.org/projects/SDL_mixer/"
  license "Zlib"
  revision 3

  stable do
    url "https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-2.0.4.tar.gz"
    sha256 "b4cf5a382c061cd75081cf246c2aa2f9df8db04bdda8dcdc6b6cca55bede2419"

    # Fix fluidsynth use-after-free bug until next release, upstream patch
    # https://github.com/libsdl-org/SDL_mixer/commit/6160668079f91d57a5d7bf0b40ffdd843be70daf
    patch do
      url "https://github.com/libsdl-org/SDL_mixer/commit/6160668079f91d57a5d7bf0b40ffdd843be70daf.patch?full_index=1"
      sha256 "73e5ebf9136818b7a65f1e8fcbeb99c350654d7d9e53629adc26887e9e169d8d"
    end
  end

  livecheck do
    url :homepage
    regex(/href=.*?SDL2_mixer[._-]v?(\d+(?:\.\d+)+)\.t/i)
  end

  head do
    url "https://github.com/libsdl-org/SDL_mixer.git"

    depends_on "autoconf" => :build
    depends_on "automake" => :build
    depends_on "libtool" => :build
  end

  depends_on "pkg-config" => :build
  depends_on "flac"
  depends_on "libmodplug"
  depends_on "libvorbis"
  depends_on "mpg123"
  depends_on "sdl2"

  def install
    inreplace "SDL2_mixer.pc.in", "@prefix@", HOMEBREW_PREFIX

    if build.head?
      mkdir "build"
      system "./autogen.sh"
    end

    args = %W[
      --prefix=#{prefix}
      --disable-dependency-tracking
      --enable-music-flac
      --disable-music-flac-shared
      --disable-music-midi-fluidsynth
      --disable-music-midi-fluidsynth-shared
      --disable-music-mod-mikmod-shared
      --disable-music-mod-modplug-shared
      --disable-music-mp3-mpg123-shared
      --disable-music-ogg-shared
      --enable-music-mod-mikmod
      --enable-music-mod-modplug
      --enable-music-ogg
      --enable-music-mp3-mpg123
    ]

    system "./configure", *args
    system "make", "install"
  end

  test do
    (testpath/"test.c").write <<~EOS
      #include <SDL2/SDL_mixer.h>

      int main()
      {
          int success = Mix_Init(0);
          Mix_Quit();
          return success;
      }
    EOS
    system ENV.cc, "-I#{Formula["sdl2"].opt_include}/SDL2",
           "test.c", "-L#{lib}", "-lSDL2_mixer", "-o", "test"
    system "./test"
  end
end

[alebcay]

Under the hood, the patch mechanism in Homebrew uses the patch command line tool. Of interest in your case is this command line flag for the patch tool:

-pnum  or  --strip=num
          Strip the smallest prefix containing num leading slashes from
          each file name found in the patch file.  A sequence of one or
          more adjacent slashes is counted as a single slash.  This
          controls how file names found in the patch file are treated,
          in case you keep your files in a different directory than the
          person who sent out the patch.  For example, supposing the
          file name in the patch file was

          /u/howard/src/blurfl/blurfl.c

       setting -p0 gives the entire file name unmodified, -p1 gives

          u/howard/src/blurfl/blurfl.c

       without the leading slash, -p4 gives

          blurfl/blurfl.c

       and not specifying -p at all just gives you blurfl.c.  Whatever
       you end up with is looked for either in the current directory, or
       the directory specified by the -d option.

Based on this, I think your patch could apply with -p3 or --strip=3 (haven't tested this with your particular patch myself). The Homebrew DSL syntax allows this argument to be specified too, and the result would look something like this (note the patch :p3 do):

patch :p3 do
  url "https://github.com/libsdl-org/SDL_mixer/commit/6160668079f91d57a5d7bf0b40ffdd843be70daf.patch?full_index=1"
  sha256 "73e5ebf9136818b7a65f1e8fcbeb99c350654d7d9e53629adc26887e9e169d8d"
end

There's also an example of a similar use in the llvm@9 formula.

[Kreeblah]

Ah, I didn't realize there was a way to specify that flag. That's exactly what I needed.

Thanks!