Fix Homebrew FFmpeg discovery on macOS

[mscheltienne]

Closes #570

This PR adds the homebrew FFmpeg path to rpath at build. It should not impact in any form the current resolution of the conda-installed libraries, but will help users of plain pip or uv with homebrew. I tested locally with the following:

$ brew install cmake pkg-config pybind11 ffmpeg
$ uv venv --python 3.13
$ source .venv/bin/activate
$ uv pip install --pre torch --index-url https://download.pytorch.org/whl/nightly/cpu
$ export TORCHCODEC_CMAKE_BUILD_DIR="${PWD}/build"
$ uv pip install -e ".[dev]" --no-build-isolation -vv

Resulting install shows:

❯ otool -l src/torchcodec/libtorchcodec_core*.dylib | grep -A2 LC_RPATH
          cmd LC_RPATH
      cmdsize 48
         path /opt/homebrew/opt/ffmpeg/lib (offset 12)

And import works:

❯ python -c "import torchcodec; print('Success!')"
Success!

And locally, default tests passes with:

$ uv pip install --pre torchvision --index-url https://download.pytorch.org/whl/nightly/cpu
$ pytest
>>> 1152 passed, 399 skipped, 30 deselected in 128.39s (0:02:08)

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[NicolasHug]

Just noting that the handling of a custom HOMEBREW_PREFIX is only going to work for users building from source. For users installing the built wheels with e.g. pip install, the HOMEBREW_PREFIX will be hard-coded to opt/homebrew/opt/ffmpeg/lib which may or may not be the current location of the user's homebrew.

[mscheltienne]

Exactly, can you think of a better alternative? I included the HOMEBREW_PREFIX path specifically for this case where a user would use homebrew in a non-default location, in which case he will be able to build from source.

[NicolasHug]

On this:

It should not impact in any form the current resolution of the conda-installed libraries

I think the change will impact the resolution of conda-installed ffmpeg, because (IIUC) the rpath will now take precedence for the path resolution order. So, if FFmpeg is installed in both the conda env and with homebrew, it's the homebrew FFmpeg that is going to be resolved first.

[mscheltienne]

I think you are right. From what I could figure out, it seems that on main, libtorchcodec_core*.dylib doesn't have any LC_RPATH entries and thus the resolution relies on torch's rpath entries (which include conda paths and thus torchcodec finds the conda FFmpeg.

With this change, libtorchcodec_core*.dylib now has one LC_RPATH which gets added to the rpath list and likely searched first. Thus, we have the following scenarios:

• Only conda FFmpeg: no change in behavior, homebrew path is searched first but no lib found, falls back to conda paths through the same path as before.
• Only homebrew FFmpeg, fails on main, works on this PR.
• Both installed, on main it would use the conda FFmpeg, on this PR it would likely resolve the homebrew FFmpeg first.

In this last scenario, torchcodec should still work and thus the change should be transparent to the user- the only way I can think of to retain control on which path to search first would be to add the conda path as well, but this is now a more invasive change, something along the lines of:

        set_target_properties(${library_name} PROPERTIES
            INSTALL_RPATH "@loader_path/../../..;${HOMEBREW_FFMPEG_LIB}"
        )