Installing a dependency using CIBW_BEFORE_BUILD doesn't prevent it being installed during wheel build

[vyasr]

I'm not sure if I am misunderstanding something about how things are supposed to work in the different cibuildwheel stages or if I am hitting an unplanned for edge case here. I have a dependency that I am installing directly from a Github URL (pip install git+https...) using CIBW_BEFORE_BUILD (more precisely CIBW_BEFORE_BUILD_LINUX, but I only have linux builds so these should be equivalent). This package is also part of the install_requires set of one of the dependencies of my package (i.e. I'm building wheels for package A, A requires B, and B requires C, where C is the package that I'm installing directly from Github).

The installation from Github goes off without a hitch, but the problem I run into is that the pip wheel command eventually tries to also install that dependency again at the step "Running command pip subprocess to install backend dependencies". I would not have expected this to happen, but it appears to be because the command there ignores already installed packages:

/opt/python/cp38-cp38/bin/python /opt/_internal/cpython-3.8.14/lib/python3.8/site-packages/pip/__pip-runner__.py install --ignore-installed --no-user --prefix /tmp/pip-build-env-jssnyqzb/normal --no-warn-script-location -v ...

Since this extra dependency is not available on PyPI or any additional server, the wheel build fails even though I have pre-installed the dependency when cibuildwheel is "Running before_build". As a result, the pre-installation of the dependency using CIBW_BEFORE_BUILD is ultimately not solving the desired problem. Is this behavior expected?

Here is an example of such a run that fails. Please ignore some of the gross hacks in there :) I now have a better solution in place that has unblocked me (I no longer need to install the package via the Github URL), but I wanted to raise the discussion anyway in case this is unexpected behavior or something useful to have a workaround for.