Looks like there might be one additional-problem to solve but it does seem to fix the original problem?

ERROR: An error occurred during the fetch of repository 'com_google_protobuf':
   Traceback (most recent call last):
	File "/root/.cache/bazel/_bazel_root/217030ec3ef64241bbb4e12700124ac3/external/bazel_tools/tools/build_defs/repo/http.bzl", line 121, column 10, in _http_archive_impl
		patch(ctx)
	File "/root/.cache/bazel/_bazel_root/217030ec3ef64241bbb4e12700124ac3/external/bazel_tools/tools/build_defs/repo/utils.bzl", line 135, column 22, in patch
		ctx.patch(patchfile, strip)
Error in patch: Unable to load package for //third_party:protobuf.patch: BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.
 - /var/local/git/grpc/bazel/test/python_test_repo/third_party
Loading: 0 packages loaded
ERROR: no such package '@com_google_protobuf//': Unable to load package for //third_party:protobuf.patch: BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.

Once that works, I can cherry-pick the commits into my PR #25131 (if @gnossen thinks this is the best way of solving this problem). If we end up using this workaround, at what point we will be able to get rid of it?

Yes. I'm glad we have the python_test_repo test. The change without my most recent commit would have broken us for downstream dependencies. I just pushed a commit to fix that.

If we end up using this workaround, at what point we will be able to get rid of it?

Quoting my response from your protobuf update PR:

So it turns out that the Envoy project dealt with this problem before us. The Protobuf team has intentionally removed their namespace package declarations from the __init__.py file, instead declaring them in their setup.py file, which we do not consume at all. It seems that this solution is only necessary until all other packages using the google namespace package (google-auth) do the same. I've implemented that solution in our repo here.

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