bazel: Migrate py_proto_library to @com_github_grpc_grpc

[sergiitk]

py_proto_library provided by @com_google_protobuf//:protobuf.bzl has been deprecated for a while now:

This is provided for backwards compatibility only. Bazel 5.3 will
introduce support for py_proto_library, which should be used instead.
https://github.com/protocolbuffers/protobuf/blob/32af7d211b85f71920acdfa9b796db008f8c2a45/protobuf.bzl#L642-L644

However, native py_proto_library has never been provided, see bazelbuild/bazel#3935. Instead @rules_python//python:proto.bzl is recommended. I attempted switching to this library, but it's not compatible with @com_google_googleapis's py_proto_library targets, see #69. I found a hacky workaround by using cc_proto_library to generate python targets, but downstream integration into Envoy failed (envoyproxy/envoy#30159).

This PR migrates py_proto_library implementation to to @com_github_grpc_grpc. This implementation is used by @com_google_googleapis's, and, more importantly, uses bazel aspects. Which decouples cncf/xds and Envoy's dependencies from concrete upstream py_proto_library implementations.

This resulted in a significant code improvement of bazel/api_build_system.bzl:

• No more custom @com_google_googleapis dependency mapping needed via py_proto_library rules override.
• No more hardcoded dependencies _xds_py_proto_library - proto dependency tree is determined from proto_library definitions via Basel aspects.
• No more EXTERNAL_PROTO_PY_BAZEL_DEP_MAP dependency map needed - for the same reason.

Similar work in Envoy: envoyproxy/envoy#30834.

[sergiitk]

@adisuissa @kyessenov this is ready for review.
@htuch - FYI this addresses a bunch of TODO's assigned to you.

[htuch]

Overall I like this approach a lot. @phlax @moderation are there any concerns here about taking the dependency from https://github.com/grpc/grpc? It's already a build dependency, and these are Bazel rules, so we should still be able to compile Envoy without linking Google gRPC, but just wanted to check.

[phlax]

overall looks fine, and appears address a number of issues - not sure iiuc re grpc dep, but from what i do seems ok

[adisuissa]

Overall LGTM, thanks for fixing this!
Will it be possible to have the same change in https://github.com/envoyproxy/envoy/tree/main/api/bazel ?

[sergiitk]

@adisuissa yes - I have a PR in progress: envoyproxy/envoy#30834.
Was out yesterday, planning to wrap it up today.

[moderation]

LGTM

[sergiitk]

Looks like to make it work nicely in Envoy, we'd have to merge this first. Otherwise, bazel complains about missing _genproto targets (generated by @com_google_protobuf//:protobuf.bzl's py_proto_library), because this cncf/xds used rules_override for googleapis before this PR.

ERROR: /private/var/tmp/_bazel_sergiitk/e60ee3f6402a793527d720007beb00cb/external/com_github_cncf_xds/xds/annotations/v3/BUILD:5:18: no such target '@com_google_googleapis//google/api:httpbody_py_proto_genproto': target 'httpbody_py_proto_genproto' not declared in package 'google/api' defined by /private/var/tmp/_bazel_sergiitk/e60ee3f6402a793527d720007beb00cb/external/com_google_googleapis/google/api/BUILD.bazel (Tip: use query "@com_google_googleapis//google/api:*" to see all the targets in that package) and referenced by '@com_github_cncf_xds//xds/annotations/v3:pkg_py_proto_genproto'

It might be possible to solve this in Envoy without merging this, but it'd be a throw-away work. Let's get this one in first.

[adisuissa]

Thanks for fixing this!
Seems that the dependency shepherds are ok with the protoc dependency, so I'm going to approve this PR.
LGTM!

[alexeagle]

@sergiitk thanks for this example - but per https://groups.google.com/g/grpc-io/c/44OcoVSmeQk?pli=1 it seems like this is using something deprecated. I'd appreciate any pointers to an "officially recommended" way to generate service stubs with Bazel, on behalf of a lot of the community looking for it.