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Bazel version?

0.14.1

Why use pubref/rules_protobuf instead of native rules?

The only Bazel native rules I am aware of are proto_library and cc_proto_library. These work well despite an issue with proto_source_root in the proto_library rule. However, there is no native support for Python and gRPC (any language?) at this time (Bazel 0.14), which is why we use pubref/rules_protobuf. The cc_grpc_library that ships with gRPC (v1.13.1) is also buggy.

What is the proto_source_root issue?

I originally had the BUILD file in the proto package and I was setting proto_source_root to "proto" in the proto_library rules. However it wasn't working because of a Bazel issue. See bazelbuild/bazel#4544 for the issue. See https://github.com/bazelbuild/bazel/commit/e8956648d1c94a3a51e1aba5d229d1f27bdf8e35 for the fix. So instead I moved the rules to the root BUILD file and I am borrowing internal_copied_filegroup from protobuf.bzl to "remove" the "proto" prefix from my paths. The proto_source_root fix may be available in Bazel 0.15.x.

What is the issue with cc_grpc_library in the gRPC repo?

It doesn't work with generated protos (which is what we have because of the proto_source_root issue). See grpc/grpc#16178.

Why use these versions of Protobuf and gRPC?

We pull in patched versions of Protobuf (v3.2.0) and gRPC (v1.3.2). These are quite old but they are the ones we have been using for a while in p4lang. The version doesn't really matter though: more recent versions should be compatible at the Protobuf protocol level. When using p4runtime as an external dependency in a Bazel project, one should be able to pull more recent versions of pubref/rules_protobuf, Protobuf and gRPC; hopefully the BUILD rules should still work. Maybe we could consider moving to a more recent pubref/rules_protobuf by default. I'd rather wait for better / working versions of the native rules.

Why did we need to patch Protobuf and gRPC?

The versions we use are quite old. They needed to be patched for recent Bazel versions (e.g. depset vs set).

Wouldn't it be better to invoke protoc manually in the BUILD file?

It would probably be less dependent on the versions of Bazel, Protobuf and gRPC. And not much more verbose. We could switch to invoking protoc directly if the current rules are too much of a headache to maintain.