Possible approach to extensibility of rules_protobuf

BUILD.py

# Later, invoke the protobuf_go_library with the language flag. This will
# trigger building of the protobuffer with the grpc-gateway plugin and 
# building of the library with the required dependencies.

load("@org_pubref_rules_protobuf//bzl:rules.bzl", "protobuf_go_library")

protobuf_go_library(
    protos = ["service.proto"],
    srcs = ["service_impl.go"],
    deps = [
        "//third_party/foo:go_default_library"
    ],
    with_grpc = True,
    with_grpc_gateway = True,
)

WORKSPACE.py

# Hypothetical WORKSPACE that demonstrates how to provide an extension
# language with pre- and post- hooks to rules_protobuf:

# ****************************************************************
# Load up the required rules dependencies
# ****************************************************************
git_repository(
  name = "org_pubref_rules_protobuf",
  remote = "https://github.com/pubref/rules_protobuf",
  tag = "0.2.0",
)

git_repository(
    name = "com_github_grpc_ecosystem_grpc_gateway",
    remote = "https://github.com/grpc-ecosystem/grpc-gateway",
    tag = "v1.1.0",
)

# ****************************************************************
# Load the plugin's skylark event hooks
# ****************************************************************

load("@com_github_grpc_ecosystem_grpc_gateway//protoc-gen-grpc-gateway/rules_protobuf_plugin.bzl",
     grpc_gateway_pre = "pre"
     grpc_gateway_post = "post"
)


load("@org_pubref_rules_protobuf//bzl:rules.bzl", "rules_protobuf")

# ****************************************************************
# Invoke rules_protobuf with the configured language.
# ****************************************************************

rules_protobuf(
    languages = [
        struct(
            lang = "grpc_gateway",
            # Provide the plugin binary here
            plugin = "@com_github_grpc_ecosystem_grpc_gateway//protoc-gen-grpc-gateway"
            # Any default options
            plugin_options = [ "foo=bar"],
            # The pre- and post- event hooks
            pre = grpc_gateway_pre,
            post = grpc_gateway_pre,
            # Additional dependencies that should be included during compilation of the outputs.
            deps = [
                {
                    # Makes //external:grpc_gateway_runtime available as a
                    # dependency for go_library
                    "kind": "bind",
                    "name": "grpc_gateway",
                    "actual": "@com_github_grpc_ecosystem_grpc_gateway//runtime",
                },
            ],
        },
    ]
)