Add gRPC Python code generation (#10937)

Users activate this on a per-target basis:

```python
# BUILD
protobuf_library(grpc=True)
```

This signals that all relevant code generators should use the gRPC plugin. For now, that is only Python. But when adding other languages like Java, it will need to respect this flag.

Closes #10497.

[ci skip-rust]
[ci skip-build-wheels]

Author: Eric-Arellano

src/python/pants/backend/codegen/protobuf/python/rules.py

@@ -5,8 +5,9 @@
 
 from pants.backend.codegen.protobuf.protoc import Protoc
 from pants.backend.codegen.protobuf.python.additional_fields import PythonSourceRootField
+from pants.backend.codegen.protobuf.python.grpc_python_plugin import GrpcPythonPlugin
 from pants.backend.codegen.protobuf.python.python_protobuf_subsystem import PythonProtobufSubsystem
-from pants.backend.codegen.protobuf.target_types import ProtobufSources
+from pants.backend.codegen.protobuf.target_types import ProtobufGrcpToggle, ProtobufSources
 from pants.backend.python.target_types import PythonSources
 from pants.backend.python.util_rules import extract_pex, pex
 from pants.backend.python.util_rules.extract_pex import ExtractedPexDistributions
@@ -52,6 +53,7 @@ class GeneratePythonFromProtobufRequest(GenerateSourcesRequest):
 async def generate_python_from_protobuf(
     request: GeneratePythonFromProtobufRequest,
     protoc: Protoc,
+    grpc_python_plugin: GrpcPythonPlugin,
     python_protobuf_subsystem: PythonProtobufSubsystem,
 ) -> GeneratedSources:
     download_protoc_request = Get(
@@ -112,13 +114,25 @@ async def generate_python_from_protobuf(
         )
         extracted_mypy_wheels = await Get(ExtractedPexDistributions, Pex, mypy_pex)
 
+    downloaded_grpc_plugin = (
+        await Get(
+            DownloadedExternalTool,
+            ExternalToolRequest,
+            grpc_python_plugin.get_request(Platform.current),
+        )
+        if request.protocol_target.get(ProtobufGrcpToggle).value
+        else None
+    )
+
     unmerged_digests = [
         all_sources_stripped.snapshot.digest,
         downloaded_protoc_binary.digest,
         empty_output_dir,
     ]
     if extracted_mypy_wheels:
         unmerged_digests.append(extracted_mypy_wheels.digest)
+    if downloaded_grpc_plugin:
+        unmerged_digests.append(downloaded_grpc_plugin.digest)
     input_digest = await Get(Digest, MergeDigests(unmerged_digests))
 
     argv = [downloaded_protoc_binary.exe, "--python_out", output_dir]
@@ -135,6 +149,10 @@ async def generate_python_from_protobuf(
                 output_dir,
             ]
         )
+    if downloaded_grpc_plugin:
+        argv.extend(
+            [f"--plugin=protoc-gen-grpc={downloaded_grpc_plugin.exe}", "--grpc_out", output_dir]
+        )
     argv.extend(target_sources_stripped.snapshot.files)
 
     env = {}

src/python/pants/backend/codegen/protobuf/python/rules_integration_test.py

@@ -232,3 +232,39 @@ def test_mypy_plugin(rule_runner: RuleRunner) -> None:
         mypy=True,
         expected_files=["src/protobuf/dir1/f_pb2.py", "src/protobuf/dir1/f_pb2.pyi"],
     )
+
+
+def test_grpc(rule_runner: RuleRunner) -> None:
+    rule_runner.create_file(
+        "src/protobuf/dir1/f.proto",
+        dedent(
+            """\
+            syntax = "proto3";
+
+            package dir1;
+
+            // The greeter service definition.
+            service Greeter {
+              // Sends a greeting
+              rpc SayHello (HelloRequest) returns (HelloReply) {}
+            }
+
+            // The request message containing the user's name.
+            message HelloRequest {
+              string name = 1;
+            }
+
+            // The response message containing the greetings
+            message HelloReply {
+              string message = 1;
+            }
+            """
+        ),
+    )
+    rule_runner.add_to_build_file("src/protobuf/dir1", "protobuf_library(grpc=True)")
+    assert_files_generated(
+        rule_runner,
+        "src/protobuf/dir1",
+        source_roots=["src/protobuf"],
+        expected_files=["src/protobuf/dir1/f_pb2.py", "src/protobuf/dir1/f_pb2_grpc.py"],
+    )

src/python/pants/backend/codegen/protobuf/target_types.py

@@ -6,6 +6,7 @@
 from pants.engine.rules import Get, collect_rules, rule
 from pants.engine.target import (
     COMMON_TARGET_FIELDS,
+    BoolField,
     Dependencies,
     InjectDependenciesRequest,
     InjectedDependencies,
@@ -30,14 +31,21 @@ class ProtobufSources(Sources):
     expected_file_extensions = (".proto",)
 
 
+class ProtobufGrcpToggle(BoolField):
+    """Whether to generate gRPC code or not."""
+
+    alias = "grpc"
+    default = False
+
+
 class ProtobufLibrary(Target):
     """Protobuf files used to generate various languages.
 
     See https://www.pantsbuild.org/docs/protobuf.
     """
 
     alias = "protobuf_library"
-    core_fields = (*COMMON_TARGET_FIELDS, ProtobufDependencies, ProtobufSources)
+    core_fields = (*COMMON_TARGET_FIELDS, ProtobufDependencies, ProtobufSources, ProtobufGrcpToggle)
 
 
 class InjectProtobufDependencies(InjectDependenciesRequest):