Code generation build plugin (#28)

## Overview
New build plugin to generate gRPC services and protobuf messages

The SwiftPM build plugin will locate protobuf files in the `Sources`
directory (with the extension `.proto`) and attempt to invoke both the
`protoc-gen-swift` and `protoc-gen-grpc-swift` `protoc` plugins on them
to automatically generate Swift source. Behavior can be modified by
specifying one or more configuration files.

* For a given protobuf definition file the tool will search for
configuration files in the same and all parent directories and will use
the file lowest in the hierarchy.
* Most configuration if not specified will use the protoc plugin's own
defaults.

Author: rnro

Package.swift

@@ -26,6 +26,10 @@ let products: [Product] = [
     name: "protoc-gen-grpc-swift",
     targets: ["protoc-gen-grpc-swift"]
   ),
+  .plugin(
+    name: "GRPCProtobufGenerator",
+    targets: ["GRPCProtobufGenerator"]
+  ),
 ]
 
 let dependencies: [Package.Dependency] = [
@@ -101,6 +105,16 @@ let targets: [Target] = [
     ],
     swiftSettings: defaultSwiftSettings
   ),
+
+  // Code generator build plugin
+  .plugin(
+    name: "GRPCProtobufGenerator",
+    capability: .buildTool(),
+    dependencies: [
+      .target(name: "protoc-gen-grpc-swift"),
+      .product(name: "protoc-gen-swift", package: "swift-protobuf"),
+    ]
+  ),
 ]
 
 let package = Package(

Plugins/GRPCProtobufGenerator/BuildPluginConfig.swift

@@ -0,0 +1,210 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+let configFileName = "grpc-swift-proto-generator-config.json"
+
+/// The config of the build plugin.
+struct BuildPluginConfig: Codable {
+  /// Config defining which components should be considered when generating source.
+  struct Generate {
+    /// Whether server code is generated.
+    ///
+    /// Defaults to `true`.
+    var servers: Bool
+    /// Whether client code is generated.
+    ///
+    /// Defaults to `true`.
+    var clients: Bool
+    /// Whether message code is generated.
+    ///
+    /// Defaults to `true`.
+    var messages: Bool
+
+    static let defaults = Self(
+      servers: true,
+      clients: true,
+      messages: true
+    )
+
+    private init(servers: Bool, clients: Bool, messages: Bool) {
+      self.servers = servers
+      self.clients = clients
+      self.messages = messages
+    }
+  }
+
+  /// Config relating to the generated code itself.
+  struct GeneratedSource {
+    /// The visibility of the generated files.
+    ///
+    /// Defaults to `Internal`.
+    var accessLevel: GenerationConfig.AccessLevel
+    /// Whether imports should have explicit access levels.
+    ///
+    /// Defaults to `false`.
+    var useAccessLevelOnImports: Bool
+
+    static let defaults = Self(
+      accessLevel: .internal,
+      useAccessLevelOnImports: false
+    )
+
+    private init(accessLevel: GenerationConfig.AccessLevel, useAccessLevelOnImports: Bool) {
+      self.accessLevel = accessLevel
+      self.useAccessLevelOnImports = useAccessLevelOnImports
+    }
+  }
+
+  /// Config relating to the protoc invocation.
+  struct Protoc {
+    /// Specify the directory in which to search for imports.
+    ///
+    /// Paths are relative to the location of the specifying config file.
+    /// Build plugins only have access to files within the target's source directory.
+    /// May be specified multiple times; directories will be searched in order.
+    /// The target source directory is always appended
+    /// to the import paths.
+    var importPaths: [String]
+
+    /// The path to the `protoc` executable binary.
+    ///
+    /// If this is not set, Swift Package Manager will try to find the tool itself.
+    var executablePath: String?
+
+    static let defaults = Self(
+      importPaths: [],
+      executablePath: nil
+    )
+
+    private init(importPaths: [String], executablePath: String?) {
+      self.importPaths = importPaths
+      self.executablePath = executablePath
+    }
+  }
+
+  /// Config defining which components should be considered when generating source.
+  var generate: Generate
+  /// Config relating to the nature of the generated code.
+  var generatedSource: GeneratedSource
+  /// Config relating to the protoc invocation.
+  var protoc: Protoc
+
+  static let defaults = Self(
+    generate: Generate.defaults,
+    generatedSource: GeneratedSource.defaults,
+    protoc: Protoc.defaults
+  )
+  private init(generate: Generate, generatedSource: GeneratedSource, protoc: Protoc) {
+    self.generate = generate
+    self.generatedSource = generatedSource
+    self.protoc = protoc
+  }
+
+  // Codable conformance with defaults
+  enum CodingKeys: String, CodingKey {
+    case generate
+    case generatedSource
+    case protoc
+  }
+
+  init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+
+    self.generate =
+      try container.decodeIfPresent(Generate.self, forKey: .generate) ?? Self.defaults.generate
+    self.generatedSource =
+      try container.decodeIfPresent(GeneratedSource.self, forKey: .generatedSource)
+      ?? Self.defaults.generatedSource
+    self.protoc =
+      try container.decodeIfPresent(Protoc.self, forKey: .protoc) ?? Self.defaults.protoc
+  }
+}
+
+extension BuildPluginConfig.Generate: Codable {
+  // Codable conformance with defaults
+  enum CodingKeys: String, CodingKey {
+    case servers
+    case clients
+    case messages
+  }
+
+  init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+
+    self.servers =
+      try container.decodeIfPresent(Bool.self, forKey: .servers) ?? Self.defaults.servers
+    self.clients =
+      try container.decodeIfPresent(Bool.self, forKey: .clients) ?? Self.defaults.clients
+    self.messages =
+      try container.decodeIfPresent(Bool.self, forKey: .messages) ?? Self.defaults.messages
+  }
+}
+
+extension BuildPluginConfig.GeneratedSource: Codable {
+  // Codable conformance with defaults
+  enum CodingKeys: String, CodingKey {
+    case accessLevel
+    case useAccessLevelOnImports
+  }
+
+  init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+
+    self.accessLevel =
+      try container.decodeIfPresent(GenerationConfig.AccessLevel.self, forKey: .accessLevel)
+      ?? Self.defaults.accessLevel
+    self.useAccessLevelOnImports =
+      try container.decodeIfPresent(Bool.self, forKey: .useAccessLevelOnImports)
+      ?? Self.defaults.useAccessLevelOnImports
+  }
+}
+
+extension BuildPluginConfig.Protoc: Codable {
+  // Codable conformance with defaults
+  enum CodingKeys: String, CodingKey {
+    case importPaths
+    case executablePath
+  }
+
+  init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+
+    self.importPaths =
+      try container.decodeIfPresent([String].self, forKey: .importPaths)
+      ?? Self.defaults.importPaths
+    self.executablePath = try container.decodeIfPresent(String.self, forKey: .executablePath)
+  }
+}
+
+extension GenerationConfig {
+  init(buildPluginConfig: BuildPluginConfig, configFilePath: URL, outputPath: URL) {
+    self.server = buildPluginConfig.generate.servers
+    self.client = buildPluginConfig.generate.clients
+    self.message = buildPluginConfig.generate.messages
+    // hard-code full-path to avoid collisions since this goes into a temporary directory anyway
+    self.fileNaming = .fullPath
+    self.visibility = buildPluginConfig.generatedSource.accessLevel
+    self.useAccessLevelOnImports = buildPluginConfig.generatedSource.useAccessLevelOnImports
+    // Generate absolute paths for the imports relative to the config file in which they are specified
+    self.importPaths = buildPluginConfig.protoc.importPaths.map { relativePath in
+      configFilePath.deletingLastPathComponent().absoluteStringNoScheme + "/" + relativePath
+    }
+    self.protocPath = buildPluginConfig.protoc.executablePath
+    self.outputPath = outputPath.absoluteStringNoScheme
+  }
+}