Code generation build plugin

Motivation:
To make code generation more convenient for adopters.

Modifications:
* New build plugin to generate gRPC services and protobuf messages

Result:
* Users will be able to make use of the build plugin.

Package.swift

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

Plugins/GRPCGeneratorPlugin/ConfigurationFile.swift

@@ -0,0 +1,79 @@
+/*
+ * 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.
+ */
+
+/// The configuration of the plugin.
+struct ConfigurationFile: Codable {
+  /// The visibility of the generated files.
+  enum Visibility: String, Codable {
+    /// The generated files should have `internal` access level.
+    case `internal`
+    /// The generated files should have `public` access level.
+    case `public`
+    /// The generated files should have `package` access level.
+    case `package`
+  }
+
+  /// The visibility of the generated files.
+  var visibility: Visibility?
+  /// Whether server code is generated.
+  var server: Bool?
+  /// Whether client code is generated.
+  var client: Bool?
+  /// Whether message code is generated.
+  var message: Bool?
+  //  /// Whether reflection data is generated.
+  //  var reflectionData: Bool?
+  /// Path to module map .asciipb file.
+  var protoPathModuleMappings: String?
+  /// Whether imports should have explicit access levels.
+  var useAccessLevelOnImports: Bool?
+
+  /// Specify the directory in which to search for
+  /// imports. 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` binary.
+  ///
+  /// If this is not set, SPM will try to find the tool itself.
+  var protocPath: String?
+}
+
+extension CommonConfiguration {
+  init(configurationFile: ConfigurationFile) {
+    if let visibility = configurationFile.visibility {
+      self.visibility = .init(visibility)
+    }
+    self.server = configurationFile.server
+    self.client = configurationFile.client
+    self.protoPathModuleMappings = configurationFile.protoPathModuleMappings
+    self.useAccessLevelOnImports = configurationFile.useAccessLevelOnImports
+    self.importPaths = configurationFile.importPaths
+    self.protocPath = configurationFile.protocPath
+  }
+}
+
+extension CommonConfiguration.Visibility {
+  init(_ configurationFileVisibility: ConfigurationFile.Visibility) {
+    switch configurationFileVisibility {
+    case .internal: self = .internal
+    case .public: self = .public
+    case .package: self = .package
+    }
+  }
+}

Plugins/GRPCGeneratorPlugin/Plugin.swift

@@ -0,0 +1,270 @@
+/*
+ * 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
+import PackagePlugin
+
+@main
+struct GRPCGeneratorPlugin {
+  /// Code common to both invocation types: package manifest Xcode project
+  func createBuildCommands(
+    pluginWorkDirectory: URL,
+    tool: (String) throws -> PluginContext.Tool,
+    inputFiles: [URL],
+    configFiles: [URL],
+    targetName: String
+  ) throws -> [Command] {
+    let configs = try readConfigurationFiles(configFiles, pluginWorkDirectory: pluginWorkDirectory)
+
+    let protocGenGRPCSwiftPath = try tool("protoc-gen-grpc-swift").url
+    let protocGenSwiftPath = try tool("protoc-gen-swift").url
+
+    var commands: [Command] = []
+    for inputFile in inputFiles {
+      guard let configFile = findApplicableConfigFor(file: inputFile, from: configs.keys.map { $0 })
+      else {
+        throw PluginError.noConfigurationFilesFound
+      }
+      guard let config = configs[configFile] else {
+        throw PluginError.expectedConfigurationNotFound(configFile.relativePath)
+      }
+
+      let protocPath = try deriveProtocPath(using: config, tool: tool)
+      let protoDirectoryPath = inputFile.deletingLastPathComponent()
+
+      // unless *explicitly* opted-out
+      if config.client != false || config.server != false {
+        let grpcCommand = try protocGenGRPCSwiftCommand(
+          inputFile: inputFile,
+          configFile: configFile,
+          config: config,
+          protoDirectoryPath: protoDirectoryPath,
+          protocPath: protocPath,
+          protocGenGRPCSwiftPath: protocGenGRPCSwiftPath
+        )
+        commands.append(grpcCommand)
+      }
+
+      // unless *explicitly* opted-out
+      if config.message != false {
+        let protoCommand = try protocGenSwiftCommand(
+          inputFile: inputFile,
+          configFile: configFile,
+          config: config,
+          protoDirectoryPath: protoDirectoryPath,
+          protocPath: protocPath,
+          protocGenSwiftPath: protocGenSwiftPath
+        )
+        commands.append(protoCommand)
+      }
+    }
+
+    return commands
+  }
+}
+
+/// Reads the configuration files at the supplied URLs into memory
+/// - Parameter configurationFiles: URLs from which to load configuration
+/// - Returns: A map of source URLs to loaded configuration
+func readConfigurationFiles(
+  _ configurationFiles: [URL],
+  pluginWorkDirectory: URL
+) throws -> [URL: CommonConfiguration] {
+  var configs: [URL: CommonConfiguration] = [:]
+  for configFile in configurationFiles {
+    let data = try Data(contentsOf: configFile)
+    let configuration = try JSONDecoder().decode(ConfigurationFile.self, from: data)
+
+    var config = CommonConfiguration(configurationFile: configuration)
+    // hard-code full-path to avoid collisions since this goes into a temporary directory anyway
+    config.fileNaming = .fullPath
+    // the output directory mandated by the plugin system
+    config.outputPath = String(pluginWorkDirectory.relativePath)
+    configs[configFile] = config
+  }
+  return configs
+}
+
+/// Finds the most precisely relevant config file for a given proto file URL.
+/// - Parameters:
+///   - file: The path to the proto file to be matched.
+///   - configFiles: The paths to all known configuration files.
+/// - Returns: The path to the most precisely relevant config file if one is found, otherwise `nil`.
+func findApplicableConfigFor(file: URL, from configFiles: [URL]) -> URL? {
+  let filePathComponents = file.pathComponents
+  for endComponent in (0 ..< filePathComponents.count).reversed() {
+    for configFile in configFiles {
+      if filePathComponents[..<endComponent]
+        == configFile.pathComponents[..<(configFile.pathComponents.count - 1)]
+      {
+        return configFile
+      }
+    }
+  }
+
+  return nil
+}
+
+/// Construct the command to invoke `protoc` with the `proto-gen-grpc-swift` plugin.
+/// - Parameters:
+///   - inputFile: The input `.proto` file.
+///   - configFile: The path file containing configuration for this operation.
+///   - config: The configuration for this operation.
+///   - protoDirectoryPath: The root path to the source `.proto` files used as the reference for relative path naming schemes.
+///   - protocPath: The path to `protoc`
+///   - protocGenGRPCSwiftPath: The path to `proto-gen-grpc-swift`.
+/// - Returns: The command to invoke `protoc` with the `proto-gen-grpc-swift` plugin.
+func protocGenGRPCSwiftCommand(
+  inputFile: URL,
+  configFile: URL,
+  config: CommonConfiguration,
+  protoDirectoryPath: URL,
+  protocPath: URL,
+  protocGenGRPCSwiftPath: URL
+) throws -> PackagePlugin.Command {
+  guard let fileNaming = config.fileNaming else {
+    assertionFailure("Missing file naming strategy - should be hard-coded.")
+    throw PluginError.missingFileNamingStrategy
+  }
+
+  guard let outputPath = config.outputPath else {
+    assertionFailure("Missing output path - should be hard-coded.")
+    throw PluginError.missingOutputPath
+  }
+  let outputPathURL = URL(fileURLWithPath: outputPath)
+
+  let outputFilePath = deriveOutputFilePath(
+    for: inputFile,
+    using: fileNaming,
+    protoDirectoryPath: protoDirectoryPath,
+    outputDirectory: outputPathURL,
+    outputExtension: "grpc.swift"
+  )
+
+  let arguments = constructProtocGenGRPCSwiftArguments(
+    config: config,
+    using: fileNaming,
+    inputFiles: [inputFile],
+    protoDirectoryPaths: [protoDirectoryPath],
+    protocGenGRPCSwiftPath: protocGenGRPCSwiftPath,
+    outputDirectory: outputPathURL
+  )
+
+  return Command.buildCommand(
+    displayName: "Generating gRPC Swift files for \(inputFile.relativePath)",
+    executable: protocPath,
+    arguments: arguments,
+    inputFiles: [inputFile, protocGenGRPCSwiftPath],
+    outputFiles: [outputFilePath]
+  )
+}
+
+/// Construct the command to invoke `protoc` with the `proto-gen-swift` plugin.
+/// - Parameters:
+///   - inputFile: The input `.proto` file.
+///   - configFile: The path file containing configuration for this operation.
+///   - config: The configuration for this operation.
+///   - protoDirectoryPath: The root path to the source `.proto` files used as the reference for relative path naming schemes.
+///   - protocPath: The path to `protoc`
+///   - protocGenSwiftPath: The path to `proto-gen-grpc-swift`.
+/// - Returns: The command to invoke `protoc` with the `proto-gen-swift` plugin.
+func protocGenSwiftCommand(
+  inputFile: URL,
+  configFile: URL,
+  config: CommonConfiguration,
+  protoDirectoryPath: URL,
+  protocPath: URL,
+  protocGenSwiftPath: URL
+) throws -> PackagePlugin.Command {
+  guard let fileNaming = config.fileNaming else {
+    assertionFailure("Missing file naming strategy - should be hard-coded.")
+    throw PluginError.missingFileNamingStrategy
+  }
+
+  guard let outputPath = config.outputPath else {
+    assertionFailure("Missing output path - should be hard-coded.")
+    throw PluginError.missingOutputPath
+  }
+  let outputPathURL = URL(fileURLWithPath: outputPath)
+
+  let outputFilePath = deriveOutputFilePath(
+    for: inputFile,
+    using: fileNaming,
+    protoDirectoryPath: protoDirectoryPath,
+    outputDirectory: outputPathURL,
+    outputExtension: "pb.swift"
+  )
+
+  let arguments = constructProtocGenSwiftArguments(
+    config: config,
+    using: fileNaming,
+    inputFiles: [inputFile],
+    protoDirectoryPaths: [protoDirectoryPath],
+    protocGenSwiftPath: protocGenSwiftPath,
+    outputDirectory: outputPathURL
+  )
+
+  return Command.buildCommand(
+    displayName: "Generating protobuf Swift files for \(inputFile.relativePath)",
+    executable: protocPath,
+    arguments: arguments,
+    inputFiles: [inputFile, protocGenSwiftPath],
+    outputFiles: [outputFilePath]
+  )
+}
+
+// Entry-point when using Package manifest
+extension GRPCGeneratorPlugin: BuildToolPlugin, LocalizedError {
+  /// Create build commands, the entry-point when using a Package manifest.
+  func createBuildCommands(context: PluginContext, target: Target) async throws -> [Command] {
+    guard let swiftTarget = target as? SwiftSourceModuleTarget else {
+      throw PluginError.incompatibleTarget(target.name)
+    }
+    let configFiles = swiftTarget.sourceFiles(withSuffix: "grpc-swift-config.json").map { $0.url }
+    let inputFiles = swiftTarget.sourceFiles(withSuffix: ".proto").map { $0.url }
+    return try createBuildCommands(
+      pluginWorkDirectory: context.pluginWorkDirectoryURL,
+      tool: context.tool,
+      inputFiles: inputFiles,
+      configFiles: configFiles,
+      targetName: target.name
+    )
+  }
+}
+
+#if canImport(XcodeProjectPlugin)
+import XcodeProjectPlugin
+
+// Entry-point when using Xcode projects
+extension GRPCGeneratorPlugin: XcodeBuildToolPlugin {
+  /// Create build commands, the entry-point when using an Xcode project.
+  func createBuildCommands(context: XcodePluginContext, target: XcodeTarget) throws -> [Command] {
+    let configFiles = target.inputFiles.filter {
+      $0.url.lastPathComponent == "grpc-swift-config.json"
+    }.map { $0.url }
+    let inputFiles = target.inputFiles.filter { $0.url.lastPathComponent.hasSuffix(".proto") }.map {
+      $0.url
+    }
+    return try createBuildCommands(
+      pluginWorkDirectory: context.pluginWorkDirectoryURL,
+      tool: context.tool,
+      inputFiles: inputFiles,
+      configFiles: configFiles,
+      targetName: target.displayName
+    )
+  }
+}
+#endif

Plugins/GRPCGeneratorPlugin/PluginsShared

@@ -0,0 +1 @@
+../PluginsShared