grpc-swift/Sources/GRPCCore/Call/Client/ClientResponse.swift

/*
 * Copyright 2023, 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.
 */

/// A response for a single message received by a client.
///
/// Single responses are used for unary and client-streaming RPCs. For streaming responses
/// see ``StreamingClientResponse``.
///
/// A single response captures every part of the response stream and distinguishes successful
/// and unsuccessful responses via the ``accepted`` property. The value for the `success` case
/// contains the initial metadata, response message, and the trailing metadata and implicitly
/// has an ``Status/Code-swift.struct/ok`` status code.
///
/// The `failure` case indicates that the server chose not to process the RPC, or the processing
/// of the RPC failed, or the client failed to execute the request. The failure case contains
/// an ``RPCError`` describing why the RPC failed, including an error code, error message and any
/// metadata sent by the server.
///
/// ### Using responses
///
/// Each response has a ``accepted`` property which contains all RPC information. You can create
/// one by calling ``init(accepted:)`` or one of the two convenience initializers:
/// - ``init(message:metadata:trailingMetadata:)`` to create a successful response, or
/// - ``init(of:error:)`` to create a failed response.
///
/// You can interrogate a response by inspecting the ``accepted`` property directly or by using
/// its convenience properties:
/// - ``metadata`` extracts the initial metadata,
/// - ``message`` extracts the message, or throws if the response failed, and
/// - ``trailingMetadata`` extracts the trailing metadata.
///
/// The following example demonstrates how you can use the API:
///
/// ```swift
/// // Create a successful response
/// let response = ClientResponse<String>(
///   message: "Hello, World!",
///   metadata: ["hello": "initial metadata"],
///   trailingMetadata: ["goodbye": "trailing metadata"]
/// )
///
/// // The explicit API:
/// switch response {
/// case .success(let contents):
///   print("Received response with message '\(try contents.message.get())'")
/// case .failure(let error):
///   print("RPC failed with code '\(error.code)'")
/// }
///
/// // The convenience API:
/// do {
///   print("Received response with message '\(try response.message)'")
/// } catch let error as RPCError {
///   print("RPC failed with code '\(error.code)'")
/// }
/// ```
public struct ClientResponse<Message: Sendable>: Sendable {
  /// The contents of an accepted response with a single message.
  public struct Contents: Sendable {
    /// Metadata received from the server at the beginning of the response.
    ///
    /// The metadata may contain transport-specific information in addition to any application
    /// level metadata provided by the service.
    public var metadata: Metadata

    /// The response message received from the server, or an error of the RPC failed with a
    /// non-ok status.
    public var message: Result<Message, RPCError>

    /// Metadata received from the server at the end of the response.
    ///
    /// The metadata may contain transport-specific information in addition to any application
    /// level metadata provided by the service.
    public var trailingMetadata: Metadata

    /// Creates a `Contents`.
    ///
    /// - Parameters:
    ///   - metadata: Metadata received from the server at the beginning of the response.
    ///   - message: The response message received from the server.
    ///   - trailingMetadata: Metadata received from the server at the end of the response.
    public init(
      metadata: Metadata,
      message: Message,
      trailingMetadata: Metadata
    ) {
      self.metadata = metadata
      self.message = .success(message)
      self.trailingMetadata = trailingMetadata
    }

    /// Creates a `Contents`.
    ///
    /// - Parameters:
    ///   - metadata: Metadata received from the server at the beginning of the response.
    ///   - error: Error received from the server.
    public init(
      metadata: Metadata,
      error: RPCError
    ) {
      self.metadata = metadata
      self.message = .failure(error)
      self.trailingMetadata = error.metadata
    }
  }

  /// Whether the RPC was accepted or rejected.
  ///
  /// The `success` case indicates the RPC completed successfully with an
  /// ``Status/Code-swift.struct/ok`` status code. The `failure` case indicates that the RPC was
  /// rejected by the server and wasn't processed or couldn't be processed successfully.
  public var accepted: Result<Contents, RPCError>

  /// Creates a new response.
  ///
  /// - Parameter accepted: The result of the RPC.
  public init(accepted: Result<Contents, RPCError>) {
    self.accepted = accepted
  }
}

/// A response for a stream of messages received by a client.
///
/// Stream responses are used for server-streaming and bidirectional-streaming RPCs. For single
/// responses see ``ClientResponse``.
///
/// A stream response captures every part of the response stream over time and distinguishes
/// accepted and rejected requests via the ``accepted`` property. An "accepted" request is one
/// where the the server responds with initial metadata and attempts to process the request. A
/// "rejected" request is one where the server responds with a status as the first and only
/// response part and doesn't process the request body.
///
/// The value for the `success` case contains the initial metadata and a ``RPCAsyncSequence`` of
/// message parts (messages followed by a single status). If the sequence completes without
/// throwing then the response implicitly has an ``Status/Code-swift.struct/ok`` status code.
/// However, the response sequence may also throw an ``RPCError`` if the server fails to complete
/// processing the request.
///
/// The `failure` case indicates that the server chose not to process the RPC or the client failed
/// to execute the request. The failure case contains an ``RPCError`` describing why the RPC
/// failed, including an error code, error message and any metadata sent by the server.
///
/// ### Using streaming responses
///
/// Each response has a ``accepted`` property which contains RPC information. You can create
/// one by calling ``init(accepted:)`` or one of the two convenience initializers:
/// - ``init(of:metadata:bodyParts:)`` to create an accepted response, or
/// - ``init(of:error:)`` to create a failed response.
///
/// You can interrogate a response by inspecting the ``accepted`` property directly or by using
/// its convenience properties:
/// - ``metadata`` extracts the initial metadata,
/// - ``messages`` extracts the sequence of response message, or throws if the response failed.
///
/// The following example demonstrates how you can use the API:
///
/// ```swift
/// // Create a failed response
/// let response = StreamingClientResponse(
///   of: String.self,
///   error: RPCError(code: .notFound, message: "The requested resource couldn't be located")
/// )
///
/// // The explicit API:
/// switch response {
/// case .success(let contents):
///   for try await part in contents.bodyParts {
///     switch part {
///     case .message(let message):
///       print("Received message '\(message)'")
///     case .trailingMetadata(let metadata):
///       print("Received trailing metadata '\(metadata)'")
///     }
///   }
/// case .failure(let error):
///   print("RPC failed with code '\(error.code)'")
/// }
///
/// // The convenience API:
/// do {
///   for try await message in response.messages {
///     print("Received message '\(message)'")
///   }
/// } catch let error as RPCError {
///   print("RPC failed with code '\(error.code)'")
/// }
/// ```
public struct StreamingClientResponse<Message: Sendable>: Sendable {
  public struct Contents: Sendable {
    /// Metadata received from the server at the beginning of the response.
    ///
    /// The metadata may contain transport-specific information in addition to any application
    /// level metadata provided by the service.
    public var metadata: Metadata

    /// A sequence of stream parts received from the server ending with metadata if the RPC
    /// succeeded.
    ///
    /// If the RPC fails then the sequence will throw an ``RPCError``.
    ///
    /// The sequence may only be iterated once.
    public var bodyParts: RPCAsyncSequence<BodyPart, any Error>

    /// Parts received from the server.
    public enum BodyPart: Sendable {
      /// A response message.
      case message(Message)
      /// Metadata. Must be the final value of the sequence unless the stream throws an error.
      case trailingMetadata(Metadata)
    }

    /// Creates a ``Contents``.
    ///
    /// - Parameters:
    ///   - metadata: Metadata received from the server at the beginning of the response.
    ///   - bodyParts: An `AsyncSequence` of parts received from the server.
    public init(
      metadata: Metadata,
      bodyParts: RPCAsyncSequence<BodyPart, any Error>
    ) {
      self.metadata = metadata
      self.bodyParts = bodyParts
    }
  }

  /// Whether the RPC was accepted or rejected.
  ///
  /// The `success` case indicates the RPC was accepted by the server for
  /// processing, however, the RPC may still fail by throwing an error from its
  /// `messages` sequence. The `failure` case indicates that the RPC was
  /// rejected by the server.
  public var accepted: Result<Contents, RPCError>

  /// Creates a new response.
  ///
  /// - Parameter accepted: The result of the RPC.
  public init(accepted: Result<Contents, RPCError>) {
    self.accepted = accepted
  }
}

// MARK: - Convenience API

extension ClientResponse {
  /// Creates a new accepted response.
  ///
  /// - Parameters:
  ///   - metadata: Metadata received from the server at the beginning of the response.
  ///   - message: The response message received from the server.
  ///   - trailingMetadata: Metadata received from the server at the end of the response.
  public init(message: Message, metadata: Metadata = [:], trailingMetadata: Metadata = [:]) {
    let contents = Contents(
      metadata: metadata,
      message: message,
      trailingMetadata: trailingMetadata
    )
    self.accepted = .success(contents)
  }

  /// Creates a new accepted response with a failed outcome.
  ///
  /// - Parameters:
  ///   - messageType: The type of message.
  ///   - metadata: Metadata received from the server at the beginning of the response.
  ///   - error: An error describing why the RPC failed.
  public init(of messageType: Message.Type = Message.self, metadata: Metadata, error: RPCError) {
    let contents = Contents(metadata: metadata, error: error)
    self.accepted = .success(contents)
  }

  /// Creates a new failed response.
  ///
  /// - Parameters:
  ///   - messageType: The type of message.
  ///   - error: An error describing why the RPC failed.
  public init(of messageType: Message.Type = Message.self, error: RPCError) {
    self.accepted = .failure(error)
  }

  /// Returns metadata received from the server at the start of the response.
  ///
  /// For rejected RPCs (in other words, where ``accepted`` is `failure`) the metadata is empty.
  public var metadata: Metadata {
    switch self.accepted {
    case let .success(contents):
      return contents.metadata
    case .failure:
      return [:]
    }
  }

  /// Returns the message received from the server.
  ///
  /// - Throws: ``RPCError`` if the request failed.
  public var message: Message {
    get throws {
      try self.accepted.flatMap { $0.message }.get()
    }
  }

  /// Returns metadata received from the server at the end of the response.
  ///
  /// Unlike ``metadata``, for rejected RPCs the metadata returned may contain values.
  public var trailingMetadata: Metadata {
    switch self.accepted {
    case let .success(contents):
      return contents.trailingMetadata
    case let .failure(error):
      return error.metadata
    }
  }
}

extension StreamingClientResponse {
  /// Creates a new accepted response.
  ///
  /// - Parameters:
  ///   - messageType: The type of message.
  ///   - metadata: Metadata received from the server at the beginning of the response.
  ///   - bodyParts: An ``RPCAsyncSequence`` of response parts received from the server.
  public init(
    of messageType: Message.Type = Message.self,
    metadata: Metadata,
    bodyParts: RPCAsyncSequence<Contents.BodyPart, any Error>
  ) {
    let contents = Contents(metadata: metadata, bodyParts: bodyParts)
    self.accepted = .success(contents)
  }

  /// Creates a new failed response.
  ///
  /// - Parameters:
  ///   - messageType: The type of message.
  ///   - error: An error describing why the RPC failed.
  public init(of messageType: Message.Type = Message.self, error: RPCError) {
    self.accepted = .failure(error)
  }

  /// Returns metadata received from the server at the start of the response.
  ///
  /// For rejected RPCs (in other words, where ``accepted`` is `failure`) the metadata is empty.
  public var metadata: Metadata {
    switch self.accepted {
    case let .success(contents):
      return contents.metadata
    case .failure:
      return [:]
    }
  }

  /// Returns the messages received from the server.
  ///
  /// For rejected RPCs the `RPCAsyncSequence` throws a `RPCError``.
  public var messages: RPCAsyncSequence<Message, any Error> {
    switch self.accepted {
    case let .success(contents):
      let filtered = contents.bodyParts.compactMap {
        switch $0 {
        case let .message(message):
          return message
        case .trailingMetadata:
          return nil
        }
      }

      return RPCAsyncSequence(wrapping: filtered)

    case let .failure(error):
      return RPCAsyncSequence.throwing(error)
    }
  }
}