grpc-swift/Sources/GRPC/ServerCallContexts/UnaryResponseCallContext.swift

/*
 * Copyright 2019, 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 Logging
import NIO
import NIOHPACK
import NIOHTTP1
import SwiftProtobuf

/// Abstract base class exposing a method that exposes a promise for the RPC response.
///
/// - When `responsePromise` is fulfilled, the call is closed and the provided response transmitted with status `responseStatus` (`.ok` by default).
/// - If `statusPromise` is failed and the error is of type `GRPCStatusTransformable`,
///   the result of `error.asGRPCStatus()` will be returned to the client.
/// - If `error.asGRPCStatus()` is not available, `GRPCStatus.processingError` is returned to the client.
///
/// For unary calls, the response is not actually provided by fulfilling `responsePromise`, but instead by completing
/// the future returned by `UnaryCallHandler.EventObserver`.
open class UnaryResponseCallContext<ResponsePayload>: ServerCallContextBase, StatusOnlyCallContext {
  typealias WrappedResponse = GRPCServerResponsePart<ResponsePayload>

  public let responsePromise: EventLoopPromise<ResponsePayload>
  public var responseStatus: GRPCStatus = .ok

  public convenience init(
    eventLoop: EventLoop,
    headers: HPACKHeaders,
    logger: Logger,
    userInfo: UserInfo = UserInfo()
  ) {
    self.init(eventLoop: eventLoop, headers: headers, logger: logger, userInfoRef: .init(userInfo))
  }

  @inlinable
  override internal init(
    eventLoop: EventLoop,
    headers: HPACKHeaders,
    logger: Logger,
    userInfoRef: Ref<UserInfo>
  ) {
    self.responsePromise = eventLoop.makePromise()
    super.init(eventLoop: eventLoop, headers: headers, logger: logger, userInfoRef: userInfoRef)
  }
}

/// Protocol variant of `UnaryResponseCallContext` that only exposes the `responseStatus` and `trailingMetadata`
/// fields, but not `responsePromise`.
///
/// Motivation: `UnaryCallHandler` already asks the call handler return an `EventLoopFuture<ResponsePayload>` which
/// is automatically cascaded into `UnaryResponseCallContext.responsePromise`, so that promise does not (and should not)
/// be fulfilled by the user.
///
/// We can use a protocol (instead of an abstract base class) here because removing the generic `responsePromise` field
/// lets us avoid associated-type requirements on the protocol.
public protocol StatusOnlyCallContext: ServerCallContext {
  var responseStatus: GRPCStatus { get set }
  var trailers: HPACKHeaders { get set }
}

/// Concrete implementation of `UnaryResponseCallContext` used for testing.
///
/// Only provided to make it clear in tests that no "real" implementation is used.
open class UnaryResponseCallContextTestStub<ResponsePayload>: UnaryResponseCallContext<ResponsePayload> {}