grpc-swift/Tests/GRPCHTTP2CoreTests/Server/GRPCServerStreamHandlerTests.swift

/*
 * 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 GRPCCore
import NIOCore
import NIOEmbedded
import NIOHPACK
import NIOHTTP2
import XCTest

@testable import GRPCHTTP2Core

@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
final class GRPCServerStreamHandlerTests: XCTestCase {
  func testH2FramesAreIgnored() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    let framesToBeIgnored: [HTTP2Frame.FramePayload] = [
      .ping(.init(), ack: false),
      .goAway(lastStreamID: .rootStream, errorCode: .cancel, opaqueData: nil),
      // TODO: add .priority(StreamPriorityData) - right now, StreamPriorityData's
      // initialiser is internal, so I can't create one of these frames.
      .rstStream(.cancel),
      .settings(.ack),
      .pushPromise(.init(pushedStreamID: .maxID, headers: [:])),
      .windowUpdate(windowSizeIncrement: 4),
      .alternativeService(origin: nil, field: nil),
      .origin([]),
    ]

    for toBeIgnored in framesToBeIgnored {
      XCTAssertNoThrow(try channel.writeInbound(toBeIgnored))
      XCTAssertNil(try channel.readInbound(as: HTTP2Frame.FramePayload.self))
    }
  }

  func testClientInitialMetadataWithoutContentTypeResultsInRejectedRPC() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata without content-type
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we have sent a trailers-only response
    let writtenTrailersOnlyResponse = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(writtenTrailersOnlyResponse.headers, [":status": "415"])
    XCTAssertTrue(writtenTrailersOnlyResponse.endStream)
  }

  func testClientInitialMetadataWithoutMethodResultsInRejectedRPC() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata without :method
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we have sent a trailers-only response
    let writtenTrailersOnlyResponse = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenTrailersOnlyResponse.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        GRPCHTTP2Keys.grpcStatus.rawValue: String(Status.Code.invalidArgument.rawValue),
        GRPCHTTP2Keys.grpcStatusMessage.rawValue:
          ":method header is expected to be present and have a value of \"POST\".",
      ]
    )
    XCTAssertTrue(writtenTrailersOnlyResponse.endStream)
  }

  func testClientInitialMetadataWithoutSchemeResultsInRejectedRPC() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata without :scheme
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we have sent a trailers-only response
    let writtenTrailersOnlyResponse = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenTrailersOnlyResponse.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        GRPCHTTP2Keys.grpcStatus.rawValue: String(Status.Code.invalidArgument.rawValue),
        GRPCHTTP2Keys.grpcStatusMessage.rawValue:
          ":scheme header must be present and one of \"http\" or \"https\".",
      ]
    )
    XCTAssertTrue(writtenTrailersOnlyResponse.endStream)
  }

  func testClientInitialMetadataWithoutPathResultsInRejectedRPC() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata without :path
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we have sent a trailers-only response
    let writtenTrailersOnlyResponse = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenTrailersOnlyResponse.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        GRPCHTTP2Keys.grpcStatus.rawValue: String(Status.Code.unimplemented.rawValue),
        GRPCHTTP2Keys.grpcStatusMessage.rawValue: "No :path header has been set.",
      ]
    )
    XCTAssertTrue(writtenTrailersOnlyResponse.endStream)
  }

  func testClientInitialMetadataWithoutTEResultsInRejectedRPC() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata without TE
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we have sent a trailers-only response
    let writtenTrailersOnlyResponse = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenTrailersOnlyResponse.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        GRPCHTTP2Keys.grpcStatus.rawValue: String(Status.Code.invalidArgument.rawValue),
        GRPCHTTP2Keys.grpcStatusMessage.rawValue:
          "\"te\" header is expected to be present and have a value of \"trailers\".",
      ]
    )
    XCTAssertTrue(writtenTrailersOnlyResponse.endStream)
  }

  func testNotAcceptedEncodingResultsInRejectedRPC() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 100
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
      GRPCHTTP2Keys.encoding.rawValue: "deflate",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we have sent a trailers-only response
    let writtenTrailersOnlyResponse = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenTrailersOnlyResponse.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        GRPCHTTP2Keys.grpcStatus.rawValue: String(Status.Code.unimplemented.rawValue),
        GRPCHTTP2Keys.grpcStatusMessage.rawValue: "Compression is not supported",
      ]
    )
    XCTAssertTrue(writtenTrailersOnlyResponse.endStream)
  }

  func testOverMaximumPayloadSize() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 1
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we haven't sent back an error response, and that we read the initial metadata
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.metadata(Metadata(headers: clientInitialMetadata))
    )

    // Write back server's initial metadata
    let headers: HPACKHeaders = [
      "some-custom-header": "some-custom-value"
    ]
    let serverInitialMetadata = RPCResponsePart.metadata(Metadata(headers: headers))
    XCTAssertNoThrow(try channel.writeOutbound(serverInitialMetadata))

    // Make sure we wrote back the initial metadata
    let writtenHeaders = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenHeaders.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        "some-custom-header": "some-custom-value",
      ]
    )

    // Receive client's message
    var buffer = ByteBuffer()
    buffer.writeInteger(UInt8(0))  // not compressed
    buffer.writeInteger(UInt32(42))  // message length
    buffer.writeRepeatingByte(0, count: 42)  // message
    let clientDataPayload = HTTP2Frame.FramePayload.Data(data: .byteBuffer(buffer), endStream: true)
    XCTAssertThrowsError(
      ofType: RPCError.self,
      try channel.writeInbound(HTTP2Frame.FramePayload.data(clientDataPayload))
    ) { error in
      XCTAssertEqual(error.code, .resourceExhausted)
      XCTAssertEqual(
        error.message,
        "Message has exceeded the configured maximum payload size (max: 1, actual: 42)"
      )
    }

    // Make sure we haven't sent a response back and that we didn't read the received message
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertNil(try channel.readInbound(as: RPCRequestPart.self))
  }

  func testClientEndsStream() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 100,
      skipStateMachineAssertions: true
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata with end stream set
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata, endStream: true))
      )
    )

    // Make sure we haven't sent back an error response, and that we read the initial metadata
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.metadata(Metadata(headers: clientInitialMetadata))
    )

    // Write back server's initial metadata
    let headers: HPACKHeaders = [
      "some-custom-header": "some-custom-value"
    ]
    let serverInitialMetadata = RPCResponsePart.metadata(Metadata(headers: headers))
    XCTAssertNoThrow(try channel.writeOutbound(serverInitialMetadata))

    // Make sure we wrote back the initial metadata
    let writtenHeaders = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenHeaders.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        "some-custom-header": "some-custom-value",
      ]
    )

    // We should throw if the client sends another message, since it's closed the stream already.
    var buffer = ByteBuffer()
    buffer.writeInteger(UInt8(0))  // not compressed
    buffer.writeInteger(UInt32(42))  // message length
    buffer.writeRepeatingByte(0, count: 42)  // message
    let clientDataPayload = HTTP2Frame.FramePayload.Data(data: .byteBuffer(buffer), endStream: true)
    XCTAssertThrowsError(
      ofType: RPCError.self,
      try channel.writeInbound(HTTP2Frame.FramePayload.data(clientDataPayload))
    ) { error in
      XCTAssertEqual(error.code, .internalError)
      XCTAssertEqual(error.message, "Client can't send a message if closed.")
    }
  }

  func testNormalFlow() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 100,
      skipStateMachineAssertions: true
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we haven't sent back an error response, and that we read the initial metadata
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.metadata(Metadata(headers: clientInitialMetadata))
    )

    // Write back server's initial metadata
    let headers: HPACKHeaders = [
      "some-custom-header": "some-custom-value"
    ]
    let serverInitialMetadata = RPCResponsePart.metadata(Metadata(headers: headers))
    XCTAssertNoThrow(try channel.writeOutbound(serverInitialMetadata))

    // Make sure we wrote back the initial metadata
    let writtenHeaders = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenHeaders.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        "some-custom-header": "some-custom-value",
      ]
    )

    // Receive client's message
    var buffer = ByteBuffer()
    buffer.writeInteger(UInt8(0))  // not compressed
    buffer.writeInteger(UInt32(42))  // message length
    buffer.writeRepeatingByte(0, count: 42)  // message
    let clientDataPayload = HTTP2Frame.FramePayload.Data(data: .byteBuffer(buffer), endStream: true)
    XCTAssertNoThrow(try channel.writeInbound(HTTP2Frame.FramePayload.data(clientDataPayload)))

    // Make sure we haven't sent back an error response, and that we read the message properly
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.message([UInt8](repeating: 0, count: 42))
    )

    // Write back response
    let serverDataPayload = RPCResponsePart.message([UInt8](repeating: 1, count: 42))
    XCTAssertNoThrow(try channel.writeOutbound(serverDataPayload))

    // Make sure we wrote back the right message
    let writtenMessage = try channel.assertReadDataOutbound()

    var expectedBuffer = ByteBuffer()
    expectedBuffer.writeInteger(UInt8(0))  // not compressed
    expectedBuffer.writeInteger(UInt32(42))  // message length
    expectedBuffer.writeRepeatingByte(1, count: 42)  // message
    XCTAssertEqual(writtenMessage.data, .byteBuffer(expectedBuffer))

    // Send back status to end RPC
    let trailers = RPCResponsePart.status(
      .init(code: .dataLoss, message: "Test data loss"),
      ["custom-header": "custom-value"]
    )
    XCTAssertNoThrow(try channel.writeOutbound(trailers))

    // Make sure we wrote back the status and trailers
    let writtenStatus = try channel.assertReadHeadersOutbound()

    XCTAssertTrue(writtenStatus.endStream)
    XCTAssertEqual(
      writtenStatus.headers,
      [
        GRPCHTTP2Keys.grpcStatus.rawValue: String(Status.Code.dataLoss.rawValue),
        GRPCHTTP2Keys.grpcStatusMessage.rawValue: "Test data loss",
        "custom-header": "custom-value",
      ]
    )

    // Try writing and assert it throws to make sure we don't allow writes
    // after closing.
    XCTAssertThrowsError(
      ofType: RPCError.self,
      try channel.writeOutbound(trailers)
    ) { error in
      XCTAssertEqual(error.code, .internalError)
      XCTAssertEqual(error.message, "Server can't send anything if closed.")
    }
  }

  func testReceiveMessageSplitAcrossMultipleBuffers() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 100
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we haven't sent back an error response, and that we read the initial metadata
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.metadata(Metadata(headers: clientInitialMetadata))
    )

    // Write back server's initial metadata
    let headers: HPACKHeaders = [
      "some-custom-header": "some-custom-value"
    ]
    let serverInitialMetadata = RPCResponsePart.metadata(Metadata(headers: headers))
    XCTAssertNoThrow(try channel.writeOutbound(serverInitialMetadata))

    // Make sure we wrote back the initial metadata
    let writtenHeaders = try channel.assertReadHeadersOutbound()

    XCTAssertEqual(
      writtenHeaders.headers,
      [
        GRPCHTTP2Keys.status.rawValue: "200",
        GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
        "some-custom-header": "some-custom-value",
      ]
    )

    // Receive client's first message
    var buffer = ByteBuffer()
    buffer.writeInteger(UInt8(0))  // not compressed
    XCTAssertNoThrow(
      try channel.writeInbound(HTTP2Frame.FramePayload.data(.init(data: .byteBuffer(buffer))))
    )
    XCTAssertNil(try channel.readInbound(as: RPCRequestPart.self))

    buffer.clear()
    buffer.writeInteger(UInt32(30))  // message length
    XCTAssertNoThrow(
      try channel.writeInbound(HTTP2Frame.FramePayload.data(.init(data: .byteBuffer(buffer))))
    )
    XCTAssertNil(try channel.readInbound(as: RPCRequestPart.self))

    buffer.clear()
    buffer.writeRepeatingByte(0, count: 10)  // first part of the message
    XCTAssertNoThrow(
      try channel.writeInbound(HTTP2Frame.FramePayload.data(.init(data: .byteBuffer(buffer))))
    )
    XCTAssertNil(try channel.readInbound(as: RPCRequestPart.self))

    buffer.clear()
    buffer.writeRepeatingByte(1, count: 10)  // second part of the message
    XCTAssertNoThrow(
      try channel.writeInbound(HTTP2Frame.FramePayload.data(.init(data: .byteBuffer(buffer))))
    )
    XCTAssertNil(try channel.readInbound(as: RPCRequestPart.self))

    buffer.clear()
    buffer.writeRepeatingByte(2, count: 10)  // third part of the message
    XCTAssertNoThrow(
      try channel.writeInbound(HTTP2Frame.FramePayload.data(.init(data: .byteBuffer(buffer))))
    )

    // Make sure we haven't sent back an error response, and that we read the message properly
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.message(
        [UInt8](repeating: 0, count: 10) + [UInt8](repeating: 1, count: 10)
          + [UInt8](repeating: 2, count: 10)
      )
    )
  }

  func testSendMultipleMessagesInSingleBuffer() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 100
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we haven't sent back an error response, and that we read the initial metadata
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.metadata(Metadata(headers: clientInitialMetadata))
    )

    // Write back server's initial metadata
    let headers: HPACKHeaders = [
      "some-custom-header": "some-custom-value"
    ]
    let serverInitialMetadata = RPCResponsePart.metadata(Metadata(headers: headers))
    XCTAssertNoThrow(try channel.writeOutbound(serverInitialMetadata))

    // Read out the metadata
    _ = try channel.readOutbound(as: HTTP2Frame.FramePayload.self)

    // This is where this test actually begins. We want to write two messages
    // without flushing, and make sure that no messages are sent down the pipeline
    // until we flush. Once we flush, both messages should be sent in the same ByteBuffer.

    // Write back first message and make sure nothing's written in the channel.
    XCTAssertNoThrow(channel.write(RPCResponsePart.message([UInt8](repeating: 1, count: 4))))
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))

    // Write back second message and make sure nothing's written in the channel.
    XCTAssertNoThrow(channel.write(RPCResponsePart.message([UInt8](repeating: 2, count: 4))))
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))

    // Now flush and check we *do* write the data.
    channel.flush()

    let writtenMessage = try channel.assertReadDataOutbound()

    // Make sure both messages have been framed together in the ByteBuffer.
    XCTAssertEqual(
      writtenMessage.data,
      .byteBuffer(
        .init(bytes: [
          // First message
          0,  // Compression disabled
          0, 0, 0, 4,  // Message length
          1, 1, 1, 1,  // First message data

          // Second message
          0,  // Compression disabled
          0, 0, 0, 4,  // Message length
          2, 2, 2, 2,  // Second message data
        ])
      )
    )
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
  }

  func testMessageAndStatusAreNotReordered() throws {
    let handler = GRPCServerStreamHandler(
      scheme: .http,
      acceptedEncodings: [],
      maximumPayloadSize: 100
    )

    let channel = EmbeddedChannel(handler: handler)

    // Receive client's initial metadata
    let clientInitialMetadata: HPACKHeaders = [
      GRPCHTTP2Keys.path.rawValue: "test/test",
      GRPCHTTP2Keys.scheme.rawValue: "http",
      GRPCHTTP2Keys.method.rawValue: "POST",
      GRPCHTTP2Keys.contentType.rawValue: "application/grpc",
      GRPCHTTP2Keys.te.rawValue: "trailers",
    ]
    XCTAssertNoThrow(
      try channel.writeInbound(
        HTTP2Frame.FramePayload.headers(.init(headers: clientInitialMetadata))
      )
    )

    // Make sure we haven't sent back an error response, and that we read the initial metadata
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
    XCTAssertEqual(
      try channel.readInbound(as: RPCRequestPart.self),
      RPCRequestPart.metadata(Metadata(headers: clientInitialMetadata))
    )

    // Write back server's initial metadata
    let serverInitialMetadata = RPCResponsePart.metadata(Metadata(headers: [:]))
    XCTAssertNoThrow(try channel.writeOutbound(serverInitialMetadata))

    // Read out the metadata
    _ = try channel.readOutbound(as: HTTP2Frame.FramePayload.self)

    // This is where this test actually begins. We want to write a message followed
    // by status and trailers, and only flush after both writes.
    // Because messages are buffered and potentially bundled together in a single
    // ByteBuffer by the GPRCMessageFramer, we want to make sure that the status
    // and trailers won't be written before the messages.

    // Write back message and make sure nothing's written in the channel.
    XCTAssertNoThrow(channel.write(RPCResponsePart.message([UInt8](repeating: 1, count: 4))))
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))

    // Write status + metadata and make sure nothing's written.
    XCTAssertNoThrow(channel.write(RPCResponsePart.status(.init(code: .ok, message: ""), [:])))
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))

    // Now flush and check we *do* write the data in the right order: message first,
    // trailers second.
    channel.flush()

    let writtenMessage = try channel.assertReadDataOutbound()

    // Make sure we first get message.
    XCTAssertEqual(
      writtenMessage.data,
      .byteBuffer(
        .init(bytes: [
          // First message
          0,  // Compression disabled
          0, 0, 0, 4,  // Message length
          1, 1, 1, 1,  // First message data
        ])
      )
    )
    XCTAssertFalse(writtenMessage.endStream)

    // Make sure we get trailers.
    let writtenTrailers = try channel.assertReadHeadersOutbound()
    XCTAssertEqual(writtenTrailers.headers, ["grpc-status": "0"])
    XCTAssertTrue(writtenTrailers.endStream)

    // Make sure we get nothing else.
    XCTAssertNil(try channel.readOutbound(as: HTTP2Frame.FramePayload.self))
  }
}

extension EmbeddedChannel {
  fileprivate func assertReadHeadersOutbound() throws -> HTTP2Frame.FramePayload.Headers {
    guard
      case .headers(let writtenHeaders) = try XCTUnwrap(
        try self.readOutbound(as: HTTP2Frame.FramePayload.self)
      )
    else {
      throw TestError.assertionFailure("Expected to write headers")
    }
    return writtenHeaders
  }

  fileprivate func assertReadDataOutbound() throws -> HTTP2Frame.FramePayload.Data {
    guard
      case .data(let writtenMessage) = try XCTUnwrap(
        try self.readOutbound(as: HTTP2Frame.FramePayload.self)
      )
    else {
      throw TestError.assertionFailure("Expected to write data")
    }
    return writtenMessage
  }
}

private enum TestError: Error {
  case assertionFailure(String)
}