gRPC-webがどのようにリクエストをシリアライズしているのか

AAAAAAcKBWhlbGxvgAAAAehncnBjLX....

echo "AAAAAAcKBWhlbGxvgAAAAehncnBjLXN0YXR1czowDQpncnBjLW1lc3NhZ2U6T0sNCnNlYy1mZXRjaC1tb2RlOmNvcnMNCngtdXNlci1hZ2VudDpncnBjLXdlYi1qYXZhc2NyaXB0LzAuMQ0KY3VzdG9tLWhlYWRlci0xOnZhbHVlMQ0KdXNlci1hZ2VudDpNb3ppbGxhLzUuMCAoWDExOyBMaW51eCB4ODZfNjQpIEFwcGxlV2ViS2l0LzUzNy4zNiAoS0hUTUwsIGxpa2UgR2Vja28pIENocm9tZS83Ny4wLjM4NjUuOTAgU2FmYXJpLzUzNy4zNg0KYWNjZXB0OmFwcGxpY2F0aW9uL2dycGMtd2ViLXRleHQNCngtZ3JwYy13ZWI6MQ0Kb3JpZ2luOmh0dHA6Ly9sb2NhbGhvc3Q6ODA4MQ0Kc2VjLWZldGNoLXNpdGU6c2FtZS1zaXRlDQpyZWZlcmVyOmh0dHA6Ly9sb2NhbGhvc3Q6ODA4MS9lY2hvdGVzdC5odG1sDQphY2NlcHQtbGFuZ3VhZ2U6ZW4tVVMsZW47cT0wLjkNCngtZm9yd2FyZGVkLXByb3RvOmh0dHANCngtcmVxdWVzdC1pZDo5OTM2YmVjMS0zZjVmLTRkZGMtYjg1OS1iZDAxZTdjOGQ1YjUNCg==" | base64 -d | xxd
00000000: 0000 0000 070a 0568 656c 6c6f 8000 0001  .......hello....
00000010: e867 7270 632d 7374 6174 7573 3a30 0d0a  .grpc-status:0..
00000020: 6772 7063 2d6d 6573 7361 6765 3a4f 4b0d  grpc-message:OK.
00000030: 0a73 6563 2d66 6574 6368 2d6d 6f64 653a  .sec-fetch-mode:
...(以下省略)

const toBytesInt32 = (num: number): Uint8Array => {
  // an Int32 takes 4 bytes
  const arr = new ArrayBuffer(4)
  const view = new DataView(arr)
  // byteOffset = 0; litteEndian = false
  view.setUint32(0, num, false)
  return new Uint8Array(arr)
}

const createHeader = (name: string, value: string): Buffer => {
  const buffers: Array<number> = []
  for (const char of name) {
    buffers.push(char.charCodeAt(0))
  }
  buffers.push(':'.charCodeAt(0))
  for (const char of value) {
    buffers.push(char.charCodeAt(0))
  }
  buffers.push('\r'.charCodeAt(0))
  buffers.push('\n'.charCodeAt(0))
  return Buffer.from(buffers)
}

export const encodeProtobuf = <
  GRPCReply extends { serializeBinary: () => Uint8Array },
>(
  reply: GRPCReply,
): string => {
  const replyBuffer = reply.serializeBinary()
  const replyLength = toBytesInt32(replyBuffer.length)

  const headers = Buffer.from([
    ...createHeader('grpc-status', '0'),
    ...createHeader('grpc-message', 'OK'),
  ])
  const headersLength = toBytesInt32(headers.length)

  const response = Buffer.from([
    0, // mark response start ("data" frame coming next)
    ...replyLength,
    ...replyBuffer,
    128, // end of "data" frame, trailer frame coming next
    ...headersLength,
    ...headers,
  ])

  return response.toString('base64')
}

function main(): void {
  // UserPB is protobuf object created with protoc-gen-grpc
  // Any protobuf object with a serializeBinary function 
  // which returns an Uint8Array will do
  const reply = new UserPB.SetUserReply()
  reply.setUserId('mock-user-id')
  reply.setFirstName('foo')
  reply.setLastName('bar')
  const encoded = encodeProtobuf(reply)
  console.log(encoded)
}

main()