func _read()

in Sources/NIOPosix/NonBlockingFileIO.swift [181:217]
36 lines of code
9 McCabe index (conditional complexity)

        func _read(remainingReads: Int, bytesReadSoFar: Int64) {
            if remainingReads > 1 || (remainingReads == 1 && lastReadSize > 0) {
                let readSize = remainingReads > 1 ? chunkSize : lastReadSize
                assert(readSize > 0)
                let readFuture = self.read0(fileHandle: fileHandle,
                                            fromOffset: fromOffset.map { $0 + bytesReadSoFar },
                                            byteCount: readSize,
                                            allocator: allocator,
                                            eventLoop: eventLoop)
                readFuture.whenComplete { (result) in
                    switch result {
                    case .success(let buffer):
                        guard buffer.readableBytes > 0 else {
                            // EOF, call `chunkHandler` one more time.
                            let handlerFuture = chunkHandler(buffer)
                            handlerFuture.cascade(to: promise)
                            return
                        }
                        let bytesRead = Int64(buffer.readableBytes)
                        chunkHandler(buffer).whenComplete { result in
                            switch result {
                            case .success(_):
                                eventLoop.assertInEventLoop()
                                _read(remainingReads: remainingReads - 1,
                                      bytesReadSoFar: bytesReadSoFar + bytesRead)
                            case .failure(let error):
                                promise.fail(error)
                            }
                        }
                  case .failure(let error):
                      promise.fail(error)
                  }
              }
            } else {
                promise.succeed(())
            }
        }