Method flushFrame

conn.go:556–641

flushFrame writes buffered data and extra as a frame to the network. The final argument indicates that this is the last frame in the message.

(final bool, extra []byte)

// flushFrame writes buffered data and extra as a frame to the network. The
// final argument indicates that this is the last frame in the message.
func (w *messageWriter) flushFrame(final bool, extra []byte) error {
	c := w.c
	length := w.pos - maxFrameHeaderSize + len(extra)

	// Check for invalid control frames.
	if isControl(w.frameType) &&
		(!final || length > maxControlFramePayloadSize) {
		return w.endMessage(errInvalidControlFrame)
	}

	b0 := byte(w.frameType)
	if final {
		b0 |= finalBit
	}
	if w.compress {
		b0 |= rsv1Bit
	}
	w.compress = false

	b1 := byte(0)
	if !c.isServer {
		b1 |= maskBit
	}

	// Assume that the frame starts at beginning of c.writeBuf.
	framePos := 0
	if c.isServer {
		// Adjust up if mask not included in the header.
		framePos = 4
	}

	switch {
	case length >= 65536:
		c.writeBuf[framePos] = b0
		c.writeBuf[framePos+1] = b1 | 127
		binary.BigEndian.PutUint64(c.writeBuf[framePos+2:], uint64(length))
	case length > 125:
		framePos += 6
		c.writeBuf[framePos] = b0
		c.writeBuf[framePos+1] = b1 | 126
		binary.BigEndian.PutUint16(c.writeBuf[framePos+2:], uint16(length))
	default:
		framePos += 8
		c.writeBuf[framePos] = b0
		c.writeBuf[framePos+1] = b1 | byte(length)
	}

	if !c.isServer {
		key := newMaskKey()
		copy(c.writeBuf[maxFrameHeaderSize-4:], key[:])
		maskBytes(key, 0, c.writeBuf[maxFrameHeaderSize:w.pos])
		if len(extra) > 0 {
			return w.endMessage(c.writeFatal(errors.New("websocket: internal error, extra used in client mode")))
		}
	}

	// Write the buffers to the connection with best-effort detection of
	// concurrent writes. See the concurrency section in the package