why unix domain socket listener read expectedPacketLength?

[zdyj3170101136]

in uds listener:

var expectedPacketLength uint32
		var maxPacketLength uint32
		if l.transport == "unix" {
			// Read the expected packet length (in stream mode)
			b := []byte{0, 0, 0, 0}
			_, err = io.ReadFull(conn, b)
			expectedPacketLength := binary.LittleEndian.Uint32(b)

			switch {
			case err == io.EOF, errors.Is(err, io.ErrUnexpectedEOF):
				log.Debugf("dogstatsd-uds: %s connection closed", l.transport)
				return nil
			}
			if expectedPacketLength > uint32(len(packet.Buffer)) {
				log.Info("dogstatsd-uds: packet length too large, dropping connection")
				return nil
			}
			maxPacketLength = expectedPacketLength
		} else {
			maxPacketLength = uint32(len(packet.Buffer))
		}

		for err == nil {
			if oob != nil {
				n, oobn, _, _, err = conn.ReadMsgUnix(packet.Buffer[n:maxPacketLength], oobS[oobn:])
			} else {
				n, _, err = conn.ReadFromUnix(packet.Buffer[n:maxPacketLength])
			}

why we read expectedPacketLength?
each system call is expensize, we should not read it!.

why we use api conn.ReadFromUnix but not conn.Read()?
there is no difference.

[zdyj3170101136]

@sschepens

[zdyj3170101136]

@remh