Something weird is definitely happening with datagram sizes

[bjeanes]

lifx-snoop output:

DATA: 58 00 00 14 00 00 00 00 d0 73 d5 00 00 00 00 00 
      d0 73 d5 00 00 00 00 00 00 00 00 00 00 00 00 00 
      6b 00 00 00 43 44 f2 a8 ff 
MSG:  LIFXMessage(lightState){Color:{Hue:17475 Saturation:43250
                                     Brightness:65535 Kelvin:3500} 
                              Dim:0 Power:65535 Label:emu Tags:0}

DATA: 58 00 00 14 00 00 00 00 d0 73 d5 00 00 00 00 00 
      d0 73 d5 00 00 00 00 00 00 00 00 00 00 00 00 00 
      6b 00 00 00 43 44 f2 a8 ff ff ac 0d 00 00 ff ff 
      65 6d 75 00 00 00 00 00 00 00 00 00 00 00 00 00 
      00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
      00 00 00 00 00 00 00 00 
MSG:  LIFXMessage(lightState){Color:{Hue:17475 Saturation:43250 
                                     Brightness:65535 Kelvin:3500}
                              Dim:0 Power:65535 Label:emu Tags:0}

Same message type (lightState), same declared message size (0x58 0x00: little endian uint16 -> 0x58 -> 88). The second datagram has the correct 88 bytes, but the first only has 41.

I think we're reading incomplete UDP packets but still parsing them. I assumed that this code would avoid this, but clearly not:

if reader.Len() != binary.Size(payload) {
    return message{}, errors.New(fmt.Sprintf("Unexpected payload size for %T", payload))
}

[bjeanes]

This time I got what I expected (from this error handler):

DATA: length=41 (0029)
      58 00 00 54 00 00 00 00 d0 73 d5 00 14 cd 00 00 
      4c 49 46 58 56 32 00 00 00 00 00 00 00 00 00 00 
      6b 00 00 00 00 00 00 00 ee 
ERR:  Incorrect message size (data: 41, header: 88)

I wonder if lifx-snoop is mismatching the bytes with the message. It's possible since the error sender can drop errors if the channel is blocked

[bjeanes]

Yup, they're getting mismatched:

DATA: length=88
      000  58 00 00 14 00 00 00 00  d0 73 d5 00 00 00 00 00  |X........s......|
      010  d0 73 d5 00 00 00 00 00  00 00 00 00 00 00 00 00  |.s..............|
      020  6b 00 00 00 43 44 f2 a8  ff ff ac 0d 00 00 ff ff  |k...CD..........|
      030  65 6d 75 00 00 00 00 00  00 00 00 00 00 00 00 00  |emu.............|
      040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      050  00 00 00 00 00 00 00 00                           |........|
MSG:  LIFXMessage(lightState){Color:{Hue:17475 Saturation:43250 Brightness:65535 Kelvin:3500} Dim:0 Power:65535 Label:emu Tags:0}

DATA: length=88
      000  58 00 00 14 00 00 00 00  d0 73 d5 00 00 00 00 00  |X........s......|
      010  d0 73 d5 00 00 00 00 00  00 00 00 00 00 00 00 00  |.s..............|
      020  6b 00 00 00 43 44 f2 a8  ff ff ac 0d 00 00 ff ff  |k...CD..........|
      030  65 6d 75 00 00 00 00 00  00 00 00 00 00 00 00 00  |emu.............|
      040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      050  00 00 00 00 00 00 00 00                           |........|
MSG:  LIFXMessage(lightState){Color:{Hue:0 Saturation:0 Brightness:28142 Kelvin:3700} Dim:0 Power:65535 Label:📈 Tags:1}

Note the emu (the label for an emulator bulb) in both hex dumps, but the second decoded message has the label for one of my other bulbs (the emoji 📈).

Damn.