max_packet_length check doesn't account for 4 byte radiohead prepend

[nateinaction]

Adafruit_CircuitPython_RFM/adafruit_rfm/rfm_common.py

Lines 294 to 299 in 91b9767

	assert 0 < len(data) <= self.max_packet_length
	# pylint: enable=len-as-condition
	self.idle() # Stop receiving to clear FIFO and keep it clear.
	# Combine header and data to form payload
	if self.radiohead:
	payload = bytearray(4)

In asyncio_send() we assert that the passed data arg is no larger than the chip's FIFO (252 bytes) but then when radiohead is enabled, we prepend 4 bytes to data potentially making it larger than 252 bytes. This behavior results in data not being sent via the radio when it is equal to self.max_packet_length and radio head is enabled.

It feels like we should assert max packet length is 248 when radiohead is enabled. Even with the comment detailing how self.max_packet_length is used on async_send() I think this would also be a helpful note above all the locations where self.max_packet_length is set.

Thoughts?

[jerryneedell]

Good catch! It may be simplest to move the assert to check len(payload) instead of len(data) just before the call to self.fill_fifo(payload) (line 321). That way, it does not matter how payload was formed. It could be data + 4 or data + 1 or just data.