More descriptive decoder exceptions

[alexrudd2]

Closes #2259.

Three current tests are relevant for badly encoded Response.

(1)
test_client_factory_fails() calls ClientDecoder.decode(None) and expects None.
This would cause a TypeError, but it's monkey-patched to throw a ModbusException instead. (The monkey patching was never necessary with the catch-all Except)

(2) test_server_factory_fails() calls ServerDecoder.decode(None) and expects None.
This would cause a TypeError, but it's monkey-patched to throw a ModbusException instead. This is caught normally.

I'm assuming that in (1) and (2) we should actually throw the TypeError to the caller, so I changed as such. I see no valid use case for decoding None...

(3) test_processincomingpacket_not_ok() calls ClientDecoder.decode(b'\x03\x01\x00\n') and expects a ModbusIOException. The real exception is a struct.error because the payload has only 3 bytes. The register values must be an even number of bytes i.e. 6 bytes for 3 registers.

This bad message is caught by decode(), losing information on the error. Later on frameProcessIncomingPacket notices that decode returned None and raise ModbusIOException.

I think it's better to explicitly raise a descriptive exception in this case.

[alexrudd2]

Regarding (3)

pymodbus/test/framers/test_old_framers.py

Lines 499 to 512 in d329475

	@pytest.mark.parametrize(
	("framer", "message"),
	[
	(ModbusAsciiFramer, b':01270001000ACD\r\n',),
	(ModbusRtuFramer, b"\x01\x03\x03\x01\x00\n\x94\x49",),
	(ModbusSocketFramer, b'\x00\x00\x00\x00\x00\x06\x01\x27\x00\x01\x00\n',),
	(ModbusTlsFramer, b'\x54\x00\x7c\x00\x02',),
	]
	)
	def test_processincomingpacket_not_ok(self, framer, message):
	"""Test processIncomingPacket."""
	test_framer = framer(ClientDecoder())
	with pytest.raises(ModbusIOException):
	test_framer.processIncomingPacket(message, mock.Mock(), 0x01)

RapidScada's parser claims it's valid.... but I don't trust it. It's parsing the last two bytes twice (once as data and once as CRC!)

Anyways, this is all in test_old_framer so maybe it's going to get deleted... anyways I got distracted hunting down #2261

[janiversen]

Another way to reach the same goal, but nicer.