"rpc" decorator for virtual devices fails to pack response if resp_format is not specified.

[dvksirin]

Here is truncated example from tutorial:

from iotile.core.hw.virtual import SimpleVirtualDevice, rpc

class DemoVirtualDevice(SimpleVirtualDevice):
    def __init__(self, args):
        super(DemoVirtualDevice, self).__init__(1, 'Demo01')
        self.temp = 273

    @rpc(8, 0x8002, "L")
    def set_temperature(self, new_temp):
        """Set the current temperature of the device in degrees kelvin"""

        self.temp = new_temp
        return []

When "resp_format" keyword argument for "rpc" is not specified it returns not changed value returned by decorated method or "[]" if returned value is None. But should return bytes according to its docstring description

....
Similarly, the function being decorated should return an iterable of results that
will be encoded into a 20 byte response buffer by struct.pack using resp_format as
the format string.

It causes an error.
Here is traceback I got when executed the example code from the tutorial

(coretools) dvk@sirin-dvk:~/test/test_component$ iotile hw --port=virtual:./demo_device.py connect 1 controller set_temperature 10
Traceback (most recent call last):
  File "/home/dvk/projects/coretools/iotilecore/iotile/core/scripts/iotile_script.py", line 206, in main
    finished = shell.invoke(line)
  File "/home/dvk/.virtualenvs/coretools/lib/python3.6/site-packages/typedargs/shell.py", line 489, in invoke
    val, line, finished = self.invoke_one(line)
  File "/home/dvk/.virtualenvs/coretools/lib/python3.6/site-packages/typedargs/shell.py", line 452, in invoke_one
    val = func(*posargs, **kwargs)
  File "</home/dvk/.virtualenvs/coretools/lib/python3.6/site-packages/decorator.py:decorator-gen-41>", line 2, in set_temperature
  File "/home/dvk/.virtualenvs/coretools/lib/python3.6/site-packages/typedargs/utils.py", line 32, in _check_and_execute
    retval = func(*convargs, **convkw)
  File "/home/dvk/test/test_component/python/demo_proxy.py", line 31, in set_temperature
    self.rpc_v2(0x8002, "L", "", new_temp)
  File "/home/dvk/projects/coretools/iotilecore/iotile/core/hw/proxy/proxy.py", line 99, in rpc_v2
    return unpack_rpc_payload(result_format, payload)
  File "/home/dvk/projects/coretools/iotilecore/iotile/core/hw/virtual/common_types.py", line 140, in unpack_rpc_payload
    return struct.unpack(code, payload)
TypeError: a bytes-like object is required, not 'list'

[timburke]

Yes, this is a clear bug. The oversight seems to be here:

coretools/iotilecore/iotile/core/hw/virtual/common_types.py

Line 194 in 1ed3db2

return resp

If the user doesn't indicate an rpc response then we should:

• verify they returned None or an empty list (otherwise raise ReturnValue error)
• return an empty byte string out of the rpc decorator.

The right exception to raise is here:

coretools/iotilecore/iotile/core/hw/exceptions.py

Line 116 in 1ed3db2

class RPCInvalidReturnValueError(RPCError):

It's possible to write a quick test here by directly invoking the wrapped rpc on a virtual device class using simple_rpc (which didn't exist at the time of the original tutorial):

coretools/iotilecore/iotile/core/hw/virtual/virtualdevice_base.py

Line 377 in 1ed3db2

def simple_rpc(self, address, rpc_id, *args, **kwargs):