Handling decoding errors

[zolakeith]

When the SocketServer attempts to decode messages there is the possibility of error caused by bad data.

This commit adapts the code so that any exceptions caused by decoding are captured, and a warning is logged, and the process is stopped without causing an error.

The types of errors seen were:

• bad return value: {:error, "" is not repeatable""}
• (CondClauseError) no cond clause evaluated to a truthy value (coap_ex 0.1.0) lib/coap/message_options.ex
• (CaseClauseError) no case clause matching <<...>> (coap_ex 0.1.0) lib/coap/message_options.ex:54
• (FunctionClauseError) no function clause matching in CoAP.Block.decode/1
• (MatchError) no match of right hand side value: """"

One could argue that these "errors" should be fixed by updating the MessageOptions module to gracefully handle such conditions, and that is definitely worth considering. By doing that, we would then be able to complete the decoding, which would allow extracting the message token which is required to identify the associated connection process.

Bad Return

GenServer #PID<0.12197.249> terminating** (stop) bad return value: {:error, "" is not repeatable""}

Last message: {:udp, #Port<0.1357880>, {10, 148, 4, 179}, 1200, <<227, 3, 7, 2, 0, 0, 0, 157, 87, 43, 90, 195, 102, 208, 55, 202, 12, 234, 12, 241, 12, 217, 12, 85, 250, 113, 61, 120, 0, 140, 192, 91, 13, 39, 0, 13, 0, 169, 4, 0, 0, 131, 92, 195, 102, ...>>}

This error happens because when a non repeatable option is encountered by the MessageOptions.Decoder, it throws an exception which is not caught, resulting in the SocketServer process returning the error from the handle_info/2 genserver callback. This causes the process to stop, because {:error, "" is not repeatable""} is not a valid handle_info/2 response.

Fixing this issue would involve "catching" the exception, and ensure the SocketServer is able to return a valid response from the handle_info/2 callback.

CondClauseError

GenServer #PID<0.3708.144> terminating** (CondClauseError) no cond clause evaluated to a truthy value 
(coap_ex 0.1.0) lib/coap/message_options.ex:73: CoAP.MessageOptions.Decoder.decode_extended/4 
(coap_ex 0.1.0) lib/coap/message_options.ex:51: CoAP.MessageOptions.Decoder.decode/3 
(coap_ex 0.1.0) lib/coap/message_options.ex:42: CoAP.MessageOptions.Decoder.options_and_payload/1 
(coap_ex 0.1.0) lib/coap/message.ex:241: CoAP.Message.decode/1 
(coap_ex 0.1.0) lib/coap/socket_server.ex:94: CoAP.SocketServer.handle_info/2 
(stdlib 3.17.2.1) gen_server.erl:695: :gen_server.try_dispatch/4 
(stdlib 3.17.2.1) gen_server.erl:771: :gen_server.handle_msg/6 
(stdlib 3.17.2.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

Last message: {:udp, #Port<0.788077>, {10, 150, 51, 74}, 5683, <<100, 69, 0, 1, 239, 70, 20, 250, 68, 105, 29, 80, 244, 100, 69, 0, 1, 239, 70, 20, 250, 68, 105, 29, 80, 244>>}

This error happens because there is no clause which returns truthy - this cond could be updated to include a base case which throws an exception.