Implement proper exception handling

[amlweems]

It is possible for our client to be incredibly confused to the point of being unable to correctly parse any server messages (you have no idea how easy it is for this to happen). This can cause segfaults, buffer overflows, and generally is very bad. In these situations, it is very likely that we can detect an error.

We should implement proper exception handling in these cases (possibly even disconnecting and reconnecting the client).

[JDongian]

Also put documentation on doxygen and/or the README.md

[JDongian]

Can you give a sample implementation of exception handling? What would this look like, and what cases would exceptions cover?

[amlweems]

Not a code sample, but these are the important locations where exceptions will come in handy.

https://github.com/NosotrosNueces/mcc/blob/master/src/protocol.c#L574

// Negative packet IDs probably mean something terrible happened... so we just exit
int32_t pid = receive_packet(bot);
if (pid < 0) {
    if (pid == -1) exit(123);
    return NULL;
}

https://github.com/NosotrosNueces/mcc/blob/master/src/marshal.c#L263

varlen = varint32_encode(packet_raw - save, varint, 5);
if(packet_raw - save + varlen > len)
    return -1; // TODO: compression

https://github.com/NosotrosNueces/mcc/blob/master/src/bot.c#L152

// A packet length of zero can sometimes happen, we return -2
len = varint32(bot->_data->buf, &packet_size);
if (packet_size == 0)
    return -2;
assert(i != len);
...
// This pattern occurs 3 times in receive_packet()
ret = receive_raw(bot, bot->_data->buf + received, packet_size - received);
if (ret <= 0)
    return -1;
...
// If we can't parse a big packet correctly, we return -2
return -2;

[JDongian]

Thanks @amlweems. I'll be working on this. The right approach may be to use jumps, i.e. setjump() and longjump(), but I am skeptical.

Error handling with signals seems a lot better IMO.

idea:

set errno (global)
send interrupt
signal handler logs message based on errno
signal handler kills main?

[JDongian]

Got some basic stuff working on branch exceptions #80. Should I put the exception handler in a separate file? Where should it go? sample/ or src/? Perhaps we could macro the call?

[JDongian]

Okay, what's the status on this? I want to close this and move on.