OSS-Fuzz: Add fuzzer targeting coap_async

[arthurscchan]

This PR adds a new fuzzer and some seed corpus with valid async package targeting coap_async logic.

[mrdeep1]

Thanks for doing this.

[arthurscchan]

@mrdeep1 Just to confirm my understanding: I have examined the logic in coap_debug_set_packet_loss() and coap_debug_set_packet_fail(). I can see that during packet transmission checking, packet_loss is evaluated first, and packet_fail is only checked if the loss check passes.

Is it correct that with the following configuration:

coap_debug_set_packet_loss("50%");
coap_debug_set_packet_fail("100%");

I can achieve a 50/50 split where 50% of packets go through the loss path and the other 50% go through the fail path, whilst still guaranteeing that no packets are actually transmitted to the network?

This might provide better fuzzing coverage by exercising both error-handling code paths. WDYT?

[mrdeep1]

Your understanding is correct.

Your solution should give the ~50/50 split and worth going doing.

[arthurscchan]

Thanks, I will try making that for all the fuzzers I have created with network packet handling later.