Code coverage for the fuzzers

[folkertdev]

Backporting lessons from zlib-rs

The comment below has a table with the flags (see dropdown). The coverage percentage itself is not too interesting, but the delta may be in the future. The real advantage is that you can now see whether the fuzzer actually hits the logic you expect.

Doing some clicking through the interface, this is the coverage for fuzz-packet_parsing_sound: https://app.codecov.io/github/pendulum-project/ntpd-rs/commit/b560a6393de61168dde532292c01bf5480e22af9/tree/ntp-proto/src?flags%5B0%5D=fuzz-packet_parsing_sound&dropdown=coverage

E.g. here you can see that the fuzzer is unable to generate a valid NtsEncryptedField:

https://app.codecov.io/github/pendulum-project/ntpd-rs/commit/b560a6393de61168dde532292c01bf5480e22af9/blob/ntp-proto/src/packet/extension_fields.rs?flags%5B0%5D=fuzz-packet_parsing_sound&dropdown=coverage#L660

At this point I'm not that familiar with the details of the codebase, so you'll have to look at the results here to see if anything jumps out. It's straightforward to add more fuzzers, and they all run in parallel so CI time does not increase if you do so.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.63%. Comparing base (5cbf2c0) to head (148689b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1873      +/-   ##
==========================================
+ Coverage   81.59%   81.63%   +0.03%     
==========================================
  Files          67       68       +1     
  Lines       21998    21178     -820     
==========================================
- Hits        17950    17289     -661     
+ Misses       4048     3889     -159     

Flag	Coverage Δ
fuzz-packet_parsing_sound	5.45% <ø> (?)
fuzz-record_encode_decode	2.90% <ø> (?)
test-aarch64-apple-darwin	81.59% <ø> (?)
test-x86_64-unknown-linux-gnu	81.50% <ø> (?)
test-x86_64-unknown-linux-musl	81.50% <ø> (?)

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[davidv1992]

Thanks for doing this for us. Integration is going to take me a bit more time than I currently can spend, but this is definitely going to come in handy in the future.