change(codec): Set relay to true if the last VersionMessage byte is omitted

[arya2]

Motivation

We want to make the relay field on VersionMessage optional.

Closes #5713.

Solution

• Set relay field on VersionMessage to true when there's an UnexpectedEof error

Related changes: moves codec tests to their own module.

Review

Part of regular scheduled work.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #5835 (b39b18c) into main (2041fda) will decrease coverage by 0.02%.
The diff coverage is 60.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5835      +/-   ##
==========================================
- Coverage   78.69%   78.66%   -0.03%     
==========================================
  Files         308      308              
  Lines       38781    38503     -278     
==========================================
- Hits        30517    30289     -228     
+ Misses       8264     8214      -50     

[teor2345]

I'm having difficulty writing the test for this, ideally it might read HEADER_LEN bytes first, decrement body_len by 1 via a test impl method, and then continue reading the frame as usual, but I'm not sure how to do that yet. I'm open to alternative designs.

One possible test:

• parse a Version message with the relay field byte set to true/false, and check it is true/false
• parse a Version message with no relay field byte, and check it is true

The second version message should be one byte shorter than the other one.

(There's no need to check for unread bytes, because the true/false test makes sure that the relay byte was read.)

[arya2]

• parse a Version message with the relay field byte set to true/false, and check it is true/false

There's a test for this part here https://github.com/ZcashFoundation/zebra/blob/main/zebra-network/src/isolated/tests/vectors.rs#L177

• parse a Version message with no relay field byte, and check it is true

Ah, I was overcomplicating this part by trying to test it with a stream, I'll use Codec directly.

[teor2345]

This looks good.

We could merge it as it is, but I'd like to improve test coverage, and document the specification we're implementing to.

[teor2345]

Thanks for the extra test and docs!