fix(s2n-quic-core): enforce checked arithmetics to prevent overflows

[boquan-fang]

Release Summary:

Resolved issues:

Description of changes:

VarInt add, mul, and other arithmetics are currently doing checked arithmetics operations during tests, but use non checked operations in production. This can cause integer overflows in production.

This problem should be fixed with checked operation without the separation for production and tests. The original decision was made based on performance concerns: we believe unchecked arithmetics is faster than checked arithmetics. However, we determine that the performance regression is negligible, but the potential exposure for overflow is concerning.

Call-outs:

Testing:

Current CI should pass.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[camshaft]

the intent was to allow the application to control if it got overflow checks or not, but unfortunately, that cfg isn't stable currently: rust-lang/rust#111466, so debug_assertions was the next-best thing.

[boquan-fang]

the intent was to allow the application to control if it got overflow checks or not, but unfortunately, that cfg isn't stable currently: rust-lang/rust#111466, so debug_assertions was the next-best thing.

ACK. I think the decision right now is to always have a check for VarInt arithmetics to prevent overflow altogether.

[maddeleine]

Have we done any type of performance analysis to prove that these checks don't add overhead? I assume we didn't want them in production for a reason.

[boquan-fang]

Document this behavior on the top of the file.