program execution: Rust 1.45 introduces incompatible math runtime behavior changes

[ryoqun]

Problem

Rust 1.45 will introduce incompatible math runtime behavior changes: https://github.com/rust-lang/rust/blob/master/RELEASES.md#compatibility-notes

In one sentence, (N/0.0) as {integer} is changed from 0 to {integer}::MAX.

In our codecase, this manifested 3 times already, caught via nightly test runs in #10585 and #10445. I think this is high occurance, worth to be aware of from this observation.

At worst, this might cause a cluster consensus loss due to different handling regarding rewards, fees, fork selection, and other token calculation with some edge cases, assuming a cluster comprised of differently-built binaries.

@jackcmay Also, smart contracts could be affected? (Well, dumb question, bpf supports floating number math??) (Quick googling tells me this is not the case...)

Proposed Solution

• Don't go straightly to update 1.45. Let's see how the rust community handles the transition
• Once more, go through any suspicious fdivs in our codebase? (But this couldn't cover our depending crates)
• Build with 1.44 and 1.45 and mix them within a cluster? (= testnet)

[mvines]

testnet has been running Rust 1.45.x since July, so I think we're ok here.

[jackcmay]

@ryoqun The BPF instruction set does not support floating-point math but LLVM provides compiler intrinsics that do so currently you can perform floating-point math in our programs (though we should probably explicitly disallow that. So as it stands if anyone is using floating-point math the incompatibility mentioned in this issue could affect program execution as we update our Rust-BPF versions

[ryoqun]

Oh, let's update the milestone as well, then. :)

[jackcmay]

@ryoqun Looks like this is probably not an issue for BPF programs from a backward compatibility perspective. Programs built against the old toolchain are already immutable in their executable accounts. Any programs built against the new toolchain will be in new accounts.

[mvines]

Anything we need to do here?