Hyrum’s Law in Golang

Hah, I wrote the crypto/rsa comments. We take Hyrum's Law (and backwards compatibility [1]) extremely seriously in Go. Here are a couple more examples:

- We randomly read an extra byte from random streams in various GenerateKey functions (which are not marked like the ones in OP) with MaybeReadByte [2] to avoid having our algorithm locked in

- Just yesterday someone reported that a private ECDSA key with a nil public key used to work, and now it doesn't, so we probably have to make it work again [3]

- Iterating over a map uses a randomized order to avoid exposing the internals

- The output of rand.Rand is considered part of the compatibility promise, so we had to go to great lengths to improve it [4]

- We discuss all the time what commitments to make in docs and what behaviors to disclaim, knowing we can never change something documented and probably something that's not explicitly documented as "this may change" [6]

[1]: https://go.dev/doc/go1compat

[2]: https://pkg.go.dev/crypto/internal/randutil#MaybeReadByte

[3]: https://go.dev/issue/70468

[4]: https://go.dev/blog/randv2

[5]: https://go.dev/blog/chacha8rand

[6]: https://go-review.googlesource.com/c/go/+/598336/comment/5d6...

As a user of your code this is true, and I'm very grateful indeed that you take this approach.

I would add as a slight caveat that to benefit from this policy, users absolutely must read the release notes on major go versions before upgrading. We recently didn't, and we were burnt somewhat by the change to disallow negative serial numbers in the x509 parser without enabling the new feature flag. Completely our fault and not yours, but I add the caveat nevertheless.

We have gotten a liiiiittle more liberal ever since we introduced the new GODEBUG feature flag mechanism.

I've been meaning to write a "how to safely update Go" post for a while, because the GODEBUG mechanism is very powerful but not well-known and we could build a bit of tooling around it.

In short, you can upgrade your toolchain without changing the go.mod version, and these things will keep working like they did, and set a metric every time the behavior would have changed, but didn't. (Here's where we could build a bit of tooling to check that metric in prod/tests/CLIs more easily.) Then you can update the go.mod version, which updates the default set of GODEBUGs, and if anything breaks, try reverting GODEBUGs one by one.