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This is really helpful, thanks for putting so much effort into this! Out of curiosity, I wonder if just using the default seed and Rand source would be more efficient, since its already synchronized: https://cs.opensource.google/go/go/+/refs/tags/go1.23.2:src/math/rand/rand.go;l=378. It's not like we need to do anything to the seed or the way the rand elements get generated, and then the locking is localized to exactly where we need it: https://cs.opensource.google/go/go/+/refs/tags/go1.23.2:src/math/rand/rand.go;l=543

I'm actually wondering whether we need the seeding at all. It is used for tests (fine), and otherwise just given the current timestamp.

If that could be removed, one could start looking at other approaches here, for instance a pool of rand's, or a ringbuffer that gets filled independently and the generation just pulling out some bits ...

I think what you have proposed is a good incremental improvement. There are a few oddities about the design and implementation of this object IMO, and I think you're right about the seeding. If anything, seeding based on timestamps may actually be decreasing the randomness. Luckily, the odds of a duplicate are incredibly low. I don't see the point of testing a specific seed. This function needs to return a unique hex slice of either 8 or 16 bits in length.

If you wanted to keep playing with this, I don't mind reviewing another PR. Otherwise, we can create a risk item to address this. I think a buffered channel of pre-generated Transaction_IDs, and another for Span_IDs may do the trick. Creating a pool of generators may increase the risk of duplication depending on how they are seeded, and I am not sure the possible performance upside would be worth the added complexity of mitigating that.