Implement own xorshift PRNG to avoid rand dependency #144
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A seed of zero will generate the output zero. If that ever happened we would be stuck forever on zero. Luckily the only input that gives zero as output is zero. So we must make sure the initial seed is non-zero and we are fine. EDIT: The pointer should never be zero (null). But casting a pointer to |
core/src/parking_lot.rs
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| impl FairTimeout { | ||
| #[inline] | ||
| fn new(timeout: Instant) -> FairTimeout { | ||
| FairTimeout { timeout, rng: SmallRng::from_entropy() } | ||
| FairTimeout { timeout, seed: &timeout as *const _ as usize as u32 } |
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I'm not a big fan of using memory addresses as seeds since they could in theory be used to leak memory layout information to attackers and defeat address space randomization. Could we maybe use timeout.nanos() + 1 as the initial seed instead?
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Sure. I just now realized other downsides with the address approach as well. Fixing 👍
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Well, Instant has no nanos method. We would need to compute a Duration for that.
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We could just take the seed as argument and have HashTable::new just have an incrementing counter it sends. So first FairTimeout has seed 1 etc.
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Yes, an incrementing counter would work fine.
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I removed the word "random" from "Random time between 0 and 1ms.". This is now very deterministic so I did not want to use the word in the wrong way. |
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I will be offline from now until Monday. |
Trying to solve #138
The PRNG implementation is stolen from https://github.com/rust-lang/rust/blob/c28084ac16af4ab594b6860958df140e7c876a13/src/libcore/slice/sort.rs#L486-L493 as pointed out in the issue.
If we are happy with the PRNG algorithm, the only question is the seed. I simply used a pointer readily available when creating the
FairTimeout. We could potentially also use the nanos offset from the elapsed timeout as a seed. Something like: