[swift-evolution] [Proposal] Random Unification

Ben Cohen ben_cohen at apple.com
Wed Oct 4 18:42:44 CDT 2017

> On Oct 4, 2017, at 9:20 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> If trapping is OK, then surely returning Optional is superior; any user who is OK with trapping can make that decision for themselves by writing `random()!`. Everyone else can then see clearly that trapping is a possibility, which is important.

It’s important not to underestimate both the extent to which using `!` scares people, _and_ the extent to which beginners use it inappropriately. Both of these mean that it’s very important the standard library not include anything that makes force-unwrapping a common/routine operation (looking at you, UnsafeBufferPointer.baseAddress).

Given that arc4random, the recommended source of random numbers on Darwin, is always successful, requiring a force-unwrap when the user is using the default source doesn’t seem like the right move.

> On Wed, Oct 4, 2017 at 11:09 David Waite <david at alkaline-solutions.com <mailto:david at alkaline-solutions.com>> wrote:
>> On Oct 4, 2017, at 4:05 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> I agree with Felix’s concern, which is why I brought up the question, but ultimately the issue is unavoidable. It’s not down to global instance or not. If your source of random numbers is unseedable and may mix in additional entropy at any time, then it may fail at any time because when a hardware restart might happen may be transparent to the process. The user must know about this or else we are laying a trap (pun intended).
> I'm of the mindset (which might be controversial) that we should attempt to expose legacy and cryptographically secure random number generators, even a mixed algorithmic/entropy source like /dev/urandom, but that we should not expose /dev/random at all. If someone is trying to use /dev/random legitimately (such as to generate one-time-pads) they will have to take into account that systems like linux still use algorithmic entropy to drive /dev/random. If someone really has this sort of use case, they have exceeded the bounds of the system randomness protocol.
> Without /dev/random support as a requirement, the only failure cases I know of are reading too much random data in one operation (which could be solved by repeated calls) or calling before sufficient entropy has been set up in /dev/urandom (such as in a system startup process). I'd be fine with the second one being a special case, and such systems needing to know use of the /dev/urandom -backed generator before randomness has been set up will trap or return predictable information on certain platforms.
> -DW
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