Make unpredictableSeed use getrandom (syscall) on Linux

[0xEAB]

This patch changes unpredictableSeed to use the getrandom syscall on Linux.

Currently, unpredictableSeed calls arc4random() on applicable BSD systems;
for everything else it executes RDRAND on InlineAsm_X86_Any-compatible targets or falls back to a homebrew solution.

[dlang-bot]

Thanks for your pull request, @0xEAB!

Bugzilla references

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10623"

[rikkimax]

That function needs a warning.

It is possible that the kernel may not have enough entropy stored to give a value.

You want to call it sparingly and only after the system has been booted fully.

[0xEAB]

and only after the system has been booted fully.

I don’t think this is accurate.
Quoting random(4):

When read during early boot time, /dev/urandom may return data prior to the entropy pool being initialized.

And also:

If this is of concern in your application, use getrandom(2) […] instead.

[rikkimax]

Yes, during booting it may be empty. Once initialized the chance of it to be empty depends upon if it has been misused.

In any case, a warning is needed ;)

[0xEAB]

@rikkimax
I’ve added further notes and a randomnes-quality warning. Please check whether you consider those sufficient.

@everyone
Please spell/grammar check those new paragraphs of technobabble.

[rikkimax]

@rikkimax
I’ve added further notes and a randomnes-quality warning. Please check whether you consider those sufficient.

You have gone above and beyond what I was wanting!

Good job.

[thewilsonator]

Use "(cryptographically secure) pseudo-random number generator" and introduce the acronym, and then use it throughout (rather than using the acronym first, and then spelling it out later).

[0xEAB]

Updated to:

This function utilizes the system cryptographically-secure pseudo-random
number generator (CSPRNG) or pseudo-random number generator (PRNG)
where available and implemented (currently arc4random on applicable BSD
systems or getrandom on Linux) to generate “high quality” pseudo-random
numbers – if possible.