On 24 June 2016 at 04:34, Victor Stinner <victor.stinner@gmail.com> wrote:
2016-06-24 2:46 GMT+02:00 Donald Stufft <donald@stufft.io>:
I think the biggest argument to blocking is that there really exist two sort of situations that blocking can happen in:
* It blocks for a tiny amount (maybe <1s) and nobody ever notices and people feel like things “just work”. * It blocks for a long amount of time (possibly forever depending on where in the boot sequence Python is being used) and it hangs for a long time (or forever).
In the second case I think it’s pretty obvious that an exception is better than hanging forever, but in the first case an exception might actually cause people to go out of their way to do something bad to “stop the pain”. My personal preference is waffling back and forth between them based on which of the two above I feel are more likely to occur in practice.
Maybe I'm wrong, but *starting* to raise BlockingIOError looks like the opposite direction taken by Python with EINTR (PEP 475).
The difference I see here is that EINTR really can happen at any time, while the transition from "system RNG is not ready" to "system RNG is ready" is a once-per-boot deal (and in most cases, the operating system itself handles making sure the RNG is initialised before it starts running userspace processes). As such, the idioms I currently have in PEP 522 are wrong - the "wait for the system RNG or not" decision wouldn't be one to be made on a per-call basis, but rather on a per-__main__ execution basis, with developers choosing which user experience they want to support on systems with a non-blocking /dev/urandom: * this application will fail if you run it before the system RNG is ready (so you may need to add "ExecStartPre=python3 -c 'import secrets; secrets.wait_for_system_rng()'" in your systemd unit file) * this application implicitly calls "secrets.wait_for_system_rng()" and hence may block waiting for the system RNG if you run it before the system RNG is ready The default state of Python 3.6+ applications would be the first one, and I think that's an entirely reasonable default - if you're writing userspace code that runs before the system RNG is ready, you're out of the world of normal software development and into the world of operating system developers, system integrators and embedded system designers. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia