Fix Issue 19836 - Excessive probability of UUID collisions in std.uuid.randomUUID #6985
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Thanks for your pull request, @n8sh! Bugzilla references
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…d.randomUUID On 64-bit architectures use 64 bits of entropy to initialize thread-local `rndGen`. The motivation for this change is std.uuid defaults to using `rndGen` to generate UUIDs. If every `rndGen` starts in one of 2^^32 states then if 77000 independent programs each generate a single UUID there is a 50% chance that at least two of them generate the same initial UUID (and all subsequent UUIDs would be identical as well). Not just Phobos but also C++ boost::uuids::random_generator defaults to generating UUIDs with a Mersenne Twister initialized from a 32-bit seed, exacerbating the collision problem further. If instead there are 2^^64 possible initial states of `rndGen` there can be over 5 billion independent `rndGen`s before there is a 50% chance of two having identical initial states. This change is limited to 64-bit architectures to avoid a measurable performance decrease, because many programs are not generating UUIDs.
thewilsonator
approved these changes
Apr 29, 2019
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On 64-bit architectures use 64 bits of entropy to initialize thread-local
rndGen.The motivation for this change is std.uuid defaults to using
rndGento generate UUIDs. If everyrndGenstarts in one of 2^^32 states then if 77000 independent programs each generate a single UUID there is a 50% chance that at least two of them generate the same initial UUID (and all subsequent UUIDs would be identical as well). Not just Phobos but also C++boost::uuids::random_generatordefaults to generating UUIDs with a Mersenne Twister initialized from a 32-bit seed, exacerbating the collision problem further. If instead there are 2^^64 possible initial states ofrndGenthere can be over 5 billion independentrndGens before there is a 50% chance of two having identical initial states.This change is limited to 64-bit architectures to avoid a measurable performance decrease, because many programs are not generating UUIDs.