Use Thread.threadDictionary instead of TaskLocal for thread-local

[simonjbeaumont]

Motivation

We were using a @TaskLocal static var to hold the FiniteFieldArithmeticContext (FFAC) on each of the curve types. The intention here was this would operate as a thread-local value, since task-locals behave like thread locals when the caller is not part of a task context. However, they still require binding, (i.e. the use of e.g. P256.$__ffac.withValue { ... }¹, otherwise they just return the default value, which is globally shared. We were not doing that, nor do we have a sensible place where we could.

This causes crashes when using these values in a multithreaded environment since the same value will be used across threads.

Modifications

• Use an explicit thread-local API from Foundation to store and read the per-curve FFAC.
• Add a test that shows each thread gets the same value on each read, but distinct from all other threads.

Result

Fixes crash when using ECToolbox-based APIs in multithreaded code.

Footnotes

1. https://developer.apple.com/documentation/swift/tasklocal#Using-task-local-values-outside-of-tasks ↩