[kasik96]

Summary:

Fixes #59613.

SimpleThreadSafeCache ran the generator for a cache miss while it held its mutex. For text measurement on iOS, the generator creates an NSTextStorage, and on iOS 27 devices its editing notification waits for the main queue. If the main thread measures text at the same time, it waits for the mutex, and the app hangs.

Now the lock covers only the lookup and the insert, and the generator runs between them. If another thread inserted the same key meanwhile, its value is kept. Two threads can then generate the same key once each.

Changelog:

[IOS] [FIXED] - Fix a deadlock between the JS and main threads in text measurement on iOS 27

Test Plan:

• Added GeneratorDoesNotBlockOtherThreads: the generator waits for another thread that uses the cache. It fails without this change and passes with it.
• Added ConcurrentMissKeepsFirstInsertedValue: another thread inserts the same key while the generator runs, and the cache keeps that value.
• On an iPhone with iOS 27.0.1, the hang reproduced before this change and not after it.

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