Chapter 9: Note on Memory Corruption?

📚 Official Books Modern Concurrency in Swift
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Chapter 9 contains the following Note:

“Calling contains(_: ) on the cache keys directly, without fetching a local copy of the keys collection, produces memory corruption in release builds when there are concurrent updates. That’s why here you safely get a local copy of the keys and then you check for the existence of key.”

I’m a bit confused by this. My understanding is that memory corruption could occur here if one thread fetched the keys, while another edited the dictionary. However,

  1. cache is a property of ImageLoader
  2. ImageLoader is an actor

Which would imply (I think?) that access to cache is always serialized. What am I missing?

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