Fix data race in _PersistentReference.withLock#209
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The defer block was reading self.value without holding the lock to pass to key.save. Another thread could acquire the lock and write a new value concurrently, causing a use-after-free on the COW buffer. Capture the value under the lock using an inner defer so it is captured on both the success and throwing paths, preserving existing semantics.
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We saw this super-hard-to-repro crash for a few users (it basically returns a 21 PB allocation for the buffer count because something gets corrupted along the way).
The defer block was reading self.value without holding the lock to pass to key.save. Another thread could acquire the lock and write a new value concurrently.
This fix captures the value under the lock using an inner defer so it is captured on both the success and throwing paths, preserving existing semantics.