[XLA:Python] Fix more bugs in the weakref_lru_cache implementation. #17866
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a) MSVC's std::unordered_map says behavior is undefined if the hash function throws an exception (https://learn.microsoft.com/en-us/cpp/standard-library/unordered-map-class?view=msvc-170#emplace). That's easy to work around, though: we can just precompute all the hash functions. b) my idiom for avoiding heterogenous lookups had a use after free problem: the weakref callback is called after the object is already in an invalid state. However, there's a much simpler solution: just create the weakref object and use it as a key unconditionally. Yes, this will mean we create more weak references than perhaps we had to otherwise. But this is simple and obviously correct. PiperOrigin-RevId: 681522048
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[XLA:Python] Fix more bugs in the weakref_lru_cache implementation.
a) MSVC's std::unordered_map says behavior is undefined if the hash function throws an exception (https://learn.microsoft.com/en-us/cpp/standard-library/unordered-map-class?view=msvc-170#emplace). That's easy to work around, though: we can just precompute all the hash functions.
b) my idiom for avoiding heterogenous lookups had a use after free problem: the weakref callback is called after the object is already in an invalid state. However, there's a much simpler solution: just create the weakref object and use it as a key unconditionally. Yes, this will mean we create more weak references than perhaps we had to otherwise. But this is simple and obviously correct.