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Backport VDBObject fix - #1574
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ivanimanishi merged 2 commits intoOct 22, 2026
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This made lots of interesting sequences of events possible - here's one : 1. Thread A sees `m_hashValid == false` and starts to compute the hash. 2. Thread B also sees `m_hashValid == false` and also starts to compute the hash. 3. Threads A and B merrily stomp over `m_hash` concurrently, so the result won't be deterministic. There are further problems though... 4. Thread A finishes stomping and sets `m_hashValid = true`. Thread B continues stomping. 5. Threads C, D, E etc see `m_hashValid == true` and read whatever the current value of `m_hash` is - all getting different values from different stages of thread B's stomping. This was causing instanced in-memory VDBS to lose their instancing when exported to Arnold, because each instance got a different random version of the hash. This is mitigated by computing the hash in a temporary variable before assigning it. This isn't a full fix though, because the compiler _could_ reorder `m_hashValid = true` before `m_hash = h` from the point of view of a reading thread. Solving that needs some form of synchronisation (mutex, call_once etc), which we can't do without an ABI break.
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Fixes
hash()