Protect cached threadlocal in recursive hash#7901
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headius merged 3 commits intojruby:masterfrom Aug 22, 2023
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The optimization previously introduced here caches the byte[16] as a thread-local value, but when recursively calling Hash#hash it will be overwritten by downstream usage. This modification avoids the hash being overwritten before it can be used to calculate the aggregate hash.
The original optimization here moved the allocation of the buffer byte[16] into a thread-local to reuse it, but it kept the original inline modification of that buffer via ByteBuffer.putLong. This caused recursive Hash#hash calls to overwrite the buffer mid-hash, leading to issues like jruby#7866 where a lazily-calculated recursive hash produces a bogus value the first time through. The change here avoids accessing and modifying the shared buffer until after any recursive hash calls have completed.
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A change introduced in 60ae8ba caused the once-transient buffer to be shared during recursive hash calculation, corrupting the results. This led to #7866.
The PR here fixes that by avoiding the shared buffer until all recursive hash calls have completed.
An additional small fix avoids allocating the long[] carrier object for hashes of size zero. Caching this in a thread-local would probably be worthwhile as well, but I was unsure how to do it safely.