Fix compatibility between the cache compute methods and a load #5348
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The asMap().compute implementation did not take into account that the present value may be loading. A load does not block other writes to that entry and takes into account that it may be clobbered, causing it to automatically discard itself. This is a known design choice that breaks linearizability assumptions (google#1881). The compute should check if a load is in progress and call the appropriate internal removal method. Because a zombie entry remained in the cache and still is marked as loading, the loader could discover entry and try to wait for it to materialize. When the computation is a removal, indicated by a null value, the loader would see this as the zombie's result. Since a cache loader may not return null it would throw an exception to indicate a user bug. A new ComputingValueReference resolves both issues by indicating that the load has completed. The compute's removeEntry will then actually remove this entry and the loader will not wait on the zombie. Instead if it observes the entry, it will neither receive a non-null value or wait for it to load, but rather try to load anew under the lock. This piggybacks on the reference collection support where an entry is present but its value was garbage collected, causing the load to proceed. By the time the lock is obtained the compute method's entry was removed and the load proceeds as normal (so no unnecessary notification is produced). fixes google#5342 fixes google#2827 resolves underlying cause of google#2108
lowasser
approved these changes
Dec 7, 2020
netdpb
approved these changes
Feb 9, 2021
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The
asMap().computeimplementation did not take into account that the present value may be loading. A load does not block other writes to that entry and takes into account that it may be clobbered, causing it to automatically discard itself. This is a known design choice that breaks linearizability assumptions (#1881). The compute should check if a load is in progress and call the appropriate internal removal method.Because a zombie entry remained in the cache and still is marked as loading, the loader could discover entry and try to wait for it to materialize. When the computation is a removal, indicated by a null value, the loader would see this as the zombie's result. Since a cache loader may not return null it would throw an exception to indicate a user bug.
A new
ComputingValueReferenceresolves both issues by indicating that the load has completed. The compute'sremoveEntrywill then actually remove this entry and the loader will not wait on the zombie. Instead if it observes the entry, it will neither receive a non-null value or wait for it to load, but rather try to load anew under the lock. This piggybacks on the reference collection support where an entry is present but its value was garbage collected, causing the load to proceed. By the time the lock is obtained the compute method's entry was removed and the load proceeds as normal (so no unnecessary notification is produced).fixes #5342
fixes #2827
resolves underlying cause of #2108