Summary:
An application may accidentally write merge operands without properly configuring `merge_operator`. We should alert them as early as possible that there's an API misuse. Previously RocksDB only notified them when a query or background operation needed to merge but couldn't. With this PR, RocksDB notifies them of the problem before applying the merge operand to the memtable (although it may already be in WAL, which seems it'd cause a crash loop until they enable `merge_operator`).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7667
Reviewed By: riversand963
Differential Revision: D24933360
Pulled By: ajkr
fbshipit-source-id: 3a4a2ceb0b7aed184113dd03b8efd735a8332f7f
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