WritePrepared Txn: duplicate keys
Summary: With WriteCommitted, when the write batch has duplicate keys, the txn db simply inserts them to the db with different seq numbers and let the db ignore/merge the duplicate values at the read time. With WritePrepared all the entries of the batch are inserted with the same seq number which prevents us from benefiting from this simple solution. This patch applies a hackish solution to unblock the end-to-end testing. The hack is to be replaced with a proper solution soon. The patch simply detects the duplicate key insertions, and mark the previous one as obsolete. Then before writing to the db it rewrites the batch eliminating the obsolete keys. This would incur a memcpy cost. Furthermore handing duplicate merge would require to do FullMerge instead of simply ignoring the previous value, which is not handled by this patch. Closes https://github.com/facebook/rocksdb/pull/2969 Differential Revision: D5976337 Pulled By: maysamyabandeh fbshipit-source-id: 114e65b66f137d8454ff2d1d782b8c05da95f989
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