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vinyl: check key uniqueness before modifying tx write set
Currently, we handle INSERT/REPLACE/UPDATE requests by iterating over all space indexes starting from the primary and inserting the corresponding statements to tx write set, checking key uniqueness if necessary. This means that by the time we write a REPLACE to the write set of a secondary index, it has already been written to the primary index write set. This is OK, and vy_tx_prepare() relies on that to implement the common memory level. However, this also means that when we check uniqueness of a secondary index, the new REPLACE can be found via the primary index. This is OK now, because all indexes are fully independent, but it isn't going to fly after #2129 is implemented. The problem is in order to check if a tuple is present in a secondary index, we will have to look up the corresponding full tuple in the primary index. To illustrate the problem, consider the following situation: Primary index covers field 1. Secondary index covers field 2. Committed statements: REPLACE{1, 10, lsn=1} - present in both indexes DELETE{1, lsn=2} - present only in the primary index Transaction: REPLACE{1, 10} When we check uniqueness of the secondary index, we find committed statement REPLACE{1, 10, lsn=1}, then look up the corresponding full tuple in the primary index and find REPLACE{1, 10}. Since the two tuples match, we mistakenly assume that there's a conflict. To avoid a situation like that, let's check uniqueness before modifying the write set of any index. Needed for #2129
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