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This patch fixes a bug noted in issue #7218 - where PutItem operations sometimes lose part of the item's data - some attributes were lost, and the name of other attributes replaced by empty strings. The problem happened when the write-isolation policy was LWT and there was contention of writes to the same partition (not necessarily the same item). To use CAS (a.k.a. LWT), Alternator builds an alternator::rmw_operation object with an apply() function which takes the old contents of the item (if needed) and a timestamp, and builds a mutation that the CAS should apply. In the case of the PutItem operation, we wrongly assumed that apply() will be called only once - so as an optimization the strings saved in the put_item_operation were moved into the returned mutation. But this optimization is wrong - when there is contention, apply() may be called again when the changed proposed by the previous one was not accepted by the Paxos protocol. The fix is to change the one place where put_item_operation *moved* strings out of the saved operations into the mutations, to be a copy. But to prevent this sort of bug from reoccuring in future code, this patch enlists the compiler to help us verify that it can't happen: The apply() function is marked "const" - it can use the information in the operation to build the mutation, but it can never modify this information or move things out of it, so it will be fine to call this function twice. The single output field that apply() does write (_return_attributes) is marked "mutable" to allow the const apply() to write to it anyway. Because apply() might be called twice, it is important that if some apply() implementation sometimes sets _return_attributes, then it must always set it (even if to the default, empty, value) on every call to apply(). The const apply() means that the compiler verfies for us that I didn't forget to fix additional wrong std::move()s. Additionally, a test I wrote to easily reproduce issue #7218 (which I will submit as a dtest later) passes after this fix. Fixes #7218. Signed-off-by: Nadav Har'El <nyh@scylladb.com> Message-Id: <20200916064906.333420-1-nyh@scylladb.com>