Nadav Har'El 5e8bdf6877 alternator: fix corruption of PutItem operation in case of contention ...

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>

2020-09-16 10:30:19 +02:00

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