Fix heap-use-after-free in MultiCfIteratorImpl #12784
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Summary
When changing the direction of the multi-cf-iter, we do this by
Seek(current_key)
(if changing from backward to forward) orSeekForPrev(current_key)
(if forward -> backward) in the child iters and rebuild the heap.Slice target
is just a pointer and contents are not guaranteed to be the same after re-init the heap.Test Plan
I was able to steadily repro by building with
COMPILE_WITH_ASAN=1
running db_stress.heap-use-after-free
was no longer happening with the same command after making the change.