PS-11308: Fix read-view reuse and clone race - #6021
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Issue ===== RC autocommit transactions can reuse a closed read view while another session clones that transaction's read view. If the donor transaction is reopening the view for the next consistent read, the read_view pointer may already be untagged while the underlying ReadView is still the stale reusable view that purge does not protect. Solution ======== Protect the read-view reuse/clone boundary so a cloned view cannot observe an unsafe stale reusable view, and add a regression test that pauses the donor in the MVCC::view_open() reuse window to cover the race.
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Issue
RC autocommit transactions can reuse a closed read view while another session clones that transaction's read view. If the donor transaction is reopening the view for the next consistent read, the read_view pointer may already be untagged while the underlying ReadView is still the stale reusable view that purge does not protect.
Solution
Protect the read-view reuse/clone boundary so a cloned view cannot observe an unsafe stale reusable view, and add a regression test that pauses the donor in the MVCC::view_open() reuse window to cover the race.