c34787f910
Add some additional defensive checks in the second phase of index deletion to detect and report index corruption during VACUUM, and to avoid having VACUUM become stuck in more cases. The code is still not robust in the presence of a circular chain of sibling links, though it's not clear whether that really matters. This is follow-up work to commit 3a01f68e. The new defensive checks rely on the assumption that there can be no more than one VACUUM operation running for an index at any given time. Remove an old comment suggesting that multiple concurrent VACUUMs need to be considered here. This concern now seems highly unlikely to have any real validity, since we clearly rely on the same assumption in several other places. For example, there are much more recent comments that appear in the same function (added by commit efada2b8e92) that make the same assumption. Also add a CHECK_FOR_INTERRUPTS() to the relevant code path. Contrary to comments added by commit 3a01f68e, it is actually possible to handle interrupts here, at least in the common case where processing takes place at the leaf level. We only hold a pin on leafbuf/target page when stepping right at the leaf level. No backpatch due to the lack of complaints following hardening added to the same area by commit 3a01f68e.
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