pgsql: Fix GIN VACUUM posting tree root split bug.

Fix GIN VACUUM posting tree root split bug.

ginVacuumPostingTreeLeaves swaps a shared buffer lock for an exclusive
one when it encounters a leaf page. It neglected to re-verify whether a
page that was initially a leaf root page became an internal page due to
a concurrent root page split (during the window when no lock was held).
It was therefore possible for GIN VACUUM to spuriously treat an internal
page as a leaf page, leading to data corruption. VACUUM could miss dead
TIDs that it was required to remove, leaving behind dangling references
in the index.

To fix, re-verify that a leaf page is still a leaf page after an
exclusive lock is acquired. If it isn't, drop our exclusive lock and
acquire a shared lock so that the non-leaf root page gets processed in
the usual way.

Oversight in commit fd83c83d, which fixed a deadlock bug in GIN posting
tree vacuuming.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/CAH2-Wz=RBpJTQgvOxr6C=J04dExmFSt1E3F-r+cRTQ56hEotkg@mail.gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dd1634ddbdbb72deb43482c9b6435c7a9c38da9a

Modified Files
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src/backend/access/gin/ginvacuum.c | 10 ++++++++++
1 file changed, 10 insertions(+)