pgsql: Reduce page locking in GIN vacuum

Reduce page locking in GIN vacuum

GIN vacuum during cleaning posting tree can lock this whole tree for a long
time with by holding LockBufferForCleanup() on root. Patch changes it with
two ways: first, cleanup lock will be taken only if there is an empty page
(which should be deleted) and, second, it tries to lock only subtree, not the
whole posting tree.

Author: Andrey Borodin with minor editorization by me
Reviewed-by: Jeff Davis, me

https://commitfest.postgresql.org/13/896/

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/218f51584d5a9fcdf702bcc7f54b5b65e255c187

Modified Files
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src/backend/access/gin/README | 15 ++-
src/backend/access/gin/ginbtree.c | 2 +-
src/backend/access/gin/ginvacuum.c | 236 ++++++++++++++++++++-----------------
src/include/access/gin_private.h | 2 +
4 files changed, 145 insertions(+), 110 deletions(-)