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Standardize cleanup lock terminology.

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The term "super-exclusive lock" is a synonym for "buffer cleanup lock"
that first appeared in nbtree many years ago.  Standardize things by
consistently using the term cleanup lock.  This finishes work started by
commit 276db875.

There is no good reason to have two terms.  But there is a good reason
to only have one: to avoid confusion around why VACUUM acquires a full
cleanup lock (not just an ordinary exclusive lock) in index AMs, during
ambulkdelete calls.  This has nothing to do with protecting the physical
index data structure itself.  It is needed to implement a locking
protocol that ensures that TIDs pointing to the heap/table structure
cannot get marked for recycling by VACUUM before it is safe (which is
somewhat similar to how VACUUM uses cleanup locks during its first heap
pass).  Note that it isn't strictly necessary for index AMs to implement
this locking protocol -- several index AMs use an MVCC snapshot as their
sole interlock to prevent unsafe TID recycling.

In passing, update the nbtree README.  Cleanly separate discussion of
the aforementioned index vacuuming locking protocol from discussion of
the "drop leaf page pin" optimization added by commit 2ed5b87f.  We now
structure discussion of the latter by describing how individual index
scans may safely opt out of applying the standard locking protocol (and
so can avoid blocking progress by VACUUM).  Also document why the
optimization is not safe to apply during nbtree index-only scans.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzngHgQa92tz6NQihf4nxJwRzCV36yMJO_i8dS+2mgEVKw@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WzkHPgsBBvGWjz=8PjNhDefy7XRkDKiT5NxMs-n5ZCf2dA@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2021-12-08 17:24:45 -08:00
parent 6f0e6ab04d
commit bcf60585e6
9 changed files with 107 additions and 95 deletions
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8
src/backend/storage/page/bufpage.c
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@@ -701,7 +701,7 @@ compactify_tuples(itemIdCompact itemidbase, int nitems, Page page, bool presorte
* there is, in general, a good chance that even large groups of unused line
* pointers that we see here will be recycled quickly.
*
* Caller had better have a super-exclusive lock on page's buffer. As a side
* Caller had better have a full cleanup lock on page's buffer. As a side
* effect the page's PD_HAS_FREE_LINES hint bit will be set or unset as
* needed.
*/
@@ -820,9 +820,9 @@ PageRepairFragmentation(Page page)
* arbitrary, but it seems like a good idea to avoid leaving a PageIsEmpty()
* page behind.
*
* Caller can have either an exclusive lock or a super-exclusive lock on
* page's buffer. The page's PD_HAS_FREE_LINES hint bit will be set or unset
* based on whether or not we leave behind any remaining LP_UNUSED items.
* Caller can have either an exclusive lock or a full cleanup lock on page's
* buffer. The page's PD_HAS_FREE_LINES hint bit will be set or unset based
* on whether or not we leave behind any remaining LP_UNUSED items.
*/
void
PageTruncateLinePointerArray(Page page)