For inplace update durability, make heap_update() callers wait.

The previous commit fixed some ways of losing an inplace update.  It
remained possible to lose one when a backend working toward a
heap_update() copied a tuple into memory just before inplace update of
that tuple.  In catalogs eligible for inplace update, use LOCKTAG_TUPLE
to govern admission to the steps of copying an old tuple, modifying it,
and issuing heap_update().  This includes MERGE commands.  To avoid
changing most of the pg_class DDL, don't require LOCKTAG_TUPLE when
holding a relation lock sufficient to exclude inplace updaters.
Back-patch to v12 (all supported versions).  In v13 and v12, "UPDATE
pg_class" or "UPDATE pg_database" can still lose an inplace update.  The
v14+ UPDATE fix needs commit 86dc90056dfdbd9d1b891718d2e5614e3e432f35,
and it wasn't worth reimplementing that fix without such infrastructure.

Reviewed by Nitin Motiani and (in earlier versions) Heikki Linnakangas.

Discussion: https://postgr.es/m/20231027214946.79.nmisch@google.com

1 files changed, 3 insertions, 1 deletions

diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index f3f6d4754e9..52fde5cc4d4 100644
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -765,7 +765,9 @@ systable_endscan_ordered(SysScanDesc sysscan)
  *
  * Overwriting violates both MVCC and transactional safety, so the uses of
  * this function in Postgres are extremely limited.  Nonetheless we find some
- * places to use it.  Standard flow:
+ * places to use it.  See README.tuplock section "Locking to write
+ * inplace-updated tables" and later sections for expectations of readers and
+ * writers of a table that gets inplace updates.  Standard flow:
  *
  * ... [any slow preparation not requiring oldtup] ...
  * systable_inplace_update_begin([...], &tup, &inplace_state);