Make index_set_state_flags() transactional

3c84046 is the original commit that introduced index_set_state_flags(),
where the presence of SnapshotNow made necessary the use of an in-place
update.  SnapshotNow has been removed in 813fb03, so there is no actual
reasons to not make this operation transactional.

Note that while making the operation more robust, using a transactional
operation in this routine was not strictly necessary as there was no use
case for it yet.  However, some future features are going to need a
transactional behavior, like support for CREATE/DROP INDEX CONCURRENTLY
with partitioned tables, where indexes in a partition tree need to have
all their pg_index.indis* flags updated in the same transaction to make
the operation stable to the end-user by keeping partition trees
consistent, even with a failure mid-flight.

REINDEX CONCURRENTLY uses already transactional updates when swapping
the old and new indexes, making this change more consistent with the
index-swapping logic.

Author: Michael Paquier
Reviewed-by: Anastasia Lubennikova
Discussion: https://postgr.es/m/20200903080440.GA8559@paquier.xyz

Author: michaelpq

src/backend/catalog/index.c

@@ -3311,18 +3311,10 @@ validate_index_callback(ItemPointer itemptr, void *opaque)
  * index_set_state_flags - adjust pg_index state flags
  *
  * This is used during CREATE/DROP INDEX CONCURRENTLY to adjust the pg_index
- * flags that denote the index's state.  Because the update is not
- * transactional and will not roll back on error, this must only be used as
- * the last step in a transaction that has not made any transactional catalog
- * updates!
+ * flags that denote the index's state.
  *
- * Note that heap_inplace_update does send a cache inval message for the
+ * Note that CatalogTupleUpdate() sends a cache invalidation message for the
  * tuple, so other sessions will hear about the update as soon as we commit.
- *
- * NB: In releases prior to PostgreSQL 9.4, the use of a non-transactional
- * update here would have been unsafe; now that MVCC rules apply even for
- * system catalog scans, we could potentially use a transactional update here
- * instead.
  */
 void
 index_set_state_flags(Oid indexId, IndexStateFlagsAction action)
@@ -3331,9 +3323,6 @@ index_set_state_flags(Oid indexId, IndexStateFlagsAction action)
 	HeapTuple	indexTuple;
 	Form_pg_index indexForm;
 
-	/* Assert that current xact hasn't done any transactional updates */
-	Assert(GetTopTransactionIdIfAny() == InvalidTransactionId);
-
 	/* Open pg_index and fetch a writable copy of the index's tuple */
 	pg_index = table_open(IndexRelationId, RowExclusiveLock);
 
@@ -3397,8 +3386,8 @@ index_set_state_flags(Oid indexId, IndexStateFlagsAction action)
 			break;
 	}
 
-	/* ... and write it back in-place */
-	heap_inplace_update(pg_index, indexTuple);
+	/* ... and update it */
+	CatalogTupleUpdate(pg_index, &indexTuple->t_self, indexTuple);
 
 	table_close(pg_index, RowExclusiveLock);
 }