Fix assert in dimension slice lookup

Calling ts_scan_iterator_next will advance internal scanner data
structures. It is therefore not safe to use the iterator after
ts_scan_iterator_next returned NULL and expect to be able to access
the previous tuple. Under certain circumstances this may point the
iterator to tuples not normally visible.

src/dimension_slice.c

@@ -811,13 +811,20 @@ ts_dimension_slice_scan_iterator_get_by_id(ScanIterator *it, int32 slice_id,
 										   const ScanTupLock *tuplock)
 {
 	TupleInfo *ti;
+	DimensionSlice *slice = NULL;
 
 	ts_dimension_slice_scan_iterator_set_slice_id(it, slice_id, tuplock);
 	ts_scan_iterator_start_or_restart_scan(it);
 	ti = ts_scan_iterator_next(it);
 	Assert(ti);
-	Assert(ts_scan_iterator_next(it) == NULL); /* This is a heavy call, consider removing it */
-	return ti ? ts_dimension_slice_from_tuple(ti) : NULL;
+
+	if (ti)
+	{
+		slice = ts_dimension_slice_from_tuple(ti);
+		Assert(ts_scan_iterator_next(it) == NULL);
+	}
+
+	return slice;
 }
 
 DimensionSlice *

tsl/test/expected/chunk_utils_internal.out

@@ -1391,6 +1391,31 @@ SELECT _timescaledb_functions.hypertable_osm_range_update('osm_tstz', '2020-01-0
 SELECT _timescaledb_functions.hypertable_osm_range_update('osm_tstz', 9223372036854771806, 9223372036854775406);
 ERROR:  invalid time argument type "bigint"
 \set ON_ERROR_STOP 1
+-- test dimension slice tuple visibility
+\c :TEST_DBNAME :ROLE_SUPERUSER
+CREATE TABLE osm_slice_update(time int not null);
+SELECT hypertable_id AS ht_id FROM create_hypertable('osm_slice_update', 'time', chunk_time_interval => 10) \gset
+INSERT INTO osm_slice_update VALUES (1);
+UPDATE _timescaledb_catalog.hypertable SET status = 3 WHERE id = :ht_id;
+UPDATE _timescaledb_catalog.chunk SET osm_chunk = true WHERE hypertable_id = :ht_id;
+\c
+BEGIN;
+ 	SELECT _timescaledb_functions.hypertable_osm_range_update('osm_slice_update',40,50);
+ hypertable_osm_range_update 
+-----------------------------
+ f
+(1 row)
+
+ROLLBACK;
+\c
+-- new session should not be affected by previous rolled back transaction
+-- should show 0 10 as range
+\set ON_ERROR_STOP 0
+INSERT INTO osm_slice_update VALUES (1);
+ERROR:  Cannot insert into tiered chunk range of public.osm_slice_update - attempt to create new chunk with range  [0 10] failed
+INSERT INTO osm_slice_update VALUES (1);
+ERROR:  Cannot insert into tiered chunk range of public.osm_slice_update - attempt to create new chunk with range  [0 10] failed
+\set ON_ERROR_STOP 1
 -- clean up databases created
 \c :TEST_DBNAME :ROLE_SUPERUSER
 DROP DATABASE postgres_fdw_db;

tsl/test/sql/chunk_utils_internal.sql

@@ -738,8 +738,32 @@ SELECT _timescaledb_functions.hypertable_osm_range_update('osm_tstz', '2020-01-0
 -- udpate range of timestamp with bigint
 SELECT _timescaledb_functions.hypertable_osm_range_update('osm_tstz', 9223372036854771806, 9223372036854775406);
 \set ON_ERROR_STOP 1
+
+-- test dimension slice tuple visibility
+\c :TEST_DBNAME :ROLE_SUPERUSER
+CREATE TABLE osm_slice_update(time int not null);
+SELECT hypertable_id AS ht_id FROM create_hypertable('osm_slice_update', 'time', chunk_time_interval => 10) \gset
+
+INSERT INTO osm_slice_update VALUES (1);
+UPDATE _timescaledb_catalog.hypertable SET status = 3 WHERE id = :ht_id;
+UPDATE _timescaledb_catalog.chunk SET osm_chunk = true WHERE hypertable_id = :ht_id;
+
+\c
+BEGIN;
+ 	SELECT _timescaledb_functions.hypertable_osm_range_update('osm_slice_update',40,50);
+ROLLBACK;
+
+\c
+-- new session should not be affected by previous rolled back transaction
+-- should show 0 10 as range
+\set ON_ERROR_STOP 0
+INSERT INTO osm_slice_update VALUES (1);
+INSERT INTO osm_slice_update VALUES (1);
+\set ON_ERROR_STOP 1
+
 -- clean up databases created
 \c :TEST_DBNAME :ROLE_SUPERUSER
 DROP DATABASE postgres_fdw_db;
 DROP DATABASE :DATA_NODE_1;
 DROP DATABASE :DATA_NODE_2;
+