Fix misuse of table_index_fetch_tuple_check().

Commit 0d861bbb, which added deduplication to nbtree, had
_bt_check_unique() pass a TID to table_index_fetch_tuple_check() that
isn't safe to mutate.  table_index_fetch_tuple_check()'s tid argument is
modified when the TID in question is not the latest visible tuple in a
hot chain, though this wasn't documented.

To fix, go back to using a local copy of the TID in _bt_check_unique(),
and update comments above table_index_fetch_tuple_check().

Backpatch: 13-, where B-Tree deduplication was introduced.
This commit is contained in:
Peter Geoghegan 2020-06-25 10:55:28 -07:00
parent 463b808e75
commit 10f1ab2cb8
3 changed files with 9 additions and 2 deletions

View File

@ -597,7 +597,8 @@ _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
* not part of this chain because it had a different index
* entry.
*/
if (table_index_fetch_tuple_check(heapRel, &itup->t_tid,
htid = itup->t_tid;
if (table_index_fetch_tuple_check(heapRel, &htid,
SnapshotSelf, NULL))
{
/* Normal case --- it's still live */

View File

@ -196,6 +196,10 @@ table_beginscan_parallel(Relation relation, ParallelTableScanDesc parallel_scan)
* optimized, but is unlikely to matter from a performance POV. If there
* frequently are live index pointers also matching a unique index key, the
* CPU overhead of this routine is unlikely to matter.
*
* Note that *tid may be modified when we return true if the AM supports
* storing multiple row versions reachable via a single index entry (like
* heap's HOT).
*/
bool
table_index_fetch_tuple_check(Relation rel,

View File

@ -989,7 +989,9 @@ table_index_fetch_end(struct IndexFetchTableData *scan)
/*
* Fetches, as part of an index scan, tuple at `tid` into `slot`, after doing
* a visibility test according to `snapshot`. If a tuple was found and passed
* the visibility test, returns true, false otherwise.
* the visibility test, returns true, false otherwise. Note that *tid may be
* modified when we return true (see later remarks on multiple row versions
* reachable via a single index entry).
*
* *call_again needs to be false on the first call to table_index_fetch_tuple() for
* a tid. If there potentially is another tuple matching the tid, *call_again