newsql_get_snapshot: fall back to stored snapshot on ha-retry

On an ha-retry, is_snap_uid_retry sets clnt->is_hasql_retry=1 and
stores the retry snapshot LSN in clnt->snapshot_file/offset during
BEGIN handling.  Statements after BEGIN (INSERT, SELECT, COMMIT) carry
no snapshot_info in their sqlquery because only the BEGIN carries it
from the client.

initialize_shadow_trans calls newsql_get_snapshot to obtain the
snapshot LSN.  Without a fallback, newsql_get_snapshot returns file=0
for these in-transaction queries, causing trans_start_serializable to
be called with file=0 and is_ha_retry=1 — which triggers the assertion
at bdb_osqltrn.c:278 and crashes the node.

fill_snapinfo already handles this correctly by checking
clnt->snapshot_file when is_hasql_retry is set.  Add the same fallback
to newsql_get_snapshot so that initialize_shadow_trans always uses the
correct retry snapshot.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Mark Hannum <mhannum@bloomberg.net>

Author: markhannum

plugins/newsql/newsql.c

@@ -1406,6 +1406,13 @@ static int newsql_get_snapshot(struct sqlclntstate *clnt, int *file,
         *file = sqlquery->snapshot_info->file;
         *offset = sqlquery->snapshot_info->offset;
     }
+    /* On ha-retry, statements after BEGIN (e.g. INSERT) carry no
+     * snapshot_info in their sqlquery.  Fall back to the snapshot stored
+     * on the clnt during BEGIN handling, just as fill_snapinfo does. */
+    if (*file == 0 && clnt->is_hasql_retry && clnt->snapshot_file) {
+        *file = clnt->snapshot_file;
+        *offset = clnt->snapshot_offset;
+    }
     return 0;
 }