MDEV-39261 MariaDB crash on startup in presence of indexed virtual columns

[Thirunarayanan]

Problem:

A single InnoDB purge worker thread can process undo logs from different tables within the same batch. But get_purge_table(), open_purge_table() incorrectly assumes that a 1:1 relationship between a purge worker thread and a table within a single batch. Based on this wrong assumtion, InnoDB attempts to reuse TABLE objects cached in thd->open_tables for virtual column computation.

1. Purge worker opens Table A and caches the TABLE pointer in thd->open_tables. 2) Same purge worker moves to Table B in the same batch, get_purge_table() retrieves the cached pointer for Table A instead of opening Table B. 3) Because innobase::open() is ignored for Table B, the virtual column template is never initialized.
2. virtual column computation for Table B aborts the server

Solution:

• Introduced purge_table class which has the following purge_table: Stores either TABLE* (for tables with indexed virtual columns) or MDL_ticket* (for tables without) in a single union using LSB as a flag.
  For tables with indexed virtual columns: opens TABLE*, accesses MDL_ticket* via TABLE->mdl_ticket
  For tables without indexed virtual columns: stores only MDL_ticket*.

trx_purge_attach_undo_recs(): Coordinator opens both dict_table_t* and TABLE* with proper MDL protection. Workers access cached table pointers from purge_node_t->tables without opening their own handles

purge_sys.coordinator_thd: Distinguish coordinator from workers in cleanup logic. Skip innobase_reset_background_thd() for coordinator thread to prevent premature table closure during batch processing. Workers still call cleanup to release their thread-local resources

trx_purge_close_tables():
Rewrite for purge coordinator thread

1. Get MDL_tickets for the tables opened
2. Call close_thread_tables() once for all TABLE* objects 3) Close the table and release the MDL

Added table->lock_mutex protection when reading (or) writing vc_templ->mysql_table and mysql_table_query_id. Clear cached TABLE* pointers before closing tables to prevent stale pointer access

Declared open_purge_table() and close_thread_tables() in trx0purge.cc Declared reset_thd() in row0purge.cc and dict0stats_bg.cc. Removed innobase_reset_background_thd()

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[Thirunarayanan]

It is 10.11 version of pr#4914

[dr-m]

I confirmed with

diff -I^@@ -I^index <(git diff 9f6152b141c9feffbdb389acf412d0d01a1ed762{~2,}) <(git diff 357055866d52af7da0fe47f75c29bf6b2b0ad7f3{~2,})

that this is equivalent to #4914 which was already merged after my approval. The differences are necessitated by the following:

• row_purge_reset_trx_id() was removed in ef2f3d2
• dict_table_close() was refactored in 6e6a1b3
• THD::mdl_context became directly accessible in 9aca89f