MDEV-16678 Prefer MDL to dict_sys.latch for innodb background tasks

This is joint work with Thirunarayanan Balathandayuthapani.
The MDL interface between InnoDB and the rest of the server
(in storage/innobase/dict/dict0dict.cc and in include/)
is my work, while most everything else is Thiru's.

The collection of InnoDB persistent statistics and the
defragmentation were not refactored to use MDL. They will
keep relying on lower-level interlocking with
fil_check_pending_operations().

The purge of transaction history and the background operations on
fulltext indexes will use MDL. We will revert
commit 2c4844c9e76427525e8c39a2d72686085efe89c3
(MDEV-17813) because thanks to MDL, purge cannot conflict
with DDL operations anymore. For a similar reason, we will remove
the MDEV-16222 test case from gcol.innodb_virtual_debug_purge.

Purge is essentially replacing all use of the global dict_sys.latch
with MDL. Purge will skip the undo log records for tables whose names
start with #sql-ib or #sql2. Theoretically, such tables might
be renamed back to visible table names if TRUNCATE fails to
create a new table, or the final rename in ALTER TABLE...ALGORITHM=COPY
fails. In that case, purge could permanently leave some garbage
in the table. Such garbage will be tolerated; the table would not
be considered corrupted.

To avoid repeated MDL releases and acquisitions,
trx_purge_attach_undo_recs() will sort undo log records by table_id,
and purge_node_t will keep the MDL and table handle open for multiple
successive undo log records.

get_purge_table(): A new accessor, used during the purge of
history for indexed virtual columns. This interface should ideally
not exist at all.

thd_mdl_context(): Accessor of THD::mdl_context.
Wrapped in a new thd_mdl_service.

dict_get_db_name_len(): Define inline.

dict_acquire_mdl_shared(): Acquire explicit shared MDL on a table name
if needed.

dict_table_open_on_id(): Return MDL_ticket, if requested.

dict_table_close(): Release MDL ticket, if requested.

dict_fts_index_syncing(), dict_index_t::index_fts_syncing: Remove.
row_drop_table_for_mysql() no longer needs to check these, because
MDL guarantees that a fulltext index sync will not be in progress
while MDL_EXCLUSIVE is protecting a DDL operation.

dict_table_t::parse_name(): Parse the table name for acquiring MDL.

purge_node_t::undo_recs: Change the type to std::list<trx_purge_rec_t*>
(different container, and storing also roll_ptr).

purge_node_t: Add mdl_ticket, last_table_id, purge_thd, mdl_hold_recs
for acquiring MDL and for keeping the table open across multiple
undo log records.

purge_vcol_info_t, row_purge_store_vsec_cur(), row_purge_restore_vsec_cur():
Remove. We will acquire the MDL earlier.

purge_sys_t::heap: Added, for reading undo log records.

fts_sync_during_ddl(): Invoked during ALGORITHM=INPLACE operations
to ensure that fts_sync_table() will not conflict with MDL_EXCLUSIVE.
Uses fts_t::sync_message for bookkeeping.

1 files changed, 18 insertions, 0 deletions

diff --git a/sql/sql_class.cc b/sql/sql_class.cc
index 09c609cdd99..4577f1007be 100644
--- a/sql/sql_class.cc
+++ b/sql/sql_class.cc
@@ -4752,6 +4752,12 @@ TABLE *open_purge_table(THD *thd, const char *db, size_t dblen,
   DBUG_RETURN(error ? NULL : tl->table);
 }
 
+TABLE *get_purge_table(THD *thd)
+{
+  /* see above, at most one table can be opened */
+  DBUG_ASSERT(thd->open_tables == NULL || thd->open_tables->next == NULL);
+  return thd->open_tables;
+}
 
 /** Find an open table in the list of prelocked tabled
 
@@ -5303,6 +5309,18 @@ extern "C" void thd_wait_end(MYSQL_THD thd)
 
 #endif // INNODB_COMPATIBILITY_HOOKS */
 
+
+/**
+  MDL_context accessor
+  @param thd   the current session
+  @return pointer to thd->mdl_context
+*/
+extern "C" void *thd_mdl_context(MYSQL_THD thd)
+{
+  return &thd->mdl_context;
+}
+
+
 /****************************************************************************
   Handling of statement states in functions and triggers.