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, 3 insertions, 9 deletions

diff --git a/storage/innobase/include/trx0purge.h b/storage/innobase/include/trx0purge.h
index ec445cd4d0c..ceecb369337 100644
--- a/storage/innobase/include/trx0purge.h
+++ b/storage/innobase/include/trx0purge.h
@@ -140,7 +140,6 @@ public:
 	MY_ALIGNED(CACHE_LINE_SIZE)
 	rw_lock_t	latch;
 private:
-	bool m_initialized;
 	/** whether purge is enabled; protected by latch and std::atomic */
 	std::atomic<bool>		m_enabled;
 	/** number of pending stop() calls without resume() */
@@ -213,6 +212,8 @@ public:
 		fil_space_t*	last;
 	} truncate;
 
+	/** Heap for reading the undo log records */
+	mem_heap_t*	heap;
   /**
     Constructor.
 
@@ -220,8 +221,7 @@ public:
     uninitialised. Real initialisation happens in create().
   */
 
-  purge_sys_t(): m_initialized(false), m_enabled(false) {}
-
+  purge_sys_t(): m_enabled(false), heap(nullptr) {}
 
   /** Create the instance */
   void create();
@@ -261,12 +261,6 @@ public:
 /** The global data structure coordinating a purge */
 extern purge_sys_t	purge_sys;
 
-/** Info required to purge a record */
-struct trx_purge_rec_t {
-	trx_undo_rec_t*	undo_rec;	/*!< Record to purge */
-	roll_ptr_t	roll_ptr;	/*!< File pointr to UNDO record */
-};
-
 #include "trx0purge.ic"
 
 #endif /* trx0purge_h */