MDEV-32050: Allow table to be guarded by an MDL of another thread

Add a debug-only field MDL_context::lock_warrant. This field can be set
to the MDL context different from the one the current execution is done in.

The lock warrantor has to hold an MDL for at least a duration of a table
lifetime.

This is needed in the subsequent commit so that the shared MDL acquired by
the InnoDB purge_coordinator_task can be shared by purge_worker_task
that access index records that include virtual columns.

Reviewed by: Vladislav Vaintroub

sql/mdl.h

@@ -1100,6 +1100,26 @@ class MDL_context
 
   /* metadata_lock_info plugin */
   friend int i_s_metadata_lock_info_fill_row(MDL_ticket*, void*);
+#ifndef DBUG_OFF
+public:
+  /**
+    This is for the case when the thread opening the table does not acquire
+    the lock itself, but utilizes a lock guarantee from another MDL context.
+
+    For example, in InnoDB, MDL is acquired by the purge_coordinator_task,
+    but the table may be opened and used in a purge_worker_task.
+    The coordinator thread holds the lock for the duration of worker's purge
+    job, or longer, possibly reusing shared MDL for different workers and jobs.
+  */
+  MDL_context *lock_warrant= NULL;
+
+  inline bool is_lock_warrantee(MDL_key::enum_mdl_namespace ns,
+                                const char *db, const char *name,
+                                enum_mdl_type mdl_type) const
+  {
+    return lock_warrant && lock_warrant->is_lock_owner(ns, db, name, mdl_type);
+  }
+#endif
 };
 
 

sql/sql_base.cc

@@ -976,6 +976,10 @@ void close_thread_table(THD *thd, TABLE **table_ptr)
     the table to the table cache.
   */
   DBUG_ASSERT(thd->mdl_context.is_lock_owner(MDL_key::TABLE,
+                                             table->s->db.str,
+                                             table->s->table_name.str,
+                                             MDL_SHARED) ||
+              thd->mdl_context.is_lock_warrantee(MDL_key::TABLE,
                                                  table->s->db.str,
                                                  table->s->table_name.str,
                                                  MDL_SHARED));