A fix for Bug#29431 killing an insert delayed thread causes crash

No test case, since the bug requires a stress case with 30 INSERT DELAYED
threads and 1 killer thread to repeat. The patch is verified
manually.
Review fixes.

The server that is running DELAYED inserts would deadlock itself
or crash under high load if some of the delayed threads were KILLed
in the meanwhile.

The fix is to change internal lock acquisition order of delayed inserts
subsystem and to ensure that
Delayed_insert::table_list::db does not point to volatile memory in some 
cases.
For details, please see a comment for sql_insert.cc.

sql/sql_insert.cc

@@ -1705,8 +1705,8 @@ Delayed_insert *find_handler(THD *thd, TABLE_LIST *table_list)
   Delayed_insert *tmp;
   while ((tmp=it++))
   {
-    if (!strcmp(tmp->thd.db,table_list->db) &&
-	!strcmp(table_list->table_name,tmp->table->s->table_name))
+    if (!strcmp(table_list->db, tmp->table_list.db) &&
+	!strcmp(table_list->table_name, tmp->table_list.table_name))
     {
       tmp->lock();
       break;
@@ -1739,7 +1739,27 @@ Delayed_insert *find_handler(THD *thd, TABLE_LIST *table_list)
                 Two latter cases indicate a request for lock upgrade.
 
   XXX: why do we regard INSERT DELAYED into a view as an error and
-  do not simply a lock upgrade?
+  do not simply perform a lock upgrade?
+
+  TODO: The approach with using two mutexes to work with the
+  delayed thread list -- LOCK_delayed_insert and
+  LOCK_delayed_create -- is redundant, and we only need one of
+  them to protect the list.  The reason we have two locks is that
+  we do not want to block look-ups in the list while we're waiting
+  for the newly created thread to open the delayed table. However,
+  this wait itself is redundant -- we always call get_local_table
+  later on, and there wait again until the created thread acquires
+  a table lock.
+
+  As is redundant the concept of locks_in_memory, since we already
+  have another counter with similar semantics - tables_in_use,
+  both of them are devoted to counting the number of producers for
+  a given consumer (delayed insert thread), only at different
+  stages of producer-consumer relationship.
+
+  'dead' and 'status' variables in Delayed_insert are redundant
+  too, since there is already 'di->thd.killed' and
+  di->stacked_inserts.
 */
 
 static
@@ -1788,7 +1808,9 @@ bool delayed_get_table(THD *thd, TABLE_LIST *table_list)
         goto end_create;
       }
       tmp->table_list= *table_list;			// Needed to open table
+      /* Replace volatile strings with local copies */
       tmp->table_list.alias= tmp->table_list.table_name= tmp->thd.query;
+      tmp->table_list.db= tmp->thd.db;
       tmp->lock();
       pthread_mutex_lock(&tmp->mutex);
       if ((error=pthread_create(&tmp->thd.real_id,&connection_attrib,
@@ -1834,6 +1856,9 @@ bool delayed_get_table(THD *thd, TABLE_LIST *table_list)
 	tmp->unlock();
         goto end_create;
       }
+      pthread_mutex_lock(&LOCK_delayed_insert);
+      delayed_threads.append(tmp);
+      pthread_mutex_unlock(&LOCK_delayed_insert);
     }
     pthread_mutex_unlock(&LOCK_delayed_create);
   }
@@ -2176,11 +2201,6 @@ pthread_handler_t handle_delayed_insert(void *arg)
   }
   di->table->copy_blobs=1;
 
-  /* One can now use this */
-  pthread_mutex_lock(&LOCK_delayed_insert);
-  delayed_threads.append(di);
-  pthread_mutex_unlock(&LOCK_delayed_insert);
-
   /* Tell client that the thread is initialized */
   pthread_cond_signal(&di->cond_client);