Thread safe statistics loading

Previously multiple threads were allowed to load statistics concurrently.
There were no known problems caused by this. But given amount of data
races in this code, it'd happen sooner or later.

To avoid scalability bottleneck, statistics loading is protected by
per-TABLE_SHARE atomic variable.

Whenever statistics were loaded by preceding statement (hot-path), a
scalable load-acquire check is performed.

Whenever statistics have to be loaded anew, mutual exclusion for loaders
is established by atomic variable. If statistics are being loaded
concurrently, statement waits until load is completed.

TABLE_STATISTICS_CB::stats_can_be_read and
TABLE_STATISTICS_CB::stats_is_read are replaced with a tri state atomic
variable.

Part of MDEV-19061 - table_share used for reading statistical tables is
                     not protected

1 files changed, 45 insertions, 54 deletions

diff --git a/sql/sql_statistics.cc b/sql/sql_statistics.cc
index 0f8daa6e52f..a99dcca89e4 100644
--- a/sql/sql_statistics.cc
+++ b/sql/sql_statistics.cc
@@ -2204,27 +2204,13 @@ static int alloc_statistics_for_table_share(THD* thd, TABLE_SHARE *table_share)
 
   DBUG_ENTER("alloc_statistics_for_table_share");
 
-  DEBUG_SYNC(thd, "statistics_mem_alloc_start1");
-  DEBUG_SYNC(thd, "statistics_mem_alloc_start2");
-
-  mysql_mutex_lock(&table_share->LOCK_share);
-
-  if (stats_cb->stats_can_be_read)
-  {
-    mysql_mutex_unlock(&table_share->LOCK_share);
-    DBUG_RETURN(0);
-  }
-
   Table_statistics *table_stats= stats_cb->table_stats;
   if (!table_stats)
   {
     table_stats=  (Table_statistics *) alloc_root(&stats_cb->mem_root,
                                                   sizeof(Table_statistics));
     if (!table_stats)
-    {
-      mysql_mutex_unlock(&table_share->LOCK_share);
       DBUG_RETURN(1);
-    }
     memset(table_stats, 0, sizeof(Table_statistics));
     stats_cb->table_stats= table_stats;
   }
@@ -2290,13 +2276,7 @@ static int alloc_statistics_for_table_share(THD* thd, TABLE_SHARE *table_share)
       }
     }   
   }
-
-  if (column_stats && index_stats && idx_avg_frequency)
-    stats_cb->stats_can_be_read= TRUE;
-
-  mysql_mutex_unlock(&table_share->LOCK_share);
-
-  DBUG_RETURN(0);
+  DBUG_RETURN(column_stats && index_stats && idx_avg_frequency ? 0 : 1);
 }
 
 
@@ -2913,11 +2893,22 @@ int read_statistics_for_table(THD *thd, TABLE *table, TABLE_LIST *stat_tables)
   Field **field_ptr;
   KEY *key_info, *key_info_end;
   TABLE_SHARE *table_share= table->s;
-  Table_statistics *read_stats= table_share->stats_cb.table_stats;
 
   DBUG_ENTER("read_statistics_for_table");
+  DEBUG_SYNC(thd, "statistics_mem_alloc_start1");
+  DEBUG_SYNC(thd, "statistics_mem_alloc_start2");
+
+  if (!table_share->stats_cb.start_stats_load())
+    DBUG_RETURN(table_share->stats_cb.stats_are_ready() ? 0 : 1);
+
+  if (alloc_statistics_for_table_share(thd, table_share))
+  {
+    table_share->stats_cb.abort_stats_load();
+    DBUG_RETURN(1);
+  }
 
   /* Read statistics from the statistical table table_stats */
+  Table_statistics *read_stats= table_share->stats_cb.table_stats;
   stat_table= stat_tables[TABLE_STAT].table;
   Table_stat table_stat(stat_table, table);
   table_stat.set_key_fields();
@@ -2995,9 +2986,7 @@ int read_statistics_for_table(THD *thd, TABLE *table, TABLE_LIST *stat_tables)
       }
     }
   }
-      
-  table->stats_is_read= TRUE;
-
+  table_share->stats_cb.end_stats_load();
   DBUG_RETURN(0);
 }
 
@@ -3146,6 +3135,23 @@ int read_statistics_for_tables_if_needed(THD *thd, TABLE_LIST *tables)
 }
 
 
+static void dump_stats_from_share_to_table(TABLE *table)
+{
+  TABLE_SHARE *table_share= table->s;
+  KEY *key_info= table_share->key_info;
+  KEY *key_info_end= key_info + table_share->keys;
+  KEY *table_key_info= table->key_info;
+  for ( ; key_info < key_info_end; key_info++, table_key_info++)
+    table_key_info->read_stats= key_info->read_stats;
+
+  Field **field_ptr= table_share->field;
+  Field **table_field_ptr= table->field;
+  for ( ; *field_ptr; field_ptr++, table_field_ptr++)
+    (*table_field_ptr)->read_stats= (*field_ptr)->read_stats;
+  table->stats_is_read= true;
+}
+
+
 int read_statistics_for_tables(THD *thd, TABLE_LIST *tables)
 {
   TABLE_LIST stat_tables[STATISTICS_TABLES];
@@ -3167,30 +3173,17 @@ int read_statistics_for_tables(THD *thd, TABLE_LIST *tables)
     {
       if (table_share->table_category == TABLE_CATEGORY_USER)
       {
-        if (table_share->stats_cb.stats_can_be_read ||
-            !alloc_statistics_for_table_share(thd, table_share))
+        if (table_share->stats_cb.stats_are_ready())
         {
-          if (table_share->stats_cb.stats_can_be_read)
-          {
-            KEY *key_info= table_share->key_info;
-            KEY *key_info_end= key_info + table_share->keys;
-            KEY *table_key_info= tl->table->key_info;
-            for ( ; key_info < key_info_end; key_info++, table_key_info++)
-              table_key_info->read_stats= key_info->read_stats;
-            Field **field_ptr= table_share->field;
-            Field **table_field_ptr= tl->table->field;
-            for ( ; *field_ptr; field_ptr++, table_field_ptr++)
-              (*table_field_ptr)->read_stats= (*field_ptr)->read_stats;
-            tl->table->stats_is_read= table_share->stats_cb.stats_is_read;
-            tl->table->histograms_are_read=
-              table_share->stats_cb.histograms_are_read;
-
-            if (!tl->table->stats_is_read ||
-                (!table_share->stats_cb.histograms_are_read &&
-                 thd->variables.optimizer_use_condition_selectivity > 3))
-              statistics_for_tables_is_needed= true;
-          }
+          if (!tl->table->stats_is_read)
+            dump_stats_from_share_to_table(tl->table);
+          tl->table->histograms_are_read=
+            table_share->stats_cb.histograms_are_read;
+          if (table_share->stats_cb.histograms_are_read ||
+              thd->variables.optimizer_use_condition_selectivity <= 3)
+            continue;
         }
+        statistics_for_tables_is_needed= true;
       }
       else if (is_stat_table(tl->db, tl->alias))
         found_stat_table= true;
@@ -3217,16 +3210,14 @@ int read_statistics_for_tables(THD *thd, TABLE_LIST *tables)
         table_share->tmp_table == NO_TMP_TABLE &&
         table_share->table_category == TABLE_CATEGORY_USER)
     {
-      if (table_share->stats_cb.stats_can_be_read &&
-	  !table_share->stats_cb.stats_is_read)
+      if (!tl->table->stats_is_read)
       {
-        (void) read_statistics_for_table(thd, tl->table, stat_tables);
-        table_share->stats_cb.stats_is_read= TRUE;
+        if (!read_statistics_for_table(thd, tl->table, stat_tables))
+          dump_stats_from_share_to_table(tl->table);
+        else
+          continue;
       }
-      if (table_share->stats_cb.stats_is_read)
-        tl->table->stats_is_read= TRUE;
       if (thd->variables.optimizer_use_condition_selectivity > 3 &&
-          table_share->stats_cb.stats_can_be_read &&
           !table_share->stats_cb.histograms_are_read)
       {
         (void) read_histograms_for_table(thd, tl->table, stat_tables);