Bug#52856 concurrent show columns or show full columns causes a crash!!!1

We should avoid any SHARE fields assignments as
this is shared structure and assignments may
affect other therads. To avoid this
copy of SHARE struct is created and
stored into TABLE struct which is
used in get_schema_coulumns_record later.

mysql-test/r/mdl_sync.result

@@ -2381,3 +2381,17 @@ commit;
 # Reap ALTER TABLE.
 set debug_sync= 'RESET';
 drop table t1;
+#
+# Bug#52856 concurrent show columns or show full columns causes a crash!!!
+#
+CREATE TABLE t1(a CHAR(255));
+SET DEBUG_SYNC= "get_schema_column SIGNAL waiting WAIT_FOR completed";
+SHOW FULL COLUMNS FROM t1;
+SET DEBUG_SYNC= "now WAIT_FOR waiting";
+SHOW FULL COLUMNS FROM t1;
+Field	Type	Collation	Null	Key	Default	Extra	Privileges	Comment
+a	char(255)	latin1_swedish_ci	YES		NULL		select,insert,update,references	
+SET DEBUG_SYNC= "now SIGNAL completed";
+Field	Type	Collation	Null	Key	Default	Extra	Privileges	Comment
+a	char(255)	latin1_swedish_ci	YES		NULL		select,insert,update,references	
+DROP TABLE t1;

mysql-test/t/mdl_sync.test

@@ -3468,6 +3468,26 @@ connection default;
 set debug_sync= 'RESET';
 drop table t1;
 
+--echo #
+--echo # Bug#52856 concurrent show columns or show full columns causes a crash!!!
+--echo #
+CREATE TABLE t1(a CHAR(255));
+
+connect(con1, localhost, root);
+SET DEBUG_SYNC= "get_schema_column SIGNAL waiting WAIT_FOR completed";
+--send SHOW FULL COLUMNS FROM t1
+
+connection default;
+SET DEBUG_SYNC= "now WAIT_FOR waiting";
+SHOW FULL COLUMNS FROM t1;
+SET DEBUG_SYNC= "now SIGNAL completed";
+
+connection con1;
+--reap
+connection default;
+DROP TABLE t1;
+disconnect con1;
+
 
 # Check that all connections opened by test cases in this file are really
 # gone so execution of other tests won't be affected by their presence.

sql/sql_show.cc

@@ -18,6 +18,7 @@
 
 #include "my_global.h"                          /* NO_EMBEDDED_ACCESS_CHECKS */
 #include "sql_priv.h"
+#include "debug_sync.h"
 #include "unireg.h"
 #include "sql_acl.h"                        // fill_schema_*_privileges
 #include "sql_select.h"                         // For select_describe
@@ -3292,12 +3293,17 @@ static int fill_schema_table_from_frm(THD *thd, TABLE_LIST *tables,
     goto end_share;
   }
 
+  if (!open_table_from_share(thd, share, table_name->str, 0,
+                             (EXTRA_RECORD | OPEN_FRM_FILE_ONLY),
+                             thd->open_options, &tbl, FALSE))
   {
     tbl.s= share;
     table_list.table= &tbl;
     table_list.view= (LEX*) share->is_view;
     res= schema_table->process_table(thd, &table_list, table,
                                      res, db_name, table_name);
+    free_root(&tbl.mem_root, MYF(0));
+    my_free((char*) tbl.alias, MYF(MY_ALLOW_ZERO_PTR));
   }
 
 end_share:
@@ -4024,7 +4030,6 @@ static int get_schema_column_record(THD *thd, TABLE_LIST *tables,
   const char *wild= lex->wild ? lex->wild->ptr() : NullS;
   CHARSET_INFO *cs= system_charset_info;
   TABLE *show_table;
-  TABLE_SHARE *show_table_share;
   Field **ptr, *field, *timestamp_field;
   int count;
   DBUG_ENTER("get_schema_column_record");
@@ -4047,37 +4052,11 @@ static int get_schema_column_record(THD *thd, TABLE_LIST *tables,
   }
 
   show_table= tables->table;
-  show_table_share= show_table->s;
   count= 0;
-
-  if (tables->view || tables->schema_table)
-  {
-    ptr= show_table->field;
-    timestamp_field= show_table->timestamp_field;
-    show_table->use_all_columns();               // Required for default
-  }
-  else
-  {
-    ptr= show_table_share->field;
-    timestamp_field= show_table_share->timestamp_field;
-    /*
-      read_set may be inited in case of
-      temporary table
-    */
-    if (!show_table->read_set)
-    {
-      /* to satisfy 'field->val_str' ASSERTs */
-      uchar *bitmaps;
-      uint bitmap_size= show_table_share->column_bitmap_size;
-      if (!(bitmaps= (uchar*) alloc_root(thd->mem_root, bitmap_size)))
-        DBUG_RETURN(0);
-      bitmap_init(&show_table->def_read_set,
-                  (my_bitmap_map*) bitmaps, show_table_share->fields, FALSE);
-      bitmap_set_all(&show_table->def_read_set);
-      show_table->read_set= &show_table->def_read_set;
-    }
-    bitmap_set_all(show_table->read_set);
-  }
+  ptr= show_table->field;
+  timestamp_field= show_table->timestamp_field;
+  show_table->use_all_columns();               // Required for default
+  restore_record(show_table, s->default_values);
 
   for (; (field= *ptr) ; ptr++)
   {
@@ -4086,9 +4065,7 @@ static int get_schema_column_record(THD *thd, TABLE_LIST *tables,
     String type(tmp,sizeof(tmp), system_charset_info);
     char *end;
 
-    /* to satisfy 'field->val_str' ASSERTs */
-    field->table= show_table;
-    show_table->in_use= thd;
+    DEBUG_SYNC(thd, "get_schema_column");
 
     if (wild && wild[0] &&
         wild_case_compare(system_charset_info, field->field_name,wild))