Bug #56085 Embedded server tests fails with assert in

           check_if_table_exists()

This assert was triggered when the server tried to load plugins
while running in embedded server mode. In embedded server mode,
check_if_table_exists() was used to check if mysql.plugin existed
so that ER_NO_SUCH_TABLE could be silently ignored.
The problem was that this check was done without acquiring a metadata
lock on mysql.plugin first. This triggered the assert.

This patch fixes the problem by removing the call to
check_if_table_exists() from plugin_load(). Instead an error handler
which traps ER_NO_SUCH_TABLE is installed before trying to open
mysql.plugin when running in embedded server mode.

No test coverage added since this assert was triggered by 
existing tests running in embedded server mode.

sql/sql_base.cc

@@ -59,37 +59,8 @@
 #endif
 
 
-/**
-  This internal handler is used to trap internally
-  errors that can occur when executing open table
-  during the prelocking phase.
-*/
-class Prelock_error_handler : public Internal_error_handler
-{
-public:
-  Prelock_error_handler()
-    : m_handled_errors(0), m_unhandled_errors(0)
-  {}
-
-  virtual ~Prelock_error_handler() {}
-
-  virtual bool handle_condition(THD *thd,
-                                uint sql_errno,
-                                const char* sqlstate,
-                                MYSQL_ERROR::enum_warning_level level,
-                                const char* msg,
-                                MYSQL_ERROR ** cond_hdl);
-
-  bool safely_trapped_errors();
-
-private:
-  int m_handled_errors;
-  int m_unhandled_errors;
-};
-
-
 bool
-Prelock_error_handler::handle_condition(THD *,
+No_such_table_error_handler::handle_condition(THD *,
                                               uint sql_errno,
                                               const char*,
                                               MYSQL_ERROR::enum_warning_level,
@@ -108,7 +79,7 @@ Prelock_error_handler::handle_condition(THD *,
 }
 
 
-bool Prelock_error_handler::safely_trapped_errors()
+bool No_such_table_error_handler::safely_trapped_errors()
 {
   /*
     If m_unhandled_errors != 0, something else, unanticipated, happened,
@@ -4353,11 +4324,11 @@ open_and_process_table(THD *thd, LEX *lex, TABLE_LIST *tables,
       The real failure will occur when/if a statement attempts to use
       that table.
     */
-    Prelock_error_handler prelock_handler;
-    thd->push_internal_handler(& prelock_handler);
+    No_such_table_error_handler no_such_table_handler;
+    thd->push_internal_handler(&no_such_table_handler);
     error= open_table(thd, tables, new_frm_mem, ot_ctx);
     thd->pop_internal_handler();
-    safe_to_ignore_table= prelock_handler.safely_trapped_errors();
+    safe_to_ignore_table= no_such_table_handler.safely_trapped_errors();
   }
   else
     error= open_table(thd, tables, new_frm_mem, ot_ctx);

sql/sql_base.h

@@ -526,4 +526,34 @@ private:
 };
 
 
+/**
+  This internal handler is used to trap ER_NO_SUCH_TABLE.
+*/
+
+class No_such_table_error_handler : public Internal_error_handler
+{
+public:
+  No_such_table_error_handler()
+    : m_handled_errors(0), m_unhandled_errors(0)
+  {}
+
+  bool handle_condition(THD *thd,
+                        uint sql_errno,
+                        const char* sqlstate,
+                        MYSQL_ERROR::enum_warning_level level,
+                        const char* msg,
+                        MYSQL_ERROR ** cond_hdl);
+
+  /**
+    Returns TRUE if one or more ER_NO_SUCH_TABLE errors have been
+    trapped and no other errors have been seen. FALSE otherwise.
+  */
+  bool safely_trapped_errors();
+
+private:
+  int m_handled_errors;
+  int m_unhandled_errors;
+};
+
+
 #endif /* SQL_BASE_INCLUDED */

sql/sql_plugin.cc

@@ -1393,8 +1393,9 @@ static void plugin_load(MEM_ROOT *tmp_root, int *argc, char **argv)
   READ_RECORD read_record_info;
   int error;
   THD *new_thd= &thd;
+  bool result;
 #ifdef EMBEDDED_LIBRARY
-  bool table_exists;
+  No_such_table_error_handler error_handler;
 #endif /* EMBEDDED_LIBRARY */
   DBUG_ENTER("plugin_load");
 
@@ -1410,13 +1411,18 @@ static void plugin_load(MEM_ROOT *tmp_root, int *argc, char **argv)
     When building an embedded library, if the mysql.plugin table
     does not exist, we silently ignore the missing table
   */
-  if (check_if_table_exists(new_thd, &tables, &table_exists))
-    table_exists= FALSE;
-  if (!table_exists)
+  new_thd->push_internal_handler(&error_handler);
+#endif /* EMBEDDED_LIBRARY */
+
+  result= open_and_lock_tables(new_thd, &tables, FALSE, MYSQL_LOCK_IGNORE_TIMEOUT);
+
+#ifdef EMBEDDED_LIBRARY
+  new_thd->pop_internal_handler();
+  if (error_handler.safely_trapped_errors())
     goto end;
 #endif /* EMBEDDED_LIBRARY */
 
-  if (open_and_lock_tables(new_thd, &tables, FALSE, MYSQL_LOCK_IGNORE_TIMEOUT))
+  if (result)
   {
     DBUG_PRINT("error",("Can't open plugin table"));
     sql_print_error("Can't open the mysql.plugin table. Please "