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.

1 files changed, 30 insertions, 0 deletions

diff --git a/sql/sql_base.h b/sql/sql_base.h
index 7a6f18545ad..ab11e939a50 100644
--- a/sql/sql_base.h
+++ b/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 */