Create MySQL_Connection.md

doc/internal/MySQL_Connection.md

@@ -0,0 +1,70 @@
+### Flowchart of `MySQL_Connection::async_query()`
+
+This function asynchronously executes a query on the MySQL connection.  
+It handles various states of the asynchronous query execution process and returns appropriate status codes indicating the result of the execution.
+
+Returns an integer status code indicating the result of the query execution:
+- 0: Query execution completed successfully.
+- -1: Query execution failed.
+- 1: Query execution in progress.
+- 2: Processing a multi-statement query, control needs to be transferred to MySQL_Session.
+- 3: In the middle of processing a multi-statement query.
+
+
+```mermaid
+---
+title: MySQL_Connection::async_query()
+---
+flowchart TD
+Assert["assert()"]
+ValidConnection{Valid Connection}
+ValidConnection -- no --> Assert
+IsServerOffline{"IsServerOffline()"}
+ValidConnection -- yes --> IsServerOffline
+IsServerOffline -- yes --> ReturnMinus1
+asyncStateMachine1{async_state_machine}
+asyncStateMachine2{async_state_machine}
+IsServerOffline -- no --> asyncStateMachine1
+asyncStateMachine1 -- ASYNC_QUERY_END --> Return0
+handler["handler()"]
+asyncStateMachine1 --> handler
+handler --> asyncStateMachine2
+asyncStateMachine2 -- ASYNC_QUERY_END --> mysql_error{"mysql_error"}
+asyncStateMachine2 -- ASYNC_STMT_EXECUTE_END --> mysql_error
+asyncStateMachine2 -- ASYNC_STMT_PREPARE_FAILED --> ReturnMinus1
+asyncStateMachine2 -- ASYNC_STMT_PREPARE_SUCCESSFUL --> Return0
+mysql_error -- yes --> ReturnMinus1
+mysql_error -- no --> Return0
+asyncStateMachine2 -- ASYNC_NEXT_RESULT_START --> Return2
+processing_multi_statement{"processing_multi_statement"}
+asyncStateMachine2 --> processing_multi_statement
+processing_multi_statement -- yes --> Return3
+processing_multi_statement -- no --> Return1
+ReturnMinus1["return -1"]
+Return0["return 0"]
+Return1["return 1"]
+Return2["return 2"]
+Return3["return 3"]
+```
+
+### Flowchart of `MySQL_Connection::IsServerOffline()`
+
+```mermaid
+---
+title: MySQL_Connection::IsServerOffline()
+---
+flowchart TD
+True[true]
+False[false]
+SS1{"server_status"}
+SA{"shunned_automatic"}
+SB{"shunned_and_kill_all_connections"}
+SS1 -- OFFLINE_HARD --> True
+SS1 -- REPLICATION_LAG --> True
+SS1 -- SHUNNED --> SA
+SA -- yes --> SB
+SB -- yes --> True
+SA -- no --> False
+SB -- no --> False
+SS1 --> False
+```