What problem does this PR solve?

Issue Number: close #16817

Problem Summary:

The root cause of the panic in #16817 is the data race.

And the data race exists because we close the transaction immediately in ExecuteStmt.
When using the RecordSet returned by ExecuteStmt, executor.Next() is called and it maybe still refering the txn, the txn is closed and set to nil however.

rs :=    ExecuteStmt --->
                runStmt --->
                    finishStmt --->
                        CommitTxn --->
                            txn set to nil !!

rs.Next() -->
    executor.Next --->
        PointGet.Next --->
            txn.Valid() ???

What is changed and how it works?

We should not commit a txn until the executor stop refering it.
The correct order is:

1. ExecuteStmt return a RecordSet rs
2. Call rs.Next(), drain the chunk data and sent result to the mysql client
3. Call rs.Close()
4. Then finishStmt should be called and txn maybe commit here.

What's Changed:

• Move finishStmt out of runStmt.
• Wrap a new execStmtResult based on RecordSet, override the Close method,
  and call finishStmt in execStmtResult.Close()

How it Works:

Txn is not commited until the executor close the RecordSet, so there aren't be any concurrent visits of txn between finishStmt and recordset.Next, the data race is fixed.

Related changes

The refactor in #16056 avoid the multiple return value []RecordSet. It makes a solid foundantion towards better code for the session processing.

And this commit is a furthur step after that. This commit solve the problem finally:

The problem is, the recordset is lazy delivered, and it references the state of the session which is always mutable.

This commit guarantee the session won't be modify until the chunk data are drained and recordset is closed.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

runStmtWrap is a copy of runStmt, that's because even a tiny modify would meet numerous CI error, I've tried.

Keep this commit as simple as possible: just move finishStmt to RecordSet.Close
And after it's merged, I'll clean old Execute method (which uses runStmt).

Release note

• No release note