server: return error for new statement after opening cursor

[YangKeao]

Signed-off-by: YangKeao yangkeao@chunibyo.icu

What problem does this PR solve?

Issue Number: close #40094, close #39148, close #38116

Problem Summary:

Run statement after opening a cursor will make the sate (session variable, etc) hard to predict. The current implementation of cursor fetch in TiDB doesn't allow creating multiple cursor at the same time.

What is changed and how it works?

This PR add a check to return error when the command other than fetching/closing the active cursor arrives. The user will have to close the existing statement before running another commands.

The user forgot to close the statement may fail to run existing program ()

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

If the user forgot to close the statement explicitly before running other queries, the program will not continue to work with this patch. However, the former behavior is both unsafe (can cause panic in TiDB) and unpredictable (can read unpredictable data).

Release note

Previously, executing queries after opening an cursor will cause unpredictable behavior when reading the cursor. From now on, TiDB will not allow executing queries other than fetch and close after opening an cursor.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[YangKeao]

/run-build
/run-unit-test

[YangKeao]

As discussed, we should fix these problems rather than returning an error 🤔 , though it may take some effort. Let me try 🤖 .