MySQL: a cancelled prepare leaves its response on the stream #4427

I have found these related issues/pull requests

#563 covers cancellation in the middle of a packet read; this reproduces with responses too small for that window.

Description

prepare_statement reads the COM_STMT_PREPARE response inline and records nothing in waiting, while run has already queued Waiting::Result. A prepare cancelled before its response is read leaves stmt-prepare-ok and the definition packets on the stream. The drain takes stmt-prepare-ok for the queued result, and the next command reads a column definition (0x03) as its reply. A failed parameter-count check leaves the same queued entry without any cancellation, and the next command hangs.

Reproduction steps

// sqlx = { version = "=0.9.0", default-features = false, features = ["mysql", "runtime-tokio"] }
// tokio = { version = "1", features = ["macros", "rt"] }
use sqlx::{Connection, MySqlConnection};

#[tokio::main(flavor = "current_thread")]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut conn = MySqlConnection::connect(&std::env::var("DATABASE_URL")?).await?;
    for i in 0..100 {
        // Poll the query once, which sends COM_STMT_PREPARE, then drop it.
        tokio::select! {
            biased;
            r = sqlx::query("SELECT ? AS a, 'b' AS b, 'c' AS c, 'd' AS d")
                .bind(1i64)
                .fetch_all(&mut conn) => { r?; }
            _ = std::future::ready(()) => {}
        }
        conn.ping().await.map_err(|e| format!("iteration {i}: {e}"))?;
    }
    println!("ok");
    Ok(())
}

On 0.9.0 and on main (b54008a) the ping fails at iteration 0 in 10 of 10 runs: expected 0x00 or 0xfe (OK_Packet) but found 0x03. Without a cancellation, sqlx::query("SELECT ?").execute(&mut conn) fails its parameter-count check, and the next conn.ping() hangs (3 of 3 runs on each).

SQLx version

0.9.0; the same on main (b54008a).

Enabled SQLx features

mysql, runtime-tokio

Database server and version

MariaDB 11.4.13

Operating system

Debian 13, Linux 6.12 x86_64

Rust version

1.97.0