fix: ensure reverse migration progress is not lost for mysql

See #1966.

Author: crepererum

sqlx-core/src/mysql/migrate.rs

@@ -261,18 +261,23 @@ CREATE TABLE IF NOT EXISTS _sqlx_migrations (
         migration: &'m Migration,
     ) -> BoxFuture<'m, Result<Duration, MigrateError>> {
         Box::pin(async move {
+            // Use a single transaction for the actual migration script and the essential bookeeping so we never
+            // execute migrations twice. See https://github.com/launchbadge/sqlx/issues/1966.
+            let mut tx = self.begin().await?;
             let start = Instant::now();
 
-            self.execute(&*migration.sql).await?;
-
-            let elapsed = start.elapsed();
+            tx.execute(&*migration.sql).await?;
 
             // language=SQL
             let _ = query(r#"DELETE FROM _sqlx_migrations WHERE version = ?"#)
                 .bind(migration.version)
-                .execute(self)
+                .execute(&mut tx)
                 .await?;
 
+            tx.commit().await?;
+
+            let elapsed = start.elapsed();
+
             Ok(elapsed)
         })
     }