feat(sqlite): support no_tx migrations

SQLite includes several SQL statements that are useful during migrations but
must be executed outside of a transaction to take effect, such as `PRAGMA
foreign_keys = ON|OFF` or `VACUUM`. Additionally, advanced migrations may want
more precise control over how statements are grouped into transactions or
savepoints to achieve the desired atomicity for different parts of the
migration.

While SQLx already supports marking migrations to run outside explicit
transactions through a `-- no-transaction` comment, this feature is currently
only available for `PgConnection`'s `Migrate` implementation, leaving SQLite and
MySQL without this capability. Although it's possible to work around this
limitation by implementing custom migration logic instead of executing
`Migrator#run`, this comes at a cost of significantly reduced developer
ergonomics: code that relies on the default migration logic, such as
`#[sqlx::test]` or `cargo sqlx database setup`, won't support these migrations.

These changes extend `SqliteConnection`'s `Migrate` implementation to support
`no_tx` migrations in the same way as PostgreSQL, addressing this feature gap. I
also considered implementing the same functionality for MySQL, but since I
haven't found a practical use case for it yet, and every
non-transaction-friendly statement I could think about in MySQL triggers
implicit commits anyway, I determined it wasn't necessary at this time and could
be considered an overreach.

Author: AlexTMjugador

sqlx-sqlite/src/migrate.rs

@@ -160,38 +160,24 @@ CREATE TABLE IF NOT EXISTS {table_name} (
         migration: &'e Migration,
     ) -> BoxFuture<'e, Result<Duration, MigrateError>> {
         Box::pin(async move {
-            let mut tx = self.begin().await?;
             let start = Instant::now();
 
-            // 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.
-            // The `execution_time` however can only be measured for the whole transaction. This value _only_ exists for
-            // data lineage and debugging reasons, so it is not super important if it is lost. So we initialize it to -1
-            // and update it once the actual transaction completed.
-            let _ = tx
-                .execute(migration.sql.clone())
-                .await
-                .map_err(|e| MigrateError::ExecuteMigration(e, migration.version))?;
-
-            // language=SQL
-            let _ = query(AssertSqlSafe(format!(
-                r#"
-    INSERT INTO {table_name} ( version, description, success, checksum, execution_time )
-    VALUES ( ?1, ?2, TRUE, ?3, -1 )
-                "#
-            )))
-            .bind(migration.version)
-            .bind(&*migration.description)
-            .bind(&*migration.checksum)
-            .execute(&mut *tx)
-            .await?;
-
-            tx.commit().await?;
+            if migration.no_tx {
+                execute_migration(self, table_name, migration).await?;
+            } else {
+                // Use a single transaction for the actual migration script and the essential bookkeeping so we never
+                // execute migrations twice. See https://github.com/launchbadge/sqlx/issues/1966.
+                // The `execution_time` however can only be measured for the whole transaction. This value _only_ exists for
+                // data lineage and debugging reasons, so it is not super important if it is lost. So we initialize it to -1
+                // and update it once the actual transaction completed.
+                let mut tx = self.begin().await?;
+                execute_migration(&mut tx, table_name, migration).await?;
+                tx.commit().await?;
+            }
 
             // Update `elapsed_time`.
             // NOTE: The process may disconnect/die at this point, so the elapsed time value might be lost. We accept
             //       this small risk since this value is not super important.
-
             let elapsed = start.elapsed();
 
             // language=SQL
@@ -218,26 +204,69 @@ CREATE TABLE IF NOT EXISTS {table_name} (
         migration: &'e Migration,
     ) -> BoxFuture<'e, 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();
 
-            let _ = tx.execute(migration.sql.clone()).await?;
-
-            // language=SQLite
-            let _ = query(AssertSqlSafe(format!(
-                r#"DELETE FROM {table_name} WHERE version = ?1"#
-            )))
-            .bind(migration.version)
-            .execute(&mut *tx)
-            .await?;
-
-            tx.commit().await?;
+            if migration.no_tx {
+                execute_migration(self, table_name, migration).await?;
+            } else {
+                let mut tx = self.begin().await?;
+                revert_migration(&mut tx, table_name, migration).await?;
+                tx.commit().await?;
+            }
 
             let elapsed = start.elapsed();
 
             Ok(elapsed)
         })
     }
 }
+
+async fn execute_migration(
+    conn: &mut SqliteConnection,
+    table_name: &str,
+    migration: &Migration,
+) -> Result<(), MigrateError> {
+    let _ = conn
+        .execute(migration.sql.clone())
+        .await
+        .map_err(|e| MigrateError::ExecuteMigration(e, migration.version))?;
+
+    // language=SQL
+    let _ = query(AssertSqlSafe(format!(
+        r#"
+    INSERT INTO {table_name} ( version, description, success, checksum, execution_time )
+    VALUES ( ?1, ?2, TRUE, ?3, -1 )
+        "#
+    )))
+    .bind(migration.version)
+    .bind(&*migration.description)
+    .bind(&*migration.checksum)
+    .execute(conn)
+    .await?;
+
+    Ok(())
+}
+
+async fn revert_migration(
+    conn: &mut SqliteConnection,
+    table_name: &str,
+    migration: &Migration,
+) -> Result<(), MigrateError> {
+    let _ = conn
+        .execute(migration.sql.clone())
+        .await
+        .map_err(|e| MigrateError::ExecuteMigration(e, migration.version))?;
+
+    // language=SQL
+    let _ = query(AssertSqlSafe(format!(
+        r#"
+    DELETE FROM {table_name}
+    WHERE version = ?1
+        "#
+    )))
+    .bind(migration.version)
+    .execute(conn)
+    .await?;
+
+    Ok(())
+}