Shared Test Transactions

[mcasper]

When running tests in a multithreaded environment (like running tests against a webserver), currently the only way to ensure that your test database is clean across threads is to drop/truncate for every test. This works fine, but is tedious, and it would be nicer if connections and their test transactions could be shared, so that data is all localized and automatically cleaned up.

One way to accomplish this that I've been thinking about is to implement Sync for connections and Connection for Arc<T: Connection>, allowing pointers to a shared connection to be passed around and data within a test transaction to be visible. I know we've previously talked about not wanting to implement Sync for connections because they're not meant to be passed around, which is why I'd want to hide this away behind a tests or testing feature, included only in [dev-dependencies].

Some changes would be necessary in other supporting libraries (like r2d2/r2d2-diesel for example), but could lead to code like the following for testing (example using Rocket):

#[test]
fn test_index() {
  let pool = database::pool();
  let conn = pool.get().unwrap();
  let rocket_app = app::launch(pool);

  conn.begin_test_transaction().unwrap();

  // insert data into connection
  // build and send request to app
  // assert on response
}

If we like this, I'm happy to take on the feature work

[sgrif]

The solution I've been using in crates.io is to set the connection pool size to 1, but I agree that it'd be nice to make it easier. I do not want to implement Sync on connections, as they are explicitly not thread safe and several of them rely on being guaranteed to not be concurrently used across threads. We'd need a mutex for sure.

[fluffysquirrels]

Arc<Mutex<T: Send>> already implements Sync automagically (Mutex docs) and Connection requires Send. So can you just wrap transactions in Arc<Mutex<T>> in your app?