RUST-1719 Fix panic after SessionCursor::with_type is called

[mlokr]

Hello there,

when using the transaction API, I encountered unconditional panic and also panic while dropping in SessionCursor::with_type. The problem occurs because the .state is moved into new cursor and subsequently, .unwrap() is called on it when marking it exhausted and also when checking for exhaustion. This is the minimal code change to fix this problem

[isabelatkinson]

Hi @mlokr thank you for reporting this issue! Would you mind providing a reproduction of the code that caused this panic and let us know what version of the driver you're using? I will look into this in the meantime.

[isabelatkinson]

Update: I was able to reproduce this issue with the following test:

#[cfg_attr(feature = "tokio-runtime", tokio::test)]
#[cfg_attr(feature = "async-std-runtime", async_std::test)]
async fn session_cursor_with_type() {
    let _guard: RwLockReadGuard<()> = LOCK.run_concurrently().await;
    let client = TestClient::new().await;

    let mut session = client.start_session(None).await.unwrap();
    let coll = client.database("db").collection("coll");
    coll.drop_with_session(None, &mut session).await.unwrap();

    coll.insert_many_with_session(
        vec![doc! { "x": 1 }, doc! { "x": 2 }, doc! { "x": 3 }],
        None,
        &mut session,
    )
    .await
    .unwrap();

    let mut cursor: crate::SessionCursor<bson::Document> = coll
        .find_with_session(doc! {}, None, &mut session)
        .await
        .unwrap();

    let _ = cursor.next(&mut session).await.unwrap().unwrap();

    let mut cursor_with_type: crate::SessionCursor<bson::RawDocumentBuf> = cursor.with_type();

    let _ = cursor_with_type.next(&mut session).await.unwrap().unwrap();
}

Would you mind adding a test to verify the changes here fix this issue (feel free to paste in the one above)? (EDIT: the test can go in src/test/cursor.rs.)

Otherwise I just have one small comment!

[mlokr]

@isabelatkinson I added the test and made other requested changes, the newly added test passes for me. Thanks for providing it!

[isabelatkinson]

Hey @mlokr, I rebased this onto main and fixed a small clippy failure. The changes look great! I am going to tag in the other member of the team for review; once he approves we'll merge this in and put out a patch release. Thanks again for bringing this bug to our attention!

[abr-egn]

LGTM!