fork can only be called on the fiber: it can't be called externally. So if it's called at all, the fiber must be running. It will keep running until after fork is called and the main interpretation loop continues.

Are you sure we're hitting this case?

We're definitely hitting that case in the example from the issue. I added a debug statement for that, published a local version of ZIO, depended on that in the reproducer from the original issue, and verified that we were hitting that case.

Correct me if I'm wrong, but if we are using an Executor with an underlying thread pool with multiple threads, couldn't each of them be executing the main interpretation loop at the same time for different fibers? So say first thread is interpreting fiber A and begins executing fork with scope X. At the same time a second thread is executing fiber B, has a reference to scope X and closes it. First thread continues its interpretation of fork and tries to add the finalizer to the scope, which is now closed.

Correct me if I'm wrong, but if we are using an Executor with an underlying thread pool with multiple threads, couldn't each of them be executing the main interpretation loop at the same time for different fibers?

No. The main interpretation loop for a given FiberContext (evaluateNow) will only ever be executed by one thread. Within the life of a single fiber, it's executor is "single threaded". The fork operation should only be called from the fiber itself, as it interprets Fork and RaceWith instructions. If that's not happening, if somehow evaluateNow is being executed concurrently, well that would be a serious error which would lead to not only this problem but many others (including random class cast exception errors and stack errors as two threads concurrently pull continuations from the fiber's single stack).

Maybe there is some bug around termination conditions which is causing a fiber already being executed to do some fork, but if that's the case, we should fix that bug rather than work around it here.

Right, but two different threads could be executing the interpretation loops for two different fibers. And one fiber could have a reference to the scope of the other fiber since there are all these ways to pass around scopes now. So the scope that one fiber is acting on could be closed part way through.