TaskHandle::is_finished == true does not mean that TaskHandle is Poll::Ready

[tillrohrmann]

During a verification run (https://github.com/restatedev/e2e-verification-runner/actions/runs/13742949412/job/38434376749#step:5:3082), a Restate server panicked here

restate/crates/metadata-server/src/raft/network/networking.rs

Line 110 in 41e606e

match task_handle.now_or_never().expect("should be finished") {

. It seems that the assumption that if TaskHandle::is_finished() == true, then the TaskHandle is Poll::Ready, does not hold true.

What's the exact state of the system at the time of panicking, is unclear from the logs.

[tillrohrmann]

TaskHandle::is_finished is indeed not sufficient for task_handle.now_or_never() to always succeed. The reason is that a tokio::JoinHandle participates in the cooperative task scheduling budget and polls the coop budget before accessing the task's result. If the budget was depleted, then the JoinHandle would return Poll::Pending even if the corresponding task has completed.