Module on_panic 

The guard that undoes what an unwinding callback would otherwise leave behind.

Everything an operator keeps on its stack — a subscription, an observer, a queued value — is released by the unwind itself, so it needs no guard. What the unwind does not undo is a change already written to a shared state that outlives the panicking call: a delivery left in its delivering state, a subscriber counted into a ref count, a source left subscribed after its termination. Those states are reached by writing them before the callback and finishing the transaction after it, which is exactly the step a panic skips.

OnPanic runs that finishing step on the unwinding thread instead. Its action therefore runs under the constraints of a Drop during a panic:

• It must not panic. A panic while panicking aborts the process. It may drop values, and run the ordinary disposals that dropping them entails, but it should not call further into code that is free to unwind.
• It must not take a lock the panicking thread already holds, which in this crate means it is only ever wrapped around callbacks that run with no lock held — observer notifications and external subscriptions. A guard that has to lock is safe exactly where the returning path locks too.

The guard is armed for the scope it is bound to, so it must be bound: let _guard = …, and drop(guard) where the scope ends before the enclosing block. When the returning path needs something back from the guard, park it in the guard’s state and take it with OnPanic::disarm, which ends the scope and hands the state over.

Structs

OnPanic

Runs action with the guarded state if the current scope unwinds, and nothing otherwise.

Functions

on_panic

Runs action if the current scope unwinds, for a guard that carries no state.