Switch PersistenceNotifierGuard persist closure to FnOnce #3835
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| @@ -2815,10 +2815,10 @@ enum NotifyOption { | |||
| /// We allow callers to either always notify by constructing with `notify_on_drop` or choose to | |||
| /// notify or not based on whether relevant changes have been made, providing a closure to | |||
| /// `optionally_notify` which returns a `NotifyOption`. | |||
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Background question: it seems that in some instances, the persist_check isn't just checking, but also making the actual changes to the chan mgr state. Is that fine, or indicative of the PersistenceNotifierGuard not fully matching its desired use cases?
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In almost all cases it is making actual changes to the internal state, that's how we have the context to know whether we should persist or not via the returned NotifyOption
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You'd almost think that the guard itself isn't needed then anymore? I don't have the overview of all uses though.
Would an alternative (not in scope of this PR obviously) be to obtain the guard and then also get some kind of function to set the internal state to needs_persist?
| @@ -2833,20 +2833,20 @@ impl<'a> PersistenceNotifierGuard<'a, fn() -> NotifyOption> { | |||
| /// isn't ideal. | |||
| fn notify_on_drop<C: AChannelManager>( | |||
| cm: &'a C, | |||
| ) -> PersistenceNotifierGuard<'a, impl FnMut() -> NotifyOption> { | |||
| ) -> PersistenceNotifierGuard<'a, impl FnOnce() -> NotifyOption> { | |||
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So this means that only single-use closure may be provided now. And it seems we are lucky that nowhere a closure was reused, so no other refactoring needed? (questions for me to understand this better)
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Correct, and there should never be a closure reuse given the usage pattern
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No reuse inside PersistenceNotifierGuard obviously, but I wondered whether there could be a reason to reuse the closure higher up the call stack? (still bg q this, and learning rust)
| fn drop(&mut self) { | ||
| match (self.should_persist)() { | ||
| match (self.should_persist.take().unwrap())() { |
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We should never reach this, but how about avoiding the unwrap and just matching on Some(..)/None?
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Isn't it basically the same? unreachable would also panic in the None case
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Well, we could also have it trigger a debug_assert, no? But if you prefer the unwrap, could you add a
// unwrap safety: ..comment?
This allows us to move owned and mutable values into the closure, with the assumption that the closure can only be invoked once. As a result, we now have to track `should_persist` as an `Option`, such that we can `take` the owned closure and invoke it within the `PersistenceNotifierGuard::drop` implementation.
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| fn drop(&mut self) { | ||
| match (self.should_persist)() { | ||
| match (self.should_persist.take().unwrap())() { |
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Well, we could also have it trigger a debug_assert, no? But if you prefer the unwrap, could you add a
// unwrap safety: ..comment?
This allows us to move owned and mutable values into the closure, with the assumption that the closure can only be invoked once. As a result, we now have to track
should_persistas anOption, such that we cantakethe owned closure and invoke it within thePersistenceNotifierGuard::dropimplementation.Motivated by #3736 (comment).