voter: Save the context waker of precommited voting rounds

[lexnv]

A best round is completed when the following conditions are met:

• The best round future is completed (ie inner.best_round.poll(cx) returns Poll::Ready(()))
• The state of the best round state advances to Precommited (from Prevoting)

When one of the previous conditions is not met, the process_best_round function returns Poll::Pending without storing the waker context.

The grandpa code relies on the first context wakeup of either:

• incoming events via the global input stream (ie process_incoming)
• pruning background rounds (either a past round generated a commit, or we receive a finalization notification)
• global output wakes the waker
  

  finality-grandpa/src/voter/mod.rs

  Lines 854 to 860 in 321973e

  	fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Result<(), E::Error>> {
  	self.process_incoming(cx)?;
  	self.prune_background_rounds(cx)?;
  	let _ = self.global_out.poll(cx)?;
  	self.process_best_round(cx)
  	}

This PR ensures we save the waker and wake it up properly on state advances (instead of relying on the above components and possibly causing a delay in starting the next round).

Notes

We are also seeing a high number of grandpa debug los:

2025-06-06 20:10:51.909 DEBUG tokio-runtime-worker grandpa: Completed round 50, state = State { prevote_ghost: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), finalized: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), estimate: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), completable: true }, step = Some(Prevoting)  
	
2025-06-06 20:10:51.909 DEBUG tokio-runtime-worker grandpa: Completed round 50, state = State { prevote_ghost: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), finalized: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), estimate: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), completable: true }, step = Some(Prevoting)    

2025-06-06 20:10:51.914 DEBUG tokio-runtime-worker grandpa: Completed round 50, state = State { prevote_ghost: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), finalized: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), estimate: Some((0xa82f134d09119028e9b5c8f1846a0440d66f284292cd57ebfba0bfaf31e44a54, 4469)), completable: true }, step = Some(Prevoting)    

The previous logs are repeated ~3.4k times during one second during a finality stall, suggesting that some other component (mentioned above) was able to make progress, but not yet the best voting round.

I would leave this PR open for a while, in case we choose to revisit it at a later time and perform in-depth testing. (May we ever want to refactor the code to align with #124 this might come in handy)