client/finality-grandpa/src/until_imported: Refactor BlockGlobalMessage (#5390)

* client/finality-grandpa/src/until_imported: Refactor BlockGlobalMessage

`BlockGlobalMessage` owns an `inner` which contains (1) a count for the
amount of outstanding blocks to be waited on and (2) the message itself.

Given that both is already wrapped in an `Arc` there is no need to keep
track of the outstanding blocks, given that it simply corresponds to the
amount of strong reference counts on the `Arc` itself.

This commit removes the atomic counter within `inner` and piggy backs on
the `Arc` reference counter instead.

* client/finality-grandpa/src/until_imported: Remove useless match

* client/finality-grandpa/src/until_imported.rs: Remove unused var

Co-Authored-By: André Silva <andre.beat@gmail.com>

* client/finality-grandpa/src/until_imported: Address comment suggestion

Co-authored-by: André Silva <andre.beat@gmail.com>

substrate/client/finality-grandpa/src/until_imported.rs

@@ -40,7 +40,7 @@ use sp_runtime::traits::{Block as BlockT, Header as HeaderT, NumberFor};
 
 use std::collections::{HashMap, VecDeque};
 use std::pin::Pin;
-use std::sync::{atomic::{AtomicUsize, Ordering}, Arc};
+use std::sync::Arc;
 use std::task::{Context, Poll};
 use std::time::{Duration, Instant};
 use sp_finality_grandpa::AuthorityId;
@@ -307,8 +307,12 @@ pub(crate) type UntilVoteTargetImported<Block, BlockStatus, BlockSyncRequester,
 ///
 /// This is used for compact commits and catch up messages which have already
 /// been checked for structural soundness (e.g. valid signatures).
+///
+/// We use the `Arc`'s reference count to implicitly count the number of outstanding blocks that we
+/// are waiting on for the same message (i.e. other `BlockGlobalMessage` instances with the same
+/// `inner`).
 pub(crate) struct BlockGlobalMessage<Block: BlockT> {
-	inner: Arc<(AtomicUsize, Mutex<Option<CommunicationIn<Block>>>)>,
+	inner: Arc<Mutex<Option<CommunicationIn<Block>>>>,
 	target_number: NumberFor<Block>,
 }
 
@@ -416,7 +420,7 @@ impl<Block: BlockT> BlockUntilImported<Block> for BlockGlobalMessage<Block> {
 			return Ok(())
 		}
 
-		let locked_global = Arc::new((AtomicUsize::new(unknown_count), Mutex::new(Some(input))));
+		let locked_global = Arc::new(Mutex::new(Some(input)));
 
 		// schedule waits for all unknown messages.
 		// when the last one of these has `wait_completed` called on it,
@@ -438,30 +442,20 @@ impl<Block: BlockT> BlockUntilImported<Block> for BlockGlobalMessage<Block> {
 
 	fn wait_completed(self, canon_number: NumberFor<Block>) -> Option<Self::Blocked> {
 		if self.target_number != canon_number {
-			// if we return without deducting the counter, then none of the other
-			// handles can return the commit message.
+			// Delete the inner message so it won't ever be forwarded. Future calls to
+			// `wait_completed` on the same `inner` will ignore it.
+			*self.inner.lock() = None;
 			return None;
 		}
 
-		let mut last_count = self.inner.0.load(Ordering::Acquire);
-
-		// CAS loop to ensure that we always have a last reader.
-		loop {
-			if last_count == 1 { // we are the last one left.
-				return self.inner.1.lock().take();
-			}
-
-			let prev_value = self.inner.0.compare_and_swap(
-				last_count,
-				last_count - 1,
-				Ordering::SeqCst,
-			);
-
-			if prev_value == last_count {
-				return None;
-			} else {
-				last_count = prev_value;
-			}
+		match Arc::try_unwrap(self.inner) {
+			// This is the last reference and thus the last outstanding block to be awaited. `inner`
+			// is either `Some(_)` or `None`. The latter implies that a previous `wait_completed`
+			// call witnessed a block number mismatch (see above).
+			Ok(inner) => Mutex::into_inner(inner),
+			// There are still other strong references to this `Arc`, thus the message is blocked on
+			// other blocks to be imported.
+			Err(_) => None,
 		}
 	}
 }
@@ -941,4 +935,67 @@ mod tests {
 
 		futures::executor::block_on(test);
 	}
+
+	fn test_catch_up() -> Arc<Mutex<Option<CommunicationIn<Block>>>> {
+		let header = make_header(5);
+
+		let unknown_catch_up = finality_grandpa::CatchUp {
+			round_number: 1,
+			precommits: vec![],
+			prevotes: vec![],
+			base_hash: header.hash(),
+			base_number: *header.number(),
+		};
+
+		let catch_up = voter::CommunicationIn::CatchUp(
+			unknown_catch_up.clone(),
+			voter::Callback::Blank,
+		);
+
+		Arc::new(Mutex::new(Some(catch_up)))
+	}
+
+	#[test]
+	fn block_global_message_wait_completed_return_when_all_awaited() {
+		let msg_inner = test_catch_up();
+
+		let waiting_block_1 = BlockGlobalMessage::<Block> {
+			inner: msg_inner.clone(),
+			target_number: 1,
+		};
+
+		let waiting_block_2 = BlockGlobalMessage::<Block> {
+			inner: msg_inner,
+			target_number: 2,
+		};
+
+		// waiting_block_2 is still waiting for block 2, thus this should return `None`.
+		assert!(waiting_block_1.wait_completed(1).is_none());
+
+		// Message only depended on block 1 and 2. Both have been imported, thus this should yield
+		// the message.
+		assert!(waiting_block_2.wait_completed(2).is_some());
+	}
+
+	#[test]
+	fn block_global_message_wait_completed_return_none_on_block_number_missmatch() {
+		let msg_inner = test_catch_up();
+
+		let waiting_block_1 = BlockGlobalMessage::<Block> {
+			inner: msg_inner.clone(),
+			target_number: 1,
+		};
+
+		let waiting_block_2 = BlockGlobalMessage::<Block> {
+			inner: msg_inner,
+			target_number: 2,
+		};
+
+		// Calling wait_completed with wrong block number should yield None.
+		assert!(waiting_block_1.wait_completed(1234).is_none());
+
+		// All blocks, that the message depended on, have been imported. Still, given the above
+		// block number mismatch this should return None.
+		assert!(waiting_block_2.wait_completed(2).is_none());
+	}
 }