backend: Use master

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

subxt/src/backend/unstable/mod.rs

@@ -437,13 +437,28 @@ impl<T: Config + Send + Sync + 'static> Backend<T> for UnstableBackend<T> {
         extrinsic: &[u8],
     ) -> Result<StreamOfResults<TransactionStatus<T::Hash>>, Error> {
         // We care about new and finalized block hashes.
+        enum SeenBlock<Ref> {
+            New(Ref),
+            Finalized(Vec<Ref>),
+        }
         enum SeenBlockMarker {
             New,
             Finalized,
         }
 
         // First, subscribe to all new and finalized block refs.
-        let mut seen_blocks_sub = self.follow_handle.subscribe().events();
+        // - we subscribe to new refs so that when we see `BestChainBlockIncluded`, we
+        //   can try to return a block ref for the best block.
+        // - we subscribe to finalized refs so that when we see `Finalized`, we can
+        //   guarantee that when we return here, the finalized block we report has been
+        //   reported from chainHead_follow already.
+        let mut seen_blocks_sub = self.follow_handle.subscribe().events().filter_map(|ev| {
+            std::future::ready(match ev {
+                FollowEvent::NewBlock(ev) => Some(SeenBlock::New(ev.block_hash)),
+                FollowEvent::Finalized(ev) => Some(SeenBlock::Finalized(ev.finalized_block_hashes)),
+                _ => None,
+            })
+        });
 
         // Then, submit the transaction.
         let mut tx_progress = self
@@ -470,22 +485,23 @@ impl<T: Config + Send + Sync + 'static> Backend<T> for UnstableBackend<T> {
                 // Make a note of new or finalized blocks that have come in since we started the TX.
                 if let Poll::Ready(Some(seen_block)) = seen_blocks_sub.poll_next_unpin(cx) {
                     match seen_block {
-                        FollowEvent::NewBlock(new_block) => {
-                            seen_blocks.insert(
-                                new_block.block_hash.hash(),
-                                (SeenBlockMarker::New, new_block.block_hash),
-                            );
+                        SeenBlock::New(block_ref) => {
+                            // Optimization: once we have a `finalized_hash`, we only care about finalized
+                            // block refs now and can avoid bothering to save new blocks.
+                            if finalized_hash.is_none() {
+                                seen_blocks
+                                    .insert(block_ref.hash(), (SeenBlockMarker::New, block_ref));
+                            }
                         }
-                        FollowEvent::Finalized(finalized_block) => {
-                            for block_ref in finalized_block.finalized_block_hashes {
+                        SeenBlock::Finalized(block_refs) => {
+                            for block_ref in block_refs {
                                 seen_blocks.insert(
                                     block_ref.hash(),
                                     (SeenBlockMarker::Finalized, block_ref),
                                 );
                             }
                         }
-                        _ => (),
-                    };
+                    }
                     continue;
                 }