From 8e29a2ca40e13614055f6900e379296c9892d6c1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Agust=C3=ADn=20Rodriguez?= <agus@moonsonglabs.com>
Date: Wed, 13 Mar 2024 19:00:12 -0300
Subject: [PATCH] Limit block production to one block per slot in basic
 collator (#456)

---
 client/consensus/src/collators.rs       |  7 +++++++
 client/consensus/src/collators/basic.rs | 15 +++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/client/consensus/src/collators.rs b/client/consensus/src/collators.rs
index ebec38d5e..89ddc88a2 100644
--- a/client/consensus/src/collators.rs
+++ b/client/consensus/src/collators.rs
@@ -264,6 +264,7 @@ where
 pub struct SlotClaim<Pub> {
     author_pub: Pub,
     pre_digest: Vec<DigestItem>,
+    slot: Slot,
 }
 
 impl<Pub: Clone> SlotClaim<Pub> {
@@ -276,6 +277,7 @@ impl<Pub: Clone> SlotClaim<Pub> {
         SlotClaim {
             author_pub: author_pub.clone(),
             pre_digest: pre_digest_data::<P>(slot, author_pub),
+            slot,
         }
     }
 
@@ -288,6 +290,11 @@ impl<Pub: Clone> SlotClaim<Pub> {
     pub fn pre_digest(&self) -> &Vec<DigestItem> {
         &self.pre_digest
     }
+
+    /// Get the slot assigned to this claim.
+    pub fn slot(&self) -> Slot {
+        self.slot
+    }
 }
 
 /// Attempt to claim a slot locally.
diff --git a/client/consensus/src/collators/basic.rs b/client/consensus/src/collators/basic.rs
index 30104011a..92e0d6a69 100644
--- a/client/consensus/src/collators/basic.rs
+++ b/client/consensus/src/collators/basic.rs
@@ -130,6 +130,8 @@ pub async fn run<Block, P, BI, CIDP, Client, RClient, SO, Proposer, CS, GOH>(
         collator_util::Collator::<Block, P, _, _, _, _, _>::new(params)
     };
 
+    let mut last_processed_slot = 0;
+
     while let Some(request) = collation_requests.next().await {
         macro_rules! reject_with_error {
 				($err:expr) => {{
@@ -211,6 +213,18 @@ pub async fn run<Block, P, BI, CIDP, Client, RClient, SO, Proposer, CS, GOH>(
             Ok(Some(h)) => h,
         };
 
+        // With async backing this function will be called every relay chain block.
+        //
+        // Most parachains currently run with 12 seconds slots and thus, they would try to
+        // produce multiple blocks per slot which very likely would fail on chain. Thus, we have
+        // this "hack" to only produce on block per slot.
+        //
+        // With https://github.com/paritytech/polkadot-sdk/issues/3168 this implementation will be
+        // obsolete and also the underlying issue will be fixed.
+        if last_processed_slot >= *claim.slot() {
+            continue;
+        }
+
         let (parachain_inherent_data, other_inherent_data) = try_request!(
             collator
                 .create_inherent_data(*request.relay_parent(), validation_data, parent_hash, None,)
@@ -244,5 +258,6 @@ pub async fn run<Block, P, BI, CIDP, Client, RClient, SO, Proposer, CS, GOH>(
             request.complete(None);
             tracing::debug!(target: crate::LOG_TARGET, "No block proposal");
         }
+        last_processed_slot = *claim.slot();
     }
 }