Prune blocks in consensus engine (#275)

* Add prune older blocks call

* Prune older blocks in simulation app

* Use latest safe view for failure case

* Pick parent view from high qc

* Add safety assertion on pruning method

consensus-engine/src/lib.rs

@@ -220,7 +220,7 @@ impl<O: Overlay> Carnot<O> {
         )
     }
 
-    /// Upon a configurable amout of time has elapsed since the last view change
+    /// Upon a configurable amount of time has elapsed since the last view change
     ///
     /// Preconditions: none!
     /// Just notice that the timer only reset after a view change, i.e. a node can't timeout
@@ -396,6 +396,12 @@ impl<O: Overlay> Carnot<O> {
             Err(e) => Err(e),
         }
     }
+
+    /// Blocks newer than the last committed block are not safe to be pruned
+    pub fn prune_older_blocks_by_view(&mut self, threshold_view: View) {
+        assert!(threshold_view < self.latest_committed_block().view);
+        self.safe_blocks.retain(|_, b| b.view < threshold_view);
+    }
 }
 
 #[cfg(test)]

simulations/src/node/carnot/mod.rs

@@ -389,8 +389,20 @@ impl<L: UpdateableLeaderSelection, O: Overlay<LeaderSelection = L>> CarnotNode<O
                     parent_block=%block.parent(),
                     "receive approve message"
                 );
-                let (new, out) = self.engine.approve_block(block);
+                let block_grandparent_view = match &block.parent_qc {
+                    Qc::Standard(qc) => qc.view,
+                    Qc::Aggregated(qc) => {
+                        self.engine
+                            .safe_blocks()
+                            .get(&qc.high_qc.id)
+                            .expect("Parent block must be present")
+                            .view
+                    }
+                } - View::new(3);
+                let (mut new, out) = self.engine.approve_block(block);
                 tracing::info!(vote=?out, node=%self.id);
+                // pruning old blocks older than the grandparent block needed to check validity
+                new.prune_older_blocks_by_view(block_grandparent_view);
                 output = Some(Output::Send(out));
                 self.engine = new;
             }