From da204d371dfcedcf28fa7eff9757c707a183faf9 Mon Sep 17 00:00:00 2001
From: William Banfield <4561443+williambanfield@users.noreply.github.com>
Date: Mon, 28 Nov 2022 15:13:14 -0500
Subject: [PATCH] consensus: correctly save reference to previous height
 precommits (#9760)

This change reduces the number of Precommit messages sent to peers by 50%.

During the `ApplyNewRoundStepMessage`, we update the known state of the peer sending us the message.

We set the value of `ps.PRS.Precommits` to nil in this method if the peer is entering a new height or round.
https://github.com/tendermint/tendermint/blob/34ca3fb4741cdb205a4714d345218a0d5e9fefd0/consensus/reactor.go#L1368

We then assign `ps.PRS.LastCommit = ps.PRS.Precommits` if the peer is entering a new height only - this does not happen during just a round change. This therefore results in `ps.PRS.LastCommit` having the value `nil`.

When the `LastCommit` bit field is seen as `nil` in the reactor, an empty bit field is initialized.
https://github.com/tendermint/tendermint/blob/34ca3fb4741cdb205a4714d345218a0d5e9fefd0/consensus/reactor.go#L1273

The code responsible for gossiping votes from the previous height uses this `LastCommit` value and, seeing an empty `LastCommit` will resend every `Precommit` from the previous height since it lost the information it previously had detailing which precommits from the previous height the peer had.

This can be seen in the code responsible for gossiping precommits from the previous height:
https://github.com/tendermint/tendermint/blob/34ca3fb4741cdb205a4714d345218a0d5e9fefd0/consensus/reactor.go#L773

Where this code grabs the, previously `nil`, `LastCommit` bit field:
https://github.com/tendermint/tendermint/blob/34ca3fb4741cdb205a4714d345218a0d5e9fefd0/consensus/reactor.go#L1204-L1212

---

#### PR checklist

- [ ] Tests written/updated, or no tests needed
- [ ] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [ ] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed
---
 CHANGELOG_PENDING.md | 1 +
 consensus/reactor.go | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index 59b303e2e..25c97aff7 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -34,6 +34,7 @@
 - [p2p/pex] \#6509 Improve addrBook.hash performance (@cuonglm)
 - [crypto/merkle] \#6443 & \#6513 Improve HashAlternatives performance (@cuonglm, @marbar3778)
 - [rpc] \#9650 Enable caching of RPC responses (@JayT106)
+- [consensus] \#9760 Save peer LastCommit correctly to achieve 50% reduction in gossiped precommits. (@williambanfield)
 
 ### BUG FIXES
 
diff --git a/consensus/reactor.go b/consensus/reactor.go
index a8d672e44..6a6d95f38 100644
--- a/consensus/reactor.go
+++ b/consensus/reactor.go
@@ -1351,6 +1351,7 @@ func (ps *PeerState) ApplyNewRoundStepMessage(msg *NewRoundStepMessage) {
 	psRound := ps.PRS.Round
 	psCatchupCommitRound := ps.PRS.CatchupCommitRound
 	psCatchupCommit := ps.PRS.CatchupCommit
+	lastPrecommits := ps.PRS.Precommits
 
 	startTime := tmtime.Now().Add(-1 * time.Duration(msg.SecondsSinceStartTime) * time.Second)
 	ps.PRS.Height = msg.Height
@@ -1378,7 +1379,7 @@ func (ps *PeerState) ApplyNewRoundStepMessage(msg *NewRoundStepMessage) {
 		// Shift Precommits to LastCommit.
 		if psHeight+1 == msg.Height && psRound == msg.LastCommitRound {
 			ps.PRS.LastCommitRound = msg.LastCommitRound
-			ps.PRS.LastCommit = ps.PRS.Precommits
+			ps.PRS.LastCommit = lastPrecommits
 		} else {
 			ps.PRS.LastCommitRound = msg.LastCommitRound
 			ps.PRS.LastCommit = nil