consensus: correctly save reference to previous height precommits (ba…

…ckport #9760) (#9776) (#911)

* 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

Co-authored-by: William Banfield <[email protected]>

CHANGELOG_PENDING.md

@@ -20,4 +20,6 @@
 
 ### IMPROVEMENTS
 
+- [consensus] \#9760 Save peer LastCommit correctly to achieve 50% reduction in gossiped precommits. (@williambanfield)
+
 ### BUG FIXES

consensus/reactor.go

@@ -1375,6 +1375,7 @@ 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
@@ -1402,7 +1403,7 @@ 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