blockchain: verify +2/3 (#5278)

## Description

Verify only +2/3 of the commit. 

Closes: #5259

CHANGELOG_PENDING.md

@@ -10,10 +10,12 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 
 ## FEATURES
 
-- [privval] \#5239 Add `chainID` to requests from client.
+- [privval] \#5239 Add `chainID` to requests from client. (@marbar3778)
 
 ## IMPROVEMENTS
 
+- [blockchain] \#5278 Verify only +2/3 of the signatures in a block when fast syncing. (@marbar3778)
+
 ## BUG FIXES
 
 - [blockchain] \#5249 Fix fast sync halt with initial height > 1 (@erikgrinaker)

blockchain/v0/reactor.go

@@ -358,7 +358,7 @@ FOR_LOOP:
 			// NOTE: we can probably make this more efficient, but note that calling
 			// first.Hash() doesn't verify the tx contents, so MakePartSet() is
 			// currently necessary.
-			err := state.Validators.VerifyCommit(
+			err := state.Validators.VerifyCommitLight(
 				chainID, firstID, first.Height, second.LastCommit)
 			if err != nil {
 				bcR.Logger.Error("Error in validation", "err", err)

blockchain/v1/reactor.go

@@ -463,7 +463,7 @@ func (bcR *BlockchainReactor) processBlock() error {
 	// NOTE: we can probably make this more efficient, but note that calling
 	// first.Hash() doesn't verify the tx contents, so MakePartSet() is
 	// currently necessary.
-	err = bcR.state.Validators.VerifyCommit(chainID, firstID, first.Height, second.LastCommit)
+	err = bcR.state.Validators.VerifyCommitLight(chainID, firstID, first.Height, second.LastCommit)
 	if err != nil {
 		bcR.Logger.Error("error during commit verification", "err", err,
 			"first", first.Height, "second", second.Height)

blockchain/v2/processor_context.go

@@ -44,7 +44,7 @@ func (pc *pContext) setState(state state.State) {
 }
 
 func (pc pContext) verifyCommit(chainID string, blockID types.BlockID, height int64, commit *types.Commit) error {
-	return pc.state.Validators.VerifyCommit(chainID, blockID, height, commit)
+	return pc.state.Validators.VerifyCommitLight(chainID, blockID, height, commit)
 }
 
 func (pc *pContext) saveBlock(block *types.Block, blockParts *types.PartSet, seenCommit *types.Commit) {