commit empty blocks when needed to prove app hash

consensus/mempool_test.go

@@ -15,7 +15,7 @@ func init() {
 	config = ResetConfig("consensus_mempool_test")
 }
 
-func TestTxsAvailable(t *testing.T) {
+func TestNoProgressUntilTxsAvailable(t *testing.T) {
 	config := ResetConfig("consensus_mempool_txs_available_test")
 	config.Consensus.NoEmptyBlocks = true
 	state, privVals := randGenesisState(1, false, 10)
@@ -25,10 +25,12 @@ func TestTxsAvailable(t *testing.T) {
 	newBlockCh := subscribeToEvent(cs.evsw, "tester", types.EventStringNewBlock(), 1)
 	startTestRound(cs, height, round)
 
-	// we shouldnt make progress until theres a tx
+	ensureNewStep(newBlockCh) // first block gets committed
+	ensureNoNewStep(newBlockCh)
+	deliverTxsRange(cs, 0, 2)
+	ensureNewStep(newBlockCh) // commit txs
+	ensureNewStep(newBlockCh) // commit updated app hash
 	ensureNoNewStep(newBlockCh)
-	deliverTxsRange(cs, 0, 100)
-	ensureNewStep(newBlockCh)
 }
 
 func deliverTxsRange(cs *ConsensusState, start, end int) {

consensus/state.go

@@ -779,14 +779,29 @@ func (cs *ConsensusState) enterNewRound(height int, round int) {
 	types.FireEventNewRound(cs.evsw, cs.RoundStateEvent())
 
 	// Wait for txs to be available in the mempool
-	// before we enterPropose
-	if cs.config.NoEmptyBlocks {
+	// before we enterPropose. If the last block changed the app hash,
+	// we may need an empty "proof" block, and enterPropose immediately.
+	if cs.config.NoEmptyBlocks && !cs.needProofBlock(height) {
 		go cs.waitForTxs(height, round)
 	} else {
 		cs.enterPropose(height, round)
 	}
 }
 
+// needProofBlock returns true on the first height (so the genesis app hash is signed right away)
+// and where the last block (height-1) caused the app hash to change
+func (cs *ConsensusState) needProofBlock(height int) bool {
+	if height == 1 {
+		return true
+	}
+
+	lastBlockMeta := cs.blockStore.LoadBlockMeta(height - 1)
+	if !bytes.Equal(cs.state.AppHash, lastBlockMeta.Header.AppHash) {
+		return true
+	}
+	return false
+}
+
 func (cs *ConsensusState) waitForTxs(height, round int) {
 	// if we're the proposer, start a heartbeat routine
 	// to tell other peers we're just waiting for txs (for debugging)