test blockExec does not panic if all vals removed (#3241)

Fix for #3224
Also address #2084

blockchain/reactor.go

@@ -8,7 +8,6 @@ import (
 
 	amino "github.com/tendermint/go-amino"
 
-	cmn "github.com/tendermint/tendermint/libs/common"
 	"github.com/tendermint/tendermint/libs/log"
 	"github.com/tendermint/tendermint/p2p"
 	sm "github.com/tendermint/tendermint/state"
@@ -338,8 +337,7 @@ FOR_LOOP:
 				state, err = bcR.blockExec.ApplyBlock(state, firstID, first)
 				if err != nil {
 					// TODO This is bad, are we zombie?
-					cmn.PanicQ(fmt.Sprintf("Failed to process committed block (%d:%X): %v",
-						first.Height, first.Hash(), err))
+					panic(fmt.Sprintf("Failed to process committed block (%d:%X): %v", first.Height, first.Hash(), err))
 				}
 				blocksSynced++
 

state/execution_test.go

@@ -354,6 +354,33 @@ func TestEndBlockValidatorUpdates(t *testing.T) {
 	}
 }
 
+// TestEndBlockValidatorUpdatesResultingInEmptySet checks that processing validator updates that
+// would result in empty set causes no panic, an error is raised and NextValidators is not updated
+func TestEndBlockValidatorUpdatesResultingInEmptySet(t *testing.T) {
+	app := &testApp{}
+	cc := proxy.NewLocalClientCreator(app)
+	proxyApp := proxy.NewAppConns(cc)
+	err := proxyApp.Start()
+	require.Nil(t, err)
+	defer proxyApp.Stop()
+
+	state, stateDB := state(1, 1)
+	blockExec := NewBlockExecutor(stateDB, log.TestingLogger(), proxyApp.Consensus(), MockMempool{}, MockEvidencePool{})
+
+	block := makeBlock(state, 1)
+	blockID := types.BlockID{block.Hash(), block.MakePartSet(testPartSize).Header()}
+
+	// Remove the only validator
+	app.ValidatorUpdates = []abci.ValidatorUpdate{
+		{PubKey: types.TM2PB.PubKey(state.Validators.Validators[0].PubKey), Power: 0},
+	}
+
+	assert.NotPanics(t, func() { state, err = blockExec.ApplyBlock(state, blockID, block) })
+	assert.NotNil(t, err)
+	assert.NotEmpty(t, state.NextValidators.Validators)
+
+}
+
 //----------------------------------------------------------------------------
 
 // make some bogus txs