go/consensus: Expose read-only state via light client interface

Nodes configured as consensus RPC services workers now expose read-only
access to consensus state via the usual MKVS ReadSyncer interface, allowing
light clients to remotely query state while transparently verifying proofs.

.changelog/3077.feature.md

@@ -0,0 +1,5 @@
+go/consensus: Expose read-only state via light client interface
+
+Nodes configured as consensus RPC services workers now expose read-only
+access to consensus state via the usual MKVS ReadSyncer interface, allowing
+light clients to remotely query state while transparently verifying proofs.

go/consensus/api/api.go

@@ -20,6 +20,7 @@ import (
 	roothash "github.com/oasisprotocol/oasis-core/go/roothash/api"
 	scheduler "github.com/oasisprotocol/oasis-core/go/scheduler/api"
 	staking "github.com/oasisprotocol/oasis-core/go/staking/api"
+	mkvsNode "github.com/oasisprotocol/oasis-core/go/storage/mkvs/node"
 )
 
 const (
@@ -99,6 +100,8 @@ type Block struct {
 	Hash []byte `json:"hash"`
 	// Time is the second-granular consensus time.
 	Time time.Time `json:"time"`
+	// StateRoot is the Merkle root of the consensus state tree.
+	StateRoot mkvsNode.Root `json:"state_root"`
 	// Meta contains the consensus backend specific block metadata.
 	Meta cbor.RawMessage `json:"meta"`
 }
@@ -119,6 +122,8 @@ type Status struct {
 	LatestHash []byte `json:"latest_hash"`
 	// LatestTime is the timestamp of the latest block.
 	LatestTime time.Time `json:"latest_time"`
+	// LatestStateRoot is the Merkle root of the consensus state tree.
+	LatestStateRoot mkvsNode.Root `json:"latest_state_root"`
 
 	// GenesisHeight is the height of the genesis block.
 	GenesisHeight int64 `json:"genesis_height"`

go/consensus/api/grpc.go

@@ -10,6 +10,7 @@ import (
 	"github.com/oasisprotocol/oasis-core/go/consensus/api/transaction"
 	epochtime "github.com/oasisprotocol/oasis-core/go/epochtime/api"
 	genesis "github.com/oasisprotocol/oasis-core/go/genesis/api"
+	"github.com/oasisprotocol/oasis-core/go/storage/mkvs/syncer"
 )
 
 var (
@@ -48,6 +49,12 @@ var (
 	methodGetValidatorSet = lightServiceName.NewMethod("GetValidatorSet", int64(0))
 	// methodGetParameters is the GetParameters method.
 	methodGetParameters = lightServiceName.NewMethod("GetParameters", int64(0))
+	// methodStateSyncGet is the StateSyncGet method.
+	methodStateSyncGet = lightServiceName.NewMethod("StateSyncGet", syncer.GetRequest{})
+	// methodStateSyncGetPrefixes is the StateSyncGetPrefixes method.
+	methodStateSyncGetPrefixes = lightServiceName.NewMethod("StateSyncGetPrefixes", syncer.GetPrefixesRequest{})
+	// methodStateSyncIterate is the StateSyncIterate method.
+	methodStateSyncIterate = lightServiceName.NewMethod("StateSyncIterate", syncer.IterateRequest{})
 
 	// serviceDesc is the gRPC service descriptor.
 	serviceDesc = grpc.ServiceDesc{
@@ -121,6 +128,18 @@ var (
 				MethodName: methodGetParameters.ShortName(),
 				Handler:    handlerGetParameters,
 			},
+			{
+				MethodName: methodStateSyncGet.ShortName(),
+				Handler:    handlerStateSyncGet,
+			},
+			{
+				MethodName: methodStateSyncGetPrefixes.ShortName(),
+				Handler:    handlerStateSyncGetPrefixes,
+			},
+			{
+				MethodName: methodStateSyncIterate.ShortName(),
+				Handler:    handlerStateSyncIterate,
+			},
 		},
 	}
 )
@@ -444,6 +463,75 @@ func handlerGetParameters( // nolint: golint
 	return interceptor(ctx, height, info, handler)
 }
 
+func handlerStateSyncGet( // nolint: golint
+	srv interface{},
+	ctx context.Context,
+	dec func(interface{}) error,
+	interceptor grpc.UnaryServerInterceptor,
+) (interface{}, error) {
+	rq := new(syncer.GetRequest)
+	if err := dec(rq); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(LightClientBackend).State().SyncGet(ctx, rq)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: methodStateSyncGet.FullName(),
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(LightClientBackend).State().SyncGet(ctx, req.(*syncer.GetRequest))
+	}
+	return interceptor(ctx, rq, info, handler)
+}
+
+func handlerStateSyncGetPrefixes( // nolint: golint
+	srv interface{},
+	ctx context.Context,
+	dec func(interface{}) error,
+	interceptor grpc.UnaryServerInterceptor,
+) (interface{}, error) {
+	rq := new(syncer.GetPrefixesRequest)
+	if err := dec(rq); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(LightClientBackend).State().SyncGetPrefixes(ctx, rq)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: methodStateSyncGetPrefixes.FullName(),
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(LightClientBackend).State().SyncGetPrefixes(ctx, req.(*syncer.GetPrefixesRequest))
+	}
+	return interceptor(ctx, rq, info, handler)
+}
+
+func handlerStateSyncIterate( // nolint: golint
+	srv interface{},
+	ctx context.Context,
+	dec func(interface{}) error,
+	interceptor grpc.UnaryServerInterceptor,
+) (interface{}, error) {
+	rq := new(syncer.IterateRequest)
+	if err := dec(rq); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(LightClientBackend).State().SyncIterate(ctx, rq)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: methodStateSyncIterate.FullName(),
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(LightClientBackend).State().SyncIterate(ctx, req.(*syncer.IterateRequest))
+	}
+	return interceptor(ctx, rq, info, handler)
+}
+
 // RegisterService registers a new client backend service with the given gRPC server.
 func RegisterService(server *grpc.Server, service ClientBackend) {
 	server.RegisterService(&serviceDesc, service)
@@ -486,6 +574,42 @@ func (c *consensusLightClient) GetParameters(ctx context.Context, height int64)
 	return &rsp, nil
 }
 
+type stateReadSync struct {
+	c *consensusLightClient
+}
+
+// Implements syncer.ReadSyncer.
+func (rs *stateReadSync) SyncGet(ctx context.Context, request *syncer.GetRequest) (*syncer.ProofResponse, error) {
+	var rsp syncer.ProofResponse
+	if err := rs.c.conn.Invoke(ctx, methodStateSyncGet.FullName(), request, &rsp); err != nil {
+		return nil, err
+	}
+	return &rsp, nil
+}
+
+// Implements syncer.ReadSyncer.
+func (rs *stateReadSync) SyncGetPrefixes(ctx context.Context, request *syncer.GetPrefixesRequest) (*syncer.ProofResponse, error) {
+	var rsp syncer.ProofResponse
+	if err := rs.c.conn.Invoke(ctx, methodStateSyncGetPrefixes.FullName(), request, &rsp); err != nil {
+		return nil, err
+	}
+	return &rsp, nil
+}
+
+// Implements syncer.ReadSyncer.
+func (rs *stateReadSync) SyncIterate(ctx context.Context, request *syncer.IterateRequest) (*syncer.ProofResponse, error) {
+	var rsp syncer.ProofResponse
+	if err := rs.c.conn.Invoke(ctx, methodStateSyncIterate.FullName(), request, &rsp); err != nil {
+		return nil, err
+	}
+	return &rsp, nil
+}
+
+// Implements LightClientBackend.
+func (c *consensusLightClient) State() syncer.ReadSyncer {
+	return &stateReadSync{c}
+}
+
 type consensusClient struct {
 	consensusLightClient
 

go/consensus/api/light.go

@@ -1,6 +1,10 @@
 package api
 
-import "context"
+import (
+	"context"
+
+	"github.com/oasisprotocol/oasis-core/go/storage/mkvs/syncer"
+)
 
 // LightClientBackend is the limited consensus interface used by light clients.
 type LightClientBackend interface {
@@ -13,6 +17,10 @@ type LightClientBackend interface {
 	// GetParameters returns the consensus parameters for a specific height.
 	GetParameters(ctx context.Context, height int64) (*Parameters, error)
 
+	// State returns a MKVS read syncer that can be used to read consensus state from a remote node
+	// and verify it against the trusted local root.
+	State() syncer.ReadSyncer
+
 	// TODO: Move SubmitEvidence etc. from Backend.
 }
 

go/consensus/tendermint/abci/mux.go

@@ -263,9 +263,9 @@ func (a *ApplicationServer) EstimateGas(caller signature.PublicKey, tx *transact
 	return a.mux.EstimateGas(caller, tx)
 }
 
-// BlockHeight returns the last committed block height.
-func (a *ApplicationServer) BlockHeight() int64 {
-	return a.mux.state.BlockHeight()
+// State returns the application state.
+func (a *ApplicationServer) State() api.ApplicationQueryState {
+	return a.mux.state
 }
 
 // NewApplicationServer returns a new ApplicationServer, using the provided

go/consensus/tendermint/api/api.go

@@ -18,6 +18,7 @@ import (
 	"github.com/oasisprotocol/oasis-core/go/common/node"
 	consensus "github.com/oasisprotocol/oasis-core/go/consensus/api"
 	"github.com/oasisprotocol/oasis-core/go/consensus/tendermint/crypto"
+	mkvsNode "github.com/oasisprotocol/oasis-core/go/storage/mkvs/node"
 )
 
 // BackendName is the consensus backend name.
@@ -175,10 +176,25 @@ func NewBlock(blk *tmtypes.Block) *consensus.Block {
 	}
 	rawMeta := cbor.Marshal(meta)
 
+	var stateRoot hash.Hash
+	switch blk.Header.AppHash {
+	case nil:
+		stateRoot.Empty()
+	default:
+		if err := stateRoot.UnmarshalBinary(blk.Header.AppHash); err != nil {
+			// This should NEVER happen.
+			panic(err)
+		}
+	}
+
 	return &consensus.Block{
 		Height: blk.Header.Height,
 		Hash:   blk.Header.Hash(),
 		Time:   blk.Header.Time,
-		Meta:   rawMeta,
+		StateRoot: mkvsNode.Root{
+			Version: uint64(blk.Header.Height) - 1,
+			Hash:    stateRoot,
+		},
+		Meta: rawMeta,
 	}
 }

go/consensus/tendermint/full.go

@@ -9,6 +9,7 @@ import (
 	tmstate "github.com/tendermint/tendermint/state"
 
 	consensusAPI "github.com/oasisprotocol/oasis-core/go/consensus/api"
+	"github.com/oasisprotocol/oasis-core/go/storage/mkvs/syncer"
 )
 
 // We must use Tendermint's amino codec as some Tendermint's types are not easily unmarshallable.
@@ -73,3 +74,8 @@ func (t *tendermintService) GetParameters(ctx context.Context, height int64) (*c
 		Meta:   aminoCodec.MustMarshalBinaryBare(params.ConsensusParams),
 	}, nil
 }
+
+// Implements LightClientBackend.
+func (t *tendermintService) State() syncer.ReadSyncer {
+	return t.mux.State().Storage()
+}

go/consensus/tendermint/tendermint.go

@@ -746,6 +746,7 @@ func (t *tendermintService) GetStatus(ctx context.Context) (*consensusAPI.Status
 		status.LatestHeight = latestBlk.Height
 		status.LatestHash = latestBlk.Hash
 		status.LatestTime = latestBlk.Time
+		status.LatestStateRoot = latestBlk.StateRoot
 	case consensusAPI.ErrNoCommittedBlocks:
 		// No committed blocks yet.
 	default:
@@ -904,7 +905,7 @@ func (t *tendermintService) GetTendermintBlock(ctx context.Context, height int64
 		// Do not let Tendermint determine the latest height (e.g., by passing nil here) as that
 		// completely ignores ABCI processing so it can return a block for which local state does
 		// not yet exist. Use our mux notion of latest height instead.
-		tmHeight = t.mux.BlockHeight()
+		tmHeight = t.mux.State().BlockHeight()
 		if tmHeight == 0 {
 			// No committed blocks yet.
 			return nil, nil
@@ -939,7 +940,7 @@ func (t *tendermintService) GetBlockResults(height int64) (*tmrpctypes.ResultBlo
 	// from our mux.
 	var tmHeight int64
 	if height == consensusAPI.HeightLatest {
-		tmHeight = t.mux.BlockHeight()
+		tmHeight = t.mux.State().BlockHeight()
 		if tmHeight == 0 {
 			// No committed blocks yet.
 			return nil, consensusAPI.ErrNoCommittedBlocks

go/consensus/tests/tester.go

@@ -14,6 +14,7 @@ import (
 	"github.com/oasisprotocol/oasis-core/go/consensus/api/transaction"
 	"github.com/oasisprotocol/oasis-core/go/consensus/tendermint/apps/epochtime_mock"
 	staking "github.com/oasisprotocol/oasis-core/go/staking/api"
+	"github.com/oasisprotocol/oasis-core/go/storage/mkvs"
 )
 
 const (
@@ -43,8 +44,13 @@ func ConsensusImplementationTests(t *testing.T, backend consensus.ClientBackend)
 	require.NoError(err, "GetStatus")
 	require.NotNil(status, "returned status should not be nil")
 	require.EqualValues(1, status.GenesisHeight, "genesis height must be 1")
+
+	blk, err = backend.GetBlock(ctx, status.LatestHeight)
+	require.NoError(err, "GetBlock")
+
 	require.EqualValues(blk.Height, status.LatestHeight, "latest block heights should match")
 	require.EqualValues(blk.Hash, status.LatestHash, "latest block hashes should match")
+	require.EqualValues(blk.StateRoot, status.LatestStateRoot, "latest state roots should match")
 
 	_, err = backend.GetTransactions(ctx, consensus.HeightLatest)
 	require.NoError(err, "GetTransactions")
@@ -99,4 +105,27 @@ func ConsensusImplementationTests(t *testing.T, backend consensus.ClientBackend)
 	require.NoError(err, "GetParameters")
 	require.Equal(params.Height, blk.Height, "returned parameters height should be correct")
 	require.NotNil(params.Meta, "returned parameters should contain metadata")
+
+	// We should be able to do remote state queries. Of course the state format is backend-specific
+	// so we simply perform some usual storage operations like fetching random keys and iterating
+	// through everything.
+	state := mkvs.NewWithRoot(backend.State(), nil, blk.StateRoot)
+
+	it := state.NewIterator(ctx)
+	defer it.Close()
+
+	var keys [][]byte
+	for it.Rewind(); it.Valid(); it.Next() {
+		keys = append(keys, it.Key())
+	}
+	require.NoError(it.Err(), "iterator should not return an error")
+	require.NotEmpty(keys, "there should be some keys in consensus state")
+
+	for _, key := range keys {
+		_, err = state.Get(ctx, key)
+		require.NoError(err, "state.Get(%X)", key)
+	}
+
+	err = state.PrefetchPrefixes(ctx, keys[:1], 10)
+	require.NoError(err, "state.PrefetchPrefixes")
 }