store: refine the error handling and retry mechanism for stale read

executor/batch_point_get.go

@@ -124,6 +124,7 @@ func (e *BatchPointGetExec) Open(context.Context) error {
 		snapshot.SetOption(kv.ReplicaRead, kv.ReplicaReadFollower)
 	}
 	snapshot.SetOption(kv.TaskID, stmtCtx.TaskID)
+	snapshot.SetOption(kv.TxnScope, e.ctx.GetSessionVars().TxnCtx.TxnScope)
 	isStaleness := e.ctx.GetSessionVars().TxnCtx.IsStaleness
 	snapshot.SetOption(kv.IsStalenessReadOnly, isStaleness)
 	if isStaleness && e.ctx.GetSessionVars().TxnCtx.TxnScope != kv.GlobalTxnScope {

executor/point_get.go

@@ -149,6 +149,7 @@ func (e *PointGetExecutor) Open(context.Context) error {
 		e.snapshot.SetOption(kv.ReplicaRead, kv.ReplicaReadFollower)
 	}
 	e.snapshot.SetOption(kv.TaskID, e.ctx.GetSessionVars().StmtCtx.TaskID)
+	e.snapshot.SetOption(kv.TxnScope, e.ctx.GetSessionVars().TxnCtx.TxnScope)
 	isStaleness := e.ctx.GetSessionVars().TxnCtx.IsStaleness
 	e.snapshot.SetOption(kv.IsStalenessReadOnly, isStaleness)
 	if isStaleness && e.ctx.GetSessionVars().TxnCtx.TxnScope != kv.GlobalTxnScope {

go.mod

@@ -43,7 +43,7 @@ require (
 	github.com/pingcap/failpoint v0.0.0-20210316064728-7acb0f0a3dfd
 	github.com/pingcap/fn v0.0.0-20200306044125-d5540d389059
 	github.com/pingcap/goleveldb v0.0.0-20191226122134-f82aafb29989
-	github.com/pingcap/kvproto v0.0.0-20210507054410-a8152f8a876c
+	github.com/pingcap/kvproto v0.0.0-20210531063847-f42e582bf0bb
 	github.com/pingcap/log v0.0.0-20210317133921-96f4fcab92a4
 	github.com/pingcap/parser v0.0.0-20210525032559-c37778aff307
 	github.com/pingcap/sysutil v0.0.0-20210315073920-cc0985d983a3

go.sum

@@ -433,8 +433,8 @@ github.com/pingcap/goleveldb v0.0.0-20191226122134-f82aafb29989/go.mod h1:O17Xtb
 github.com/pingcap/kvproto v0.0.0-20191211054548-3c6b38ea5107/go.mod h1:WWLmULLO7l8IOcQG+t+ItJ3fEcrL5FxF0Wu+HrMy26w=
 github.com/pingcap/kvproto v0.0.0-20200411081810-b85805c9476c/go.mod h1:IOdRDPLyda8GX2hE/jO7gqaCV/PNFh8BZQCQZXfIOqI=
 github.com/pingcap/kvproto v0.0.0-20210219064844-c1844a4775d6/go.mod h1:IOdRDPLyda8GX2hE/jO7gqaCV/PNFh8BZQCQZXfIOqI=
-github.com/pingcap/kvproto v0.0.0-20210507054410-a8152f8a876c h1:cy87vgUJT0U4JuxC7R14PuwBrabI9fDawYhyKTbjOBQ=
-github.com/pingcap/kvproto v0.0.0-20210507054410-a8152f8a876c/go.mod h1:IOdRDPLyda8GX2hE/jO7gqaCV/PNFh8BZQCQZXfIOqI=
+github.com/pingcap/kvproto v0.0.0-20210531063847-f42e582bf0bb h1:6isHwZRl1fc9i1Mggiza2iQcfvVvYAAerFIs5t9msXg=
+github.com/pingcap/kvproto v0.0.0-20210531063847-f42e582bf0bb/go.mod h1:IOdRDPLyda8GX2hE/jO7gqaCV/PNFh8BZQCQZXfIOqI=
 github.com/pingcap/log v0.0.0-20191012051959-b742a5d432e9/go.mod h1:4rbK1p9ILyIfb6hU7OG2CiWSqMXnp3JMbiaVJ6mvoY8=
 github.com/pingcap/log v0.0.0-20200511115504-543df19646ad/go.mod h1:4rbK1p9ILyIfb6hU7OG2CiWSqMXnp3JMbiaVJ6mvoY8=
 github.com/pingcap/log v0.0.0-20201112100606-8f1e84a3abc8/go.mod h1:4rbK1p9ILyIfb6hU7OG2CiWSqMXnp3JMbiaVJ6mvoY8=

kv/kv.go

@@ -283,6 +283,8 @@ type Request struct {
 	TaskID uint64
 	// TiDBServerID is the specified TiDB serverID to execute request. `0` means all TiDB instances.
 	TiDBServerID uint64
+	// TxnScope is the scope of the current txn.
+	TxnScope string
 	// IsStaleness indicates whether the request read staleness data
 	IsStaleness bool
 	// MatchStoreLabels indicates the labels the store should be matched

store/copr/coprocessor.go

@@ -713,10 +713,11 @@ func (worker *copIteratorWorker) handleTaskOnce(bo *Backoffer, task *copTask, ch
 	if worker.kvclient.Stats == nil {
 		worker.kvclient.Stats = make(map[tikvrpc.CmdType]*tikv.RPCRuntimeStats)
 	}
+	req.TxnScope = worker.req.TxnScope
 	if worker.req.IsStaleness {
 		req.EnableStaleRead()
 	}
-	var ops []tikv.StoreSelectorOption
+	ops := make([]tikv.StoreSelectorOption, 0, 2)
 	if len(worker.req.MatchStoreLabels) > 0 {
 		ops = append(ops, tikv.WithMatchLabels(worker.req.MatchStoreLabels))
 	}

store/mockstore/unistore/tikv/server.go

@@ -618,6 +618,11 @@ func (svr *Server) BatchCoprocessor(req *coprocessor.BatchRequest, batchCopServe
 	return nil
 }
 
+// RawCoprocessor implements implements the tikvpb.TikvServer interface.
+func (svr *Server) RawCoprocessor(context.Context, *kvrpcpb.RawCoprocessorRequest) (*kvrpcpb.RawCoprocessorResponse, error) {
+	panic("unimplemented")
+}
+
 func (mrm *MockRegionManager) removeMPPTaskHandler(taskID int64, storeID uint64) error {
 	set := mrm.getMPPTaskSet(storeID)
 	if set == nil {

store/tikv/region_cache.go

@@ -172,14 +172,19 @@ func (r *RegionStore) follower(seed uint32, op *storeSelectorOp) AccessIndex {
 
 // return next leader or follower store's index
 func (r *RegionStore) kvPeer(seed uint32, op *storeSelectorOp) AccessIndex {
+	if op.leaderOnly {
+		return r.workTiKVIdx
+	}
 	candidates := make([]AccessIndex, 0, r.accessStoreNum(TiKVOnly))
 	for i := 0; i < r.accessStoreNum(TiKVOnly); i++ {
-		storeIdx, s := r.accessStore(TiKVOnly, AccessIndex(i))
-		if r.storeEpochs[storeIdx] != atomic.LoadUint32(&s.epoch) || !r.filterStoreCandidate(AccessIndex(i), op) {
+		accessIdx := AccessIndex(i)
+		storeIdx, s := r.accessStore(TiKVOnly, accessIdx)
+		if r.storeEpochs[storeIdx] != atomic.LoadUint32(&s.epoch) || !r.filterStoreCandidate(accessIdx, op) {
 			continue
 		}
-		candidates = append(candidates, AccessIndex(i))
+		candidates = append(candidates, accessIdx)
 	}
+	// If there is no candidates, send to current workTiKVIdx which generally is the leader.
 	if len(candidates) == 0 {
 		return r.workTiKVIdx
 	}
@@ -422,6 +427,8 @@ type RPCContext struct {
 	ProxyAccessIdx AccessIndex // valid when ProxyStore is not nil
 	ProxyAddr      string      // valid when ProxyStore is not nil
 	TiKVNum        int         // Number of TiKV nodes among the region's peers. Assuming non-TiKV peers are all TiFlash peers.
+
+	tryTimes int
 }
 
 func (c *RPCContext) String() string {
@@ -438,16 +445,24 @@ func (c *RPCContext) String() string {
 }
 
 type storeSelectorOp struct {
-	labels []*metapb.StoreLabel
+	leaderOnly bool
+	labels     []*metapb.StoreLabel
 }
 
 // StoreSelectorOption configures storeSelectorOp.
 type StoreSelectorOption func(*storeSelectorOp)
 
-// WithMatchLabels indicates selecting stores with matched labels
+// WithMatchLabels indicates selecting stores with matched labels.
 func WithMatchLabels(labels []*metapb.StoreLabel) StoreSelectorOption {
 	return func(op *storeSelectorOp) {
-		op.labels = labels
+		op.labels = append(op.labels, labels...)
+	}
+}
+
+// WithLeaderOnly indicates selecting stores with leader only.
+func WithLeaderOnly() StoreSelectorOption {
+	return func(op *storeSelectorOp) {
+		op.leaderOnly = true
 	}
 }
 

store/tikv/region_cache_test.go

@@ -29,6 +29,7 @@ import (
 	"github.com/pingcap/tidb/store/tikv/kv"
 	"github.com/pingcap/tidb/store/tikv/mockstore/mocktikv"
 	"github.com/pingcap/tidb/store/tikv/retry"
+	"github.com/pingcap/tidb/store/tikv/tikvrpc"
 	pd "github.com/tikv/pd/client"
 )
 
@@ -1043,7 +1044,7 @@ func (s *testRegionCacheSuite) TestRegionEpochOnTiFlash(c *C) {
 	r := ctxTiFlash.Meta
 	reqSend := NewRegionRequestSender(s.cache, nil)
 	regionErr := &errorpb.Error{EpochNotMatch: &errorpb.EpochNotMatch{CurrentRegions: []*metapb.Region{r}}}
-	reqSend.onRegionError(s.bo, ctxTiFlash, nil, regionErr)
+	reqSend.onRegionError(s.bo, ctxTiFlash, nil, regionErr, nil)
 
 	// check leader read should not go to tiflash
 	lctx, err = s.cache.GetTiKVRPCContext(s.bo, loc1.Region, kv.ReplicaReadLeader, 0)
@@ -1406,11 +1407,11 @@ func (s *testRegionCacheSuite) TestFollowerMeetEpochNotMatch(c *C) {
 	c.Assert(ctxFollower1.Store.storeID, Equals, s.store2)
 
 	regionErr := &errorpb.Error{EpochNotMatch: &errorpb.EpochNotMatch{}}
-	reqSend.onRegionError(s.bo, ctxFollower1, &followReqSeed, regionErr)
+	reqSend.onRegionError(s.bo, ctxFollower1, &tikvrpc.Request{ReplicaReadSeed: &followReqSeed}, regionErr, nil)
 	c.Assert(followReqSeed, Equals, uint32(1))
 
 	regionErr = &errorpb.Error{RegionNotFound: &errorpb.RegionNotFound{}}
-	reqSend.onRegionError(s.bo, ctxFollower1, &followReqSeed, regionErr)
+	reqSend.onRegionError(s.bo, ctxFollower1, &tikvrpc.Request{ReplicaReadSeed: &followReqSeed}, regionErr, nil)
 	c.Assert(followReqSeed, Equals, uint32(2))
 }
 
@@ -1437,7 +1438,7 @@ func (s *testRegionCacheSuite) TestMixedMeetEpochNotMatch(c *C) {
 	c.Assert(ctxFollower1.Store.storeID, Equals, s.store1)
 
 	regionErr := &errorpb.Error{EpochNotMatch: &errorpb.EpochNotMatch{}}
-	reqSend.onRegionError(s.bo, ctxFollower1, &followReqSeed, regionErr)
+	reqSend.onRegionError(s.bo, ctxFollower1, &tikvrpc.Request{ReplicaReadSeed: &followReqSeed}, regionErr, nil)
 	c.Assert(followReqSeed, Equals, uint32(1))
 }
 

store/tikv/region_request.go

@@ -36,6 +36,7 @@ import (
 	"github.com/pingcap/tidb/store/tikv/kv"
 	"github.com/pingcap/tidb/store/tikv/logutil"
 	"github.com/pingcap/tidb/store/tikv/metrics"
+	"github.com/pingcap/tidb/store/tikv/oracle"
 	"github.com/pingcap/tidb/store/tikv/retry"
 	"github.com/pingcap/tidb/store/tikv/tikvrpc"
 	"github.com/pingcap/tidb/store/tikv/util"
@@ -286,6 +287,9 @@ func (s *RegionRequestSender) SendReqCtx(
 		if err != nil {
 			return nil, nil, err
 		}
+		if rpcCtx != nil {
+			rpcCtx.tryTimes = tryTimes
+		}
 
 		failpoint.Inject("invalidCacheAndRetry", func() {
 			// cooperate with github.com/pingcap/tidb/store/gcworker/setGcResolveMaxBackoff
@@ -334,8 +338,14 @@ func (s *RegionRequestSender) SendReqCtx(
 		if err != nil {
 			return nil, nil, errors.Trace(err)
 		}
+		failpoint.Inject("mockDataIsNotReadyError", func(val failpoint.Value) {
+			regionErr = &errorpb.Error{}
+			if tryTimesLimit, ok := val.(int); ok && tryTimes <= tryTimesLimit {
+				regionErr.DataIsNotReady = &errorpb.DataIsNotReady{}
+			}
+		})
 		if regionErr != nil {
-			retry, err = s.onRegionError(bo, rpcCtx, req.ReplicaReadSeed, regionErr)
+			retry, err = s.onRegionError(bo, rpcCtx, req, regionErr, &opts)
 			if err != nil {
 				return nil, nil, errors.Trace(err)
 			}
@@ -644,14 +654,20 @@ func regionErrorToLabel(e *errorpb.Error) string {
 	return "unknown"
 }
 
-func (s *RegionRequestSender) onRegionError(bo *Backoffer, ctx *RPCContext, seed *uint32, regionErr *errorpb.Error) (shouldRetry bool, err error) {
+func (s *RegionRequestSender) onRegionError(bo *Backoffer, ctx *RPCContext, req *tikvrpc.Request, regionErr *errorpb.Error, opts *[]StoreSelectorOption) (shouldRetry bool, err error) {
 	if span := opentracing.SpanFromContext(bo.GetCtx()); span != nil && span.Tracer() != nil {
 		span1 := span.Tracer().StartSpan("tikv.onRegionError", opentracing.ChildOf(span.Context()))
 		defer span1.Finish()
 		// TODO(MyonKeminta): Make sure trace works without cloning the backoffer.
 		// bo = bo.Clone()
 		bo.SetCtx(opentracing.ContextWithSpan(bo.GetCtx(), span1))
 	}
+	// Stale Read request will retry the leader or next peer on error,
+	// if txnScope is global, we will only retry the leader by using the WithLeaderOnly option,
+	// if txnScope is local, we will retry both other peers and the leader by the incresing seed.
+	if ctx.tryTimes < 1 && req != nil && req.TxnScope == oracle.GlobalTxnScope && req.GetStaleRead() {
+		*opts = append(*opts, WithLeaderOnly())
+	}
 
 	metrics.TiKVRegionErrorCounter.WithLabelValues(regionErrorToLabel(regionErr)).Inc()
 	if notLeader := regionErr.GetNotLeader(); notLeader != nil {
@@ -687,6 +703,7 @@ func (s *RegionRequestSender) onRegionError(bo *Backoffer, ctx *RPCContext, seed
 		return true, nil
 	}
 
+	seed := req.GetReplicaReadSeed()
 	if epochNotMatch := regionErr.GetEpochNotMatch(); epochNotMatch != nil {
 		logutil.BgLogger().Debug("tikv reports `EpochNotMatch` retry later",
 			zap.Stringer("EpochNotMatch", epochNotMatch),
@@ -723,6 +740,30 @@ func (s *RegionRequestSender) onRegionError(bo *Backoffer, ctx *RPCContext, seed
 		logutil.BgLogger().Warn("tikv reports `RaftEntryTooLarge`", zap.Stringer("ctx", ctx))
 		return false, errors.New(regionErr.String())
 	}
+	// A stale read request may be sent to a peer which the data is not ready yet, we should retry in this case.
+	if regionErr.GetDataIsNotReady() != nil {
+		logutil.BgLogger().Warn("tikv reports `DataIsNotReady` retry later",
+			zap.Uint64("store-id", ctx.Store.storeID),
+			zap.Uint64("peer-id", regionErr.GetDataIsNotReady().GetPeerId()),
+			zap.Uint64("region-id", regionErr.GetDataIsNotReady().GetRegionId()),
+			zap.Uint64("safe-ts", regionErr.GetDataIsNotReady().GetSafeTs()),
+			zap.Stringer("ctx", ctx))
+		if seed != nil {
+			*seed = *seed + 1
+		}
+		return true, nil
+	}
+	// A read request may be sent to a peer which has not been initialized yet, we should retry in this case.
+	if regionErr.GetRegionNotInitialized() != nil {
+		logutil.BgLogger().Warn("tikv reports `RegionNotInitialized` retry later",
+			zap.Uint64("store-id", ctx.Store.storeID),
+			zap.Uint64("region-id", regionErr.GetRegionNotInitialized().GetRegionId()),
+			zap.Stringer("ctx", ctx))
+		if seed != nil {
+			*seed = *seed + 1
+		}
+		return true, nil
+	}
 	if regionErr.GetRegionNotFound() != nil && seed != nil {
 		logutil.BgLogger().Debug("tikv reports `RegionNotFound` in follow-reader",
 			zap.Stringer("ctx", ctx), zap.Uint32("seed", *seed))

store/tikv/region_request_test.go

@@ -24,6 +24,7 @@ import (
 
 	. "github.com/pingcap/check"
 	"github.com/pingcap/errors"
+	"github.com/pingcap/failpoint"
 	"github.com/pingcap/kvproto/pkg/coprocessor"
 	"github.com/pingcap/kvproto/pkg/coprocessor_v2"
 	"github.com/pingcap/kvproto/pkg/errorpb"
@@ -33,6 +34,7 @@ import (
 	"github.com/pingcap/kvproto/pkg/tikvpb"
 	tikverr "github.com/pingcap/tidb/store/tikv/error"
 	"github.com/pingcap/tidb/store/tikv/kv"
+	"github.com/pingcap/tidb/store/tikv/oracle"
 	"github.com/pingcap/tidb/store/tikv/retry"
 
 	"github.com/pingcap/tidb/store/tikv/config"
@@ -185,6 +187,92 @@ func (s *testRegionRequestToThreeStoresSuite) TestStoreTokenLimit(c *C) {
 	kv.StoreLimit.Store(oldStoreLimit)
 }
 
+// Test whether the Stale Read request will retry the leader or other peers on error.
+func (s *testRegionRequestToThreeStoresSuite) TestStaleReadRetry(c *C) {
+	var seed uint32 = 0
+	req := tikvrpc.NewReplicaReadRequest(tikvrpc.CmdGet, &kvrpcpb.GetRequest{}, kv.ReplicaReadMixed, &seed)
+	req.EnableStaleRead()
+
+	// Test whether a global Stale Read request will only retry on the leader.
+	req.TxnScope = oracle.GlobalTxnScope
+	region, err := s.cache.LocateRegionByID(s.bo, s.regionID)
+	c.Assert(err, IsNil)
+	c.Assert(region, NotNil)
+	// Retry 1 time.
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError", `return(1)`), IsNil)
+	resp, ctx, err := s.regionRequestSender.SendReqCtx(s.bo, req, region.Region, time.Second, tikvrpc.TiKV)
+	c.Assert(err, IsNil)
+	c.Assert(resp.Resp, NotNil)
+	c.Assert(ctx, NotNil)
+	c.Assert(ctx.Peer.GetId(), Equals, s.leaderPeer)
+
+	seed = 0
+	region, err = s.cache.LocateRegionByID(s.bo, s.regionID)
+	c.Assert(err, IsNil)
+	c.Assert(region, NotNil)
+	// Retry 2 times.
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError", `return(2)`), IsNil)
+	resp, ctx, err = s.regionRequestSender.SendReqCtx(s.bo, req, region.Region, time.Second, tikvrpc.TiKV)
+	c.Assert(err, IsNil)
+	c.Assert(resp.Resp, NotNil)
+	c.Assert(ctx, NotNil)
+	c.Assert(ctx.Peer.GetId(), Equals, s.leaderPeer)
+
+	seed = 0
+	region, err = s.cache.LocateRegionByID(s.bo, s.regionID)
+	c.Assert(err, IsNil)
+	c.Assert(region, NotNil)
+	// Retry 3 times.
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError", `return(3)`), IsNil)
+	resp, ctx, err = s.regionRequestSender.SendReqCtx(s.bo, req, region.Region, time.Second, tikvrpc.TiKV)
+	c.Assert(err, IsNil)
+	c.Assert(resp.Resp, NotNil)
+	c.Assert(ctx, NotNil)
+	c.Assert(ctx.Peer.GetId(), Equals, s.leaderPeer)
+
+	// Test whether a local Stale Read request will retry on the leader and other peers.
+	req.TxnScope = "local"
+	seed = 0
+	region, err = s.cache.LocateRegionByID(s.bo, s.regionID)
+	c.Assert(err, IsNil)
+	c.Assert(region, NotNil)
+	// Retry 1 time.
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError", `return(1)`), IsNil)
+	resp, ctx, err = s.regionRequestSender.SendReqCtx(s.bo, req, region.Region, time.Second, tikvrpc.TiKV)
+	c.Assert(err, IsNil)
+	c.Assert(resp.Resp, NotNil)
+	c.Assert(ctx, NotNil)
+	peerID1 := ctx.Peer.GetId()
+
+	seed = 0
+	region, err = s.cache.LocateRegionByID(s.bo, s.regionID)
+	c.Assert(err, IsNil)
+	c.Assert(region, NotNil)
+	// Retry 2 times.
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError", `return(2)`), IsNil)
+	resp, ctx, err = s.regionRequestSender.SendReqCtx(s.bo, req, region.Region, time.Second, tikvrpc.TiKV)
+	c.Assert(err, IsNil)
+	c.Assert(resp.Resp, NotNil)
+	c.Assert(ctx, NotNil)
+	peerID2 := ctx.Peer.GetId()
+	c.Assert(peerID2, Not(Equals), peerID1)
+
+	seed = 0
+	region, err = s.cache.LocateRegionByID(s.bo, s.regionID)
+	c.Assert(err, IsNil)
+	c.Assert(region, NotNil)
+	// Retry 3 times.
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError", `return(3)`), IsNil)
+	resp, ctx, err = s.regionRequestSender.SendReqCtx(s.bo, req, region.Region, time.Second, tikvrpc.TiKV)
+	c.Assert(err, IsNil)
+	c.Assert(resp.Resp, NotNil)
+	c.Assert(ctx, NotNil)
+	peerID3 := ctx.Peer.GetId()
+	c.Assert(peerID3, Not(Equals), peerID1)
+	c.Assert(peerID3, Not(Equals), peerID2)
+	c.Assert(failpoint.Disable("github.com/pingcap/tidb/store/tikv/mockDataIsNotReadyError"), IsNil)
+}
+
 func (s *testRegionRequestToSingleStoreSuite) TestOnSendFailedWithStoreRestart(c *C) {
 	req := tikvrpc.NewRequest(tikvrpc.CmdRawPut, &kvrpcpb.RawPutRequest{
 		Key:   []byte("key"),
@@ -435,6 +523,9 @@ func (s *mockTikvGrpcServer) Coprocessor(context.Context, *coprocessor.Request)
 func (s *mockTikvGrpcServer) BatchCoprocessor(*coprocessor.BatchRequest, tikvpb.Tikv_BatchCoprocessorServer) error {
 	return errors.New("unreachable")
 }
+func (s *mockTikvGrpcServer) RawCoprocessor(context.Context, *kvrpcpb.RawCoprocessorRequest) (*kvrpcpb.RawCoprocessorResponse, error) {
+	return nil, errors.New("unreachable")
+}
 func (s *mockTikvGrpcServer) DispatchMPPTask(context.Context, *mpp.DispatchTaskRequest) (*mpp.DispatchTaskResponse, error) {
 	return nil, errors.New("unreachable")
 }