// BootstrapCluster bootstraps a multiple stores using the provided engines and
// cluster ID. The first bootstrapped store contains a single range spanning
// all keys. Initial range lookup metadata is populated for the range.
//
// Returns a KV client for unittest purposes. Caller should close the returned
// client.
func BootstrapCluster(clusterID string, engines []engine.Engine, stopper *stop.Stopper) (*client.DB, error) {
	ctx := storage.StoreContext{}
	ctx.ScanInterval = 10 * time.Minute
	ctx.Clock = hlc.NewClock(hlc.UnixNano)
	// Create a KV DB with a local sender.
	lSender := kv.NewLocalSender()
	sender := kv.NewTxnCoordSender(lSender, ctx.Clock, false, nil, stopper)
	var err error
	if ctx.DB, err = client.Open("//[email protected]", client.SenderOpt(sender)); err != nil {
		return nil, err
	}
	ctx.Transport = multiraft.NewLocalRPCTransport(stopper)
	for i, eng := range engines {
		sIdent := proto.StoreIdent{
			ClusterID: clusterID,
			NodeID:    1,
			StoreID:   proto.StoreID(i + 1),
		}

		// The bootstrapping store will not connect to other nodes so its
		// StoreConfig doesn't really matter.
		s := storage.NewStore(ctx, eng, &proto.NodeDescriptor{NodeID: 1})

		// Verify the store isn't already part of a cluster.
		if len(s.Ident.ClusterID) > 0 {
			return nil, util.Errorf("storage engine already belongs to a cluster (%s)", s.Ident.ClusterID)
		}

		// Bootstrap store to persist the store ident.
		if err := s.Bootstrap(sIdent, stopper); err != nil {
			return nil, err
		}
		// Create first range, writing directly to engine. Note this does
		// not create the range, just its data.  Only do this if this is the
		// first store.
		if i == 0 {
			if err := s.BootstrapRange(); err != nil {
				return nil, err
			}
		}
		if err := s.Start(stopper); err != nil {
			return nil, err
		}

		lSender.AddStore(s)

		// Initialize node and store ids.  Only initialize the node once.
		if i == 0 {
			if nodeID, err := allocateNodeID(ctx.DB); nodeID != sIdent.NodeID || err != nil {
				return nil, util.Errorf("expected to initialize node id allocator to %d, got %d: %s",
					sIdent.NodeID, nodeID, err)
			}
		}
		if storeID, err := allocateStoreIDs(sIdent.NodeID, 1, ctx.DB); storeID != sIdent.StoreID || err != nil {
			return nil, util.Errorf("expected to initialize store id allocator to %d, got %d: %s",
				sIdent.StoreID, storeID, err)
		}
	}
	return ctx.DB, nil
}

// createTestNode creates an rpc server using the specified address,
// gossip instance, KV database and a node using the specified slice
// of engines. The server, clock and node are returned. If gossipBS is
// not nil, the gossip bootstrap address is set to gossipBS.
func createTestNode(addr net.Addr, engines []engine.Engine, gossipBS net.Addr, t *testing.T) (
	*rpc.Server, *hlc.Clock, *Node, *stop.Stopper) {
	var err error
	ctx := storage.StoreContext{}

	stopper := stop.NewStopper()
	ctx.Clock = hlc.NewClock(hlc.UnixNano)
	nodeRPCContext := rpc.NewContext(nodeTestBaseContext, ctx.Clock, stopper)
	ctx.ScanInterval = 10 * time.Hour
	rpcServer := rpc.NewServer(addr, nodeRPCContext)
	if err := rpcServer.Start(); err != nil {
		t.Fatal(err)
	}
	g := gossip.New(nodeRPCContext, testContext.GossipInterval, testContext.GossipBootstrapResolvers)
	if gossipBS != nil {
		// Handle possibility of a :0 port specification.
		if gossipBS == addr {
			gossipBS = rpcServer.Addr()
		}
		g.SetResolvers([]resolver.Resolver{resolver.NewResolverFromAddress(gossipBS)})
		g.Start(rpcServer, stopper)
	}
	ctx.Gossip = g
	sender := kv.NewDistSender(&kv.DistSenderContext{Clock: ctx.Clock}, g)
	if ctx.DB, err = client.Open("//[email protected]", client.SenderOpt(sender)); err != nil {
		t.Fatal(err)
	}
	// TODO(bdarnell): arrange to have the transport closed.
	// (or attach LocalRPCTransport.Close to the stopper)
	ctx.Transport = multiraft.NewLocalRPCTransport(stopper)
	ctx.EventFeed = util.NewFeed(stopper)
	node := NewNode(ctx)
	return rpcServer, ctx.Clock, node, stopper
}

// createTestStoreWithoutStart creates a test store using an in-memory
// engine without starting the store. It returns the store, the store
// clock's manual unix nanos time and a stopper. The caller is
// responsible for stopping the stopper upon completion.
func createTestStoreWithoutStart(t *testing.T) (*Store, *hlc.ManualClock, *stop.Stopper) {
	stopper := stop.NewStopper()
	rpcContext := rpc.NewContext(rootTestBaseContext, hlc.NewClock(hlc.UnixNano), stopper)
	ctx := TestStoreContext
	ctx.Gossip = gossip.New(rpcContext, gossip.TestInterval, gossip.TestBootstrap)
	manual := hlc.NewManualClock(0)
	ctx.Clock = hlc.NewClock(manual.UnixNano)
	eng := engine.NewInMem(proto.Attributes{}, 10<<20)
	ctx.Transport = multiraft.NewLocalRPCTransport()
	stopper.AddCloser(ctx.Transport)
	sender := &testSender{}
	var err error
	if ctx.DB, err = client.Open("//[email protected]", client.SenderOpt(sender)); err != nil {
		t.Fatal(err)
	}
	store := NewStore(ctx, eng, &proto.NodeDescriptor{NodeID: 1})
	sender.store = store
	if err := store.Bootstrap(proto.StoreIdent{NodeID: 1, StoreID: 1}, stopper); err != nil {
		t.Fatal(err)
	}
	if err := store.BootstrapRange(); err != nil {
		t.Fatal(err)
	}
	return store, manual, stopper
}

// Start starts the test cluster by bootstrapping an in-memory store
// (defaults to maximum of 50M). The server is started, launching the
// node RPC server and all HTTP endpoints. Use the value of
// TestServer.Addr after Start() for client connections. Use Stop()
// to shutdown the server after the test completes.
func (ltc *LocalTestCluster) Start(t util.Tester) {
	ltc.Manual = hlc.NewManualClock(0)
	ltc.Clock = hlc.NewClock(ltc.Manual.UnixNano)
	ltc.Stopper = stop.NewStopper()
	rpcContext := rpc.NewContext(testutils.NewRootTestBaseContext(), ltc.Clock, ltc.Stopper)
	ltc.Gossip = gossip.New(rpcContext, gossip.TestInterval, gossip.TestBootstrap)
	ltc.Eng = engine.NewInMem(proto.Attributes{}, 50<<20)
	ltc.lSender = newRetryableLocalSender(NewLocalSender())
	ltc.Sender = NewTxnCoordSender(ltc.lSender, ltc.Clock, false, nil, ltc.Stopper)
	var err error
	if ltc.DB, err = client.Open("//[email protected]", client.SenderOpt(ltc.Sender)); err != nil {
		t.Fatal(err)
	}
	transport := multiraft.NewLocalRPCTransport(ltc.Stopper)
	ltc.Stopper.AddCloser(transport)
	ctx := storage.TestStoreContext
	ctx.Clock = ltc.Clock
	ctx.DB = ltc.DB
	ctx.Gossip = ltc.Gossip
	ctx.Transport = transport
	ltc.Store = storage.NewStore(ctx, ltc.Eng, &proto.NodeDescriptor{NodeID: 1})
	if err := ltc.Store.Bootstrap(proto.StoreIdent{NodeID: 1, StoreID: 1}, ltc.Stopper); err != nil {
		t.Fatalf("unable to start local test cluster: %s", err)
	}
	ltc.lSender.AddStore(ltc.Store)
	if err := ltc.Store.BootstrapRange(nil); err != nil {
		t.Fatalf("unable to start local test cluster: %s", err)
	}
	if err := ltc.Store.Start(ltc.Stopper); err != nil {
		t.Fatalf("unable to start local test cluster: %s", err)
	}
}

func (m *multiTestContext) Start(t *testing.T, numStores int) {
	m.t = t
	if m.manualClock == nil {
		m.manualClock = hlc.NewManualClock(0)
	}
	if m.clock == nil {
		m.clock = hlc.NewClock(m.manualClock.UnixNano)
	}
	if m.gossip == nil {
		rpcContext := rpc.NewContext(&base.Context{}, m.clock, nil)
		m.gossip = gossip.New(rpcContext, gossip.TestInterval, gossip.TestBootstrap)
	}
	if m.clientStopper == nil {
		m.clientStopper = stop.NewStopper()
	}
	if m.transport == nil {
		m.transport = multiraft.NewLocalRPCTransport(m.clientStopper)
	}
	if m.storePool == nil {
		m.storePool = storage.NewStorePool(m.gossip, storage.TestTimeUntilStoreDeadOff, m.clientStopper)
	}

	// Always create the first sender.
	m.senders = append(m.senders, kv.NewLocalSender())

	if m.db == nil {
		sender := kv.NewTxnCoordSender(m.senders[0], m.clock, false, nil, m.clientStopper)
		var err error
		if m.db, err = client.Open("//", client.SenderOpt(sender)); err != nil {
			t.Fatal(err)
		}
	}

	for i := 0; i < numStores; i++ {
		m.addStore()
	}
	if m.transportStopper == nil {
		m.transportStopper = stop.NewStopper()
	}
	m.transportStopper.AddCloser(m.transport)
}

// createTestStoreWithEngine creates a test store using the given engine and clock.
// The caller is responsible for closing the store on exit.
func createTestStoreWithEngine(t *testing.T, eng engine.Engine, clock *hlc.Clock,
	bootstrap bool, context *storage.StoreContext) (*storage.Store, *stop.Stopper) {
	stopper := stop.NewStopper()
	rpcContext := rpc.NewContext(&base.Context{}, hlc.NewClock(hlc.UnixNano), stopper)
	if context == nil {
		// make a copy
		ctx := storage.TestStoreContext
		context = &ctx
	}
	context.Gossip = gossip.New(rpcContext, gossip.TestInterval, gossip.TestBootstrap)
	lSender := kv.NewLocalSender()
	sender := kv.NewTxnCoordSender(lSender, clock, false, nil, stopper)
	context.Clock = clock
	var err error
	if context.DB, err = client.Open("//", client.SenderOpt(sender)); err != nil {
		t.Fatal(err)
	}
	context.Transport = multiraft.NewLocalRPCTransport(stopper)
	// TODO(bdarnell): arrange to have the transport closed.
	store := storage.NewStore(*context, eng, &proto.NodeDescriptor{NodeID: 1})
	if bootstrap {
		if err := store.Bootstrap(proto.StoreIdent{NodeID: 1, StoreID: 1}, stopper); err != nil {
			t.Fatal(err)
		}
	}
	lSender.AddStore(store)
	if bootstrap {
		if err := store.BootstrapRange(nil); err != nil {
			t.Fatal(err)
		}
	}
	if err := store.Start(stopper); err != nil {
		t.Fatal(err)
	}
	return store, stopper
}

func TestIDAllocationRetry(t *testing.T) {
	defer leaktest.AfterTest(t)
	defer setAllocRetryBackoff(0)()
	i := 98
	sender := func(_ context.Context, c proto.Call) {
		if i%2 == 0 {
			c.Reply.Header().Error = &proto.Error{Retryable: true}
		}
		c.Reply.(*proto.IncrementResponse).NewValue = int64(i)
		i++
	}
	db, err := client.Open("//[email protected]", client.SenderOpt(testSender(sender)))
	if err != nil {
		t.Fatal(err)
	}
	if n, err := allocateNodeID(db); n != 99 || err != nil {
		t.Fatalf("wanted NodeID 99, got %d (err=%s)", n, err)
	}

	if n, err := allocateStoreIDs(1, 1, db); n != 101 || err != nil {
		t.Fatalf("wanted NodeID 101, got %d (err=%s)", n, err)
	}

}

// NewServer creates a Server from a server.Context.
func NewServer(ctx *Context, stopper *stop.Stopper) (*Server, error) {
	if ctx == nil {
		return nil, util.Error("ctx must not be null")
	}

	addr := ctx.Addr
	_, err := net.ResolveTCPAddr("tcp", addr)
	if err != nil {
		return nil, util.Errorf("unable to resolve RPC address %q: %v", addr, err)
	}

	if ctx.Insecure {
		log.Warning("running in insecure mode, this is strongly discouraged. See --insecure and --certs.")
	}
	// Try loading the TLS configs before anything else.
	if _, err := ctx.GetServerTLSConfig(); err != nil {
		return nil, err
	}
	if _, err := ctx.GetClientTLSConfig(); err != nil {
		return nil, err
	}

	s := &Server{
		ctx:     ctx,
		mux:     http.NewServeMux(),
		clock:   hlc.NewClock(hlc.UnixNano),
		stopper: stopper,
	}
	s.clock.SetMaxOffset(ctx.MaxOffset)

	rpcContext := rpc.NewContext(&ctx.Context, s.clock, stopper)
	stopper.RunWorker(func() {
		rpcContext.RemoteClocks.MonitorRemoteOffsets(stopper)
	})

	s.rpc = rpc.NewServer(util.MakeUnresolvedAddr("tcp", addr), rpcContext)
	s.stopper.AddCloser(s.rpc)
	s.gossip = gossip.New(rpcContext, s.ctx.GossipInterval, s.ctx.GossipBootstrapResolvers)

	feed := util.NewFeed(stopper)
	tracer := tracer.NewTracer(feed, addr)

	ds := kv.NewDistSender(&kv.DistSenderContext{Clock: s.clock}, s.gossip)
	sender := kv.NewTxnCoordSender(ds, s.clock, ctx.Linearizable, tracer, s.stopper)
	if s.db, err = client.Open("//[email protected]", client.SenderOpt(sender)); err != nil {
		return nil, err
	}

	s.raftTransport, err = newRPCTransport(s.gossip, s.rpc, rpcContext)
	if err != nil {
		return nil, err
	}
	s.stopper.AddCloser(s.raftTransport)

	s.kvDB = kv.NewDBServer(&s.ctx.Context, sender)
	if s.ctx.ExperimentalRPCServer {
		if err = s.kvDB.RegisterRPC(s.rpc); err != nil {
			return nil, err
		}
	}

	s.sqlServer = sql.NewServer(&s.ctx.Context, s.db)

	// TODO(bdarnell): make StoreConfig configurable.
	nCtx := storage.StoreContext{
		Clock:           s.clock,
		DB:              s.db,
		Gossip:          s.gossip,
		Transport:       s.raftTransport,
		ScanInterval:    s.ctx.ScanInterval,
		ScanMaxIdleTime: s.ctx.ScanMaxIdleTime,
		EventFeed:       feed,
		Tracer:          tracer,
	}
	s.node = NewNode(nCtx)
	s.admin = newAdminServer(s.db, s.stopper)
	s.status = newStatusServer(s.db, s.gossip, ctx)
	s.tsDB = ts.NewDB(s.db)
	s.tsServer = ts.NewServer(s.tsDB)

	return s, nil
}

//.........这里部分代码省略.........
					trace.Event("coordinator spawns")
					txnMeta = &txnMetadata{
						txn:              *txn,
						keys:             cache.NewIntervalCache(cache.Config{Policy: cache.CacheNone}),
						firstUpdateNanos: tc.clock.PhysicalNow(),
						lastUpdateNanos:  tc.clock.PhysicalNow(),
						timeoutDuration:  tc.clientTimeout,
						txnEnd:           make(chan struct{}),
					}
					tc.txns[id] = txnMeta
					if !tc.stopper.RunAsyncTask(func() {
						tc.heartbeatLoop(id)
					}) {
						// The system is already draining and we can't start the
						// heartbeat. We refuse new transactions for now because
						// they're likely not going to have all intents committed.
						// In principle, we can relax this as needed though.
						call.Reply.Header().SetGoError(&proto.NodeUnavailableError{})
						tc.Unlock()
						tc.unregisterTxn(id)
						return
					}
				}
				txnMeta.addKeyRange(header.Key, header.EndKey)
			}
			// Update our record of this transaction.
			if txnMeta != nil {
				txnMeta.txn = *txn
				txnMeta.setLastUpdate(tc.clock.PhysicalNow())
			}
		}
		tc.Unlock()
	}

	// Cleanup intents and transaction map if end of transaction.
	switch t := call.Reply.Header().GoError().(type) {
	case *proto.TransactionStatusError:
		// Likely already committed or more obscure errors such as epoch or
		// timestamp regressions; consider it dead.
		tc.cleanupTxn(trace, t.Txn)
	case *proto.TransactionAbortedError:
		// If already aborted, cleanup the txn on this TxnCoordSender.
		tc.cleanupTxn(trace, t.Txn)
	case *proto.OpRequiresTxnError:
		// Run a one-off transaction with that single command.
		if log.V(1) {
			log.Infof("%s: auto-wrapping in txn and re-executing", call.Method())
		}
		// TODO(tschottdorf): this part is awkward. Consider resending here
		// without starting a new call, which is hard to trace. Plus, the
		// below depends on default configuration.
		tmpDB, err := client.Open(
			fmt.Sprintf("//%s?priority=%d",
				call.Args.Header().User, call.Args.Header().GetUserPriority()),
			client.SenderOpt(tc))
		if err != nil {
			log.Warning(err)
			return
		}
		call.Reply.Reset()
		if err := tmpDB.Txn(func(txn *client.Txn) error {
			txn.SetDebugName("auto-wrap", 0)
			b := &client.Batch{}
			b.InternalAddCall(call)
			return txn.CommitInBatch(b)
		}); err != nil {
			log.Warning(err)
		}
	case nil:
		if txn := call.Reply.Header().Txn; txn != nil {
			if _, ok := call.Args.(*proto.EndTransactionRequest); ok {
				// If the --linearizable flag is set, we want to make sure that
				// all the clocks in the system are past the commit timestamp
				// of the transaction. This is guaranteed if either
				// - the commit timestamp is MaxOffset behind startNS
				// - MaxOffset ns were spent in this function
				// when returning to the client. Below we choose the option
				// that involves less waiting, which is likely the first one
				// unless a transaction commits with an odd timestamp.
				if tsNS := txn.Timestamp.WallTime; startNS > tsNS {
					startNS = tsNS
				}
				sleepNS := tc.clock.MaxOffset() -
					time.Duration(tc.clock.PhysicalNow()-startNS)
				if tc.linearizable && sleepNS > 0 {
					defer func() {
						if log.V(1) {
							log.Infof("%v: waiting %s on EndTransaction for linearizability", txn.Short(), util.TruncateDuration(sleepNS, time.Millisecond))
						}
						time.Sleep(sleepNS)
					}()
				}
				if txn.Status != proto.PENDING {
					tc.cleanupTxn(trace, *txn)
				}

			}
		}
	}
}

// sendOne sends a single call via the wrapped sender. If the call is
// part of a transaction, the TxnCoordSender adds the transaction to a
// map of active transactions and begins heartbeating it. Every
// subsequent call for the same transaction updates the lastUpdate
// timestamp to prevent live transactions from being considered
// abandoned and garbage collected. Read/write mutating requests have
// their key or key range added to the transaction's interval tree of
// key ranges for eventual cleanup via resolved write intents.
//
// On success, and if the call is part of a transaction, the affected
// key range is recorded as live intents for eventual cleanup upon
// transaction commit. Upon successful txn commit, initiates cleanup
// of intents.
func (tc *TxnCoordSender) sendOne(call proto.Call) {
	var startNS int64
	header := call.Args.Header()
	// If this call is part of a transaction...
	if header.Txn != nil {
		// Set the timestamp to the original timestamp for read-only
		// commands and to the transaction timestamp for read/write
		// commands.
		if proto.IsReadOnly(call.Args) {
			header.Timestamp = header.Txn.OrigTimestamp
		} else {
			header.Timestamp = header.Txn.Timestamp
		}
		// EndTransaction must have its key set to that of the txn.
		if _, ok := call.Args.(*proto.EndTransactionRequest); ok {
			header.Key = header.Txn.Key
			// Remember when EndTransaction started in case we want to
			// be linearizable.
			startNS = tc.clock.PhysicalNow()
		}
	}

	// Send the command through wrapped sender.
	tc.wrapped.Send(context.TODO(), call)

	if header.Txn != nil {
		// If not already set, copy the request txn.
		if call.Reply.Header().Txn == nil {
			call.Reply.Header().Txn = gogoproto.Clone(header.Txn).(*proto.Transaction)
		}
		tc.updateResponseTxn(header, call.Reply.Header())
	}

	if txn := call.Reply.Header().Txn; txn != nil {
		tc.Lock()
		txnMeta := tc.txns[string(txn.ID)]
		// If this transactional command leaves transactional intents, add the key
		// or key range to the intents map. If the transaction metadata doesn't yet
		// exist, create it.
		if call.Reply.Header().GoError() == nil {
			if proto.IsTransactionWrite(call.Args) {
				if txnMeta == nil {
					txnMeta = &txnMetadata{
						txn:              *txn,
						keys:             cache.NewIntervalCache(cache.Config{Policy: cache.CacheNone}),
						firstUpdateNanos: tc.clock.PhysicalNow(),
						lastUpdateNanos:  tc.clock.PhysicalNow(),
						timeoutDuration:  tc.clientTimeout,
						txnEnd:           make(chan struct{}),
					}
					id := string(txn.ID)
					tc.txns[id] = txnMeta
					tc.heartbeat(id)
				}
				txnMeta.addKeyRange(header.Key, header.EndKey)
			}
			// Update our record of this transaction.
			if txnMeta != nil {
				txnMeta.txn = *txn
				txnMeta.setLastUpdate(tc.clock.PhysicalNow())
			}
		}
		tc.Unlock()
	}

	// Cleanup intents and transaction map if end of transaction.
	switch t := call.Reply.Header().GoError().(type) {
	case *proto.TransactionStatusError:
		// Likely already committed or more obscure errors such as epoch or
		// timestamp regressions; consider it dead.
		tc.cleanupTxn(t.Txn, nil)
	case *proto.TransactionAbortedError:
		// If already aborted, cleanup the txn on this TxnCoordSender.
		tc.cleanupTxn(t.Txn, nil)
	case *proto.OpRequiresTxnError:
		// Run a one-off transaction with that single command.
		if log.V(1) {
			log.Infof("%s: auto-wrapping in txn and re-executing", call.Method())
		}
		tmpDB, err := client.Open(
			fmt.Sprintf("//%s?priority=%d",
				call.Args.Header().User, call.Args.Header().GetUserPriority()),
			client.SenderOpt(tc))
		if err != nil {
			log.Warning(err)
			return
		}
//.........这里部分代码省略.........

// verifyUncertainty writes values to a key in 5ns intervals and then launches
// a transaction at each value's timestamp reading that value with
// the maximumOffset given, verifying in the process that the correct values
// are read (usually after one transaction restart).
func verifyUncertainty(concurrency int, maxOffset time.Duration, t *testing.T) {
	s := createTestDB(t)
	defer s.Stop()

	key := []byte("key-test")
	// wgStart waits for all transactions to line up, wgEnd has the main
	// function wait for them to finish.
	var wgStart, wgEnd sync.WaitGroup
	wgStart.Add(concurrency + 1)
	wgEnd.Add(concurrency)

	// Initial high offset to allow for future writes.
	s.Clock.SetMaxOffset(999 * time.Nanosecond)
	s.Manual.Set(s.Clock.MaxOffset().Nanoseconds() + 1)
	for i := 0; i < concurrency; i++ {
		value := []byte(fmt.Sprintf("value-%d", i))
		// Values will be written with 5ns spacing.
		futureTS := s.Clock.Now().Add(5, 0)
		s.Clock.Update(futureTS)
		// Expected number of versions skipped.
		skipCount := int(maxOffset) / 5
		if i+skipCount >= concurrency {
			skipCount = concurrency - i - 1
		}
		readValue := []byte(fmt.Sprintf("value-%d", i+skipCount))
		if err := s.DB.Put(key, value); err != nil {
			t.Errorf("%d: got write error: %s", i, err)
		}
		if gr, err := s.DB.Get(key); err != nil {
			t.Fatalf("%d: expected success reading value: %s", i, err)
		} else if !gr.Exists() || !bytes.Equal(gr.ValueBytes(), value) {
			t.Fatalf("%d: expected success reading value: %v", i, gr.Value)
		}

		go func(i int) {
			defer wgEnd.Done()
			wgStart.Done()
			// Wait until the other goroutines are running.
			wgStart.Wait()

			txnManual := hlc.NewManualClock(futureTS.WallTime)
			txnClock := hlc.NewClock(txnManual.UnixNano)
			// Make sure to incorporate the logical component if the wall time
			// hasn't changed (i=0). The logical component will change
			// internally in a way we can't track, but we want to be just
			// ahead.
			txnClock.Update(futureTS.Add(0, 999))
			// The written values are spaced out in intervals of 5ns, so
			// setting <5ns here should make do without any restarts while
			// higher values require roughly offset/5 restarts.
			txnClock.SetMaxOffset(maxOffset)

			sender := NewTxnCoordSender(s.lSender, txnClock, false, nil, s.Stopper)
			txnDB, err := client.Open("//", client.SenderOpt(sender))
			if err != nil {
				t.Fatal(err)
			}

			if err := txnDB.Txn(func(txn *client.Txn) error {
				// Read within the transaction.
				gr, err := txn.Get(key)
				if err != nil {
					if _, ok := err.(*proto.ReadWithinUncertaintyIntervalError); ok {
						return err
					}
					return util.Errorf("unexpected read error of type %s: %s", reflect.TypeOf(err), err)
				}
				if !gr.Exists() {
					return util.Errorf("no value read")
				}
				if !bytes.Equal(gr.ValueBytes(), readValue) {
					return util.Errorf("%d: read wrong value %v at %s, wanted %q",
						i, gr.Value, futureTS, readValue)
				}
				return nil
			}); err != nil {
				t.Error(err)
			}
		}(i)
	}
	// Kick the goroutines loose.
	wgStart.Done()
	// Wait for the goroutines to finish.
	wgEnd.Wait()
}