Merge pull request #10889 from tbg/joint-conf-change-logic

raft: internally support joint consensus

raft/confchange/confchange.go

@@ -0,0 +1,420 @@
+// Copyright 2019 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package confchange
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"go.etcd.io/etcd/raft/quorum"
+	pb "go.etcd.io/etcd/raft/raftpb"
+	"go.etcd.io/etcd/raft/tracker"
+)
+
+// Changer facilitates configuration changes. It exposes methods to handle
+// simple and joint consensus while performing the proper validation that allows
+// refusing invalid configuration changes before they affect the active
+// configuration.
+type Changer struct {
+	Tracker   tracker.ProgressTracker
+	LastIndex uint64
+}
+
+// EnterJoint verifies that the outgoing (=right) majority config of the joint
+// config is empty and initializes it with a copy of the incoming (=left)
+// majority config. That is, it transitions from
+//
+//     (1 2 3)&&()
+// to
+//     (1 2 3)&&(1 2 3).
+//
+// The supplied ConfChanges are then applied to the incoming majority config,
+// resulting in a joint configuration that in terms of the Raft thesis[1]
+// (Section 4.3) corresponds to `C_{new,old}`.
+//
+// [1]: https://github.com/ongardie/dissertation/blob/master/online-trim.pdf
+func (c Changer) EnterJoint(ccs ...pb.ConfChange) (tracker.Config, tracker.ProgressMap, error) {
+	cfg, prs, err := c.checkAndCopy()
+	if err != nil {
+		return c.err(err)
+	}
+	if joint(cfg) {
+		err := errors.New("config is already joint")
+		return c.err(err)
+	}
+	if len(incoming(cfg.Voters)) == 0 {
+		// We allow adding nodes to an empty config for convenience (testing and
+		// bootstrap), but you can't enter a joint state.
+		err := errors.New("can't make a zero-voter config joint")
+		return c.err(err)
+	}
+	// Clear the outgoing config.
+	{
+		*outgoingPtr(&cfg.Voters) = quorum.MajorityConfig{}
+
+	}
+	// Copy incoming to outgoing.
+	for id := range incoming(cfg.Voters) {
+		outgoing(cfg.Voters)[id] = struct{}{}
+	}
+
+	if err := c.apply(&cfg, prs, ccs...); err != nil {
+		return c.err(err)
+	}
+
+	return checkAndReturn(cfg, prs)
+}
+
+// LeaveJoint transitions out of a joint configuration. It is an error to call
+// this method if the configuration is not joint, i.e. if the outgoing majority
+// config Voters[1] is empty.
+//
+// The outgoing majority config of the joint configuration will be removed,
+// that is, the incoming config is promoted as the sole decision maker. In the
+// notation of the Raft thesis[1] (Section 4.3), this method transitions from
+// `C_{new,old}` into `C_new`.
+//
+// At the same time, any staged learners (LearnersNext) the addition of which
+// was held back by an overlapping voter in the former outgoing config will be
+// inserted into Learners.
+//
+// [1]: https://github.com/ongardie/dissertation/blob/master/online-trim.pdf
+func (c Changer) LeaveJoint() (tracker.Config, tracker.ProgressMap, error) {
+	cfg, prs, err := c.checkAndCopy()
+	if err != nil {
+		return c.err(err)
+	}
+	if !joint(cfg) {
+		err := errors.New("can't leave a non-joint config")
+		return c.err(err)
+	}
+	if len(outgoing(cfg.Voters)) == 0 {
+		err := fmt.Errorf("configuration is not joint: %v", cfg)
+		return c.err(err)
+	}
+	for id := range cfg.LearnersNext {
+		nilAwareAdd(&cfg.Learners, id)
+		prs[id].IsLearner = true
+	}
+	cfg.LearnersNext = nil
+
+	for id := range outgoing(cfg.Voters) {
+		_, isVoter := incoming(cfg.Voters)[id]
+		_, isLearner := cfg.Learners[id]
+
+		if !isVoter && !isLearner {
+			delete(prs, id)
+		}
+	}
+	*outgoingPtr(&cfg.Voters) = nil
+
+	return checkAndReturn(cfg, prs)
+}
+
+// Simple carries out a series of configuration changes that (in aggregate)
+// mutates the incoming majority config Voters[0] by at most one. This method
+// will return an error if that is not the case, if the resulting quorum is
+// zero, or if the configuration is in a joint state (i.e. if there is an
+// outgoing configuration).
+func (c Changer) Simple(ccs ...pb.ConfChange) (tracker.Config, tracker.ProgressMap, error) {
+	cfg, prs, err := c.checkAndCopy()
+	if err != nil {
+		return c.err(err)
+	}
+	if joint(cfg) {
+		err := errors.New("can't apply simple config change in joint config")
+		return c.err(err)
+	}
+	if err := c.apply(&cfg, prs, ccs...); err != nil {
+		return c.err(err)
+	}
+	if n := symdiff(incoming(c.Tracker.Voters), incoming(cfg.Voters)); n > 1 {
+		return tracker.Config{}, nil, errors.New("more than voter changed without entering joint config")
+	}
+	if err := checkInvariants(cfg, prs); err != nil {
+		return tracker.Config{}, tracker.ProgressMap{}, nil
+	}
+
+	return checkAndReturn(cfg, prs)
+}
+
+// apply a ConfChange to the configuration. By convention, changes to voters are
+// always made to the incoming majority config Voters[0]. Voters[1] is either
+// empty or preserves the outgoing majority configuration while in a joint state.
+func (c Changer) apply(cfg *tracker.Config, prs tracker.ProgressMap, ccs ...pb.ConfChange) error {
+	for _, cc := range ccs {
+		if cc.NodeID == 0 {
+			// etcd replaces the NodeID with zero if it decides (downstream of
+			// raft) to not apply a ConfChange, so we have to have explicit code
+			// here to ignore these.
+			continue
+		}
+		switch cc.Type {
+		case pb.ConfChangeAddNode:
+			c.makeVoter(cfg, prs, cc.NodeID)
+		case pb.ConfChangeAddLearnerNode:
+			c.makeLearner(cfg, prs, cc.NodeID)
+		case pb.ConfChangeRemoveNode:
+			c.remove(cfg, prs, cc.NodeID)
+		case pb.ConfChangeUpdateNode:
+		default:
+			return fmt.Errorf("unexpected conf type %d", cc.Type)
+		}
+	}
+	if len(incoming(cfg.Voters)) == 0 {
+		return errors.New("removed all voters")
+	}
+	return nil
+}
+
+// makeVoter adds or promotes the given ID to be a voter in the incoming
+// majority config.
+func (c Changer) makeVoter(cfg *tracker.Config, prs tracker.ProgressMap, id uint64) {
+	pr := prs[id]
+	if pr == nil {
+		c.initProgress(cfg, prs, id, false /* isLearner */)
+		return
+	}
+
+	pr.IsLearner = false
+	nilAwareDelete(&cfg.Learners, id)
+	nilAwareDelete(&cfg.LearnersNext, id)
+	incoming(cfg.Voters)[id] = struct{}{}
+	return
+}
+
+// makeLearner makes the given ID a learner or stages it to be a learner once
+// an active joint configuration is exited.
+//
+// The former happens when the peer is not a part of the outgoing config, in
+// which case we either add a new learner or demote a voter in the incoming
+// config.
+//
+// The latter case occurs when the configuration is joint and the peer is a
+// voter in the outgoing config. In that case, we do not want to add the peer
+// as a learner because then we'd have to track a peer as a voter and learner
+// simultaneously. Instead, we add the learner to LearnersNext, so that it will
+// be added to Learners the moment the outgoing config is removed by
+// LeaveJoint().
+func (c Changer) makeLearner(cfg *tracker.Config, prs tracker.ProgressMap, id uint64) {
+	pr := prs[id]
+	if pr == nil {
+		c.initProgress(cfg, prs, id, true /* isLearner */)
+		return
+	}
+	if pr.IsLearner {
+		return
+	}
+	// Remove any existing voter in the incoming config...
+	c.remove(cfg, prs, id)
+	// ... but save the Progress.
+	prs[id] = pr
+	// Use LearnersNext if we can't add the learner to Learners directly, i.e.
+	// if the peer is still tracked as a voter in the outgoing config. It will
+	// be turned into a learner in LeaveJoint().
+	//
+	// Otherwise, add a regular learner right away.
+	if _, onRight := outgoing(cfg.Voters)[id]; onRight {
+		nilAwareAdd(&cfg.LearnersNext, id)
+	} else {
+		pr.IsLearner = true
+		nilAwareAdd(&cfg.Learners, id)
+	}
+}
+
+// remove this peer as a voter or learner from the incoming config.
+func (c Changer) remove(cfg *tracker.Config, prs tracker.ProgressMap, id uint64) {
+	if _, ok := prs[id]; !ok {
+		return
+	}
+
+	delete(incoming(cfg.Voters), id)
+	nilAwareDelete(&cfg.Learners, id)
+	nilAwareDelete(&cfg.LearnersNext, id)
+
+	// If the peer is still a voter in the outgoing config, keep the Progress.
+	if _, onRight := outgoing(cfg.Voters)[id]; !onRight {
+		delete(prs, id)
+	}
+}
+
+// initProgress initializes a new progress for the given node or learner.
+func (c Changer) initProgress(cfg *tracker.Config, prs tracker.ProgressMap, id uint64, isLearner bool) {
+	if !isLearner {
+		incoming(cfg.Voters)[id] = struct{}{}
+	} else {
+		nilAwareAdd(&cfg.Learners, id)
+	}
+	prs[id] = &tracker.Progress{
+		// We initialize Progress.Next with lastIndex+1 so that the peer will be
+		// probed without an index first.
+		//
+		// TODO(tbg): verify that, this is just my best guess.
+		Next:      c.LastIndex + 1,
+		Match:     0,
+		Inflights: tracker.NewInflights(c.Tracker.MaxInflight),
+		IsLearner: isLearner,
+		// When a node is first added, we should mark it as recently active.
+		// Otherwise, CheckQuorum may cause us to step down if it is invoked
+		// before the added node has had a chance to communicate with us.
+		RecentActive: true,
+	}
+}
+
+// checkInvariants makes sure that the config and progress are compatible with
+// each other. This is used to check both what the Changer is initialized with,
+// as well as what it returns.
+func checkInvariants(cfg tracker.Config, prs tracker.ProgressMap) error {
+	// NB: intentionally allow the empty config. In production we'll never see a
+	// non-empty config (we prevent it from being created) but we will need to
+	// be able to *create* an initial config, for example during bootstrap (or
+	// during tests). Instead of having to hand-code this, we allow
+	// transitioning from an empty config into any other legal and non-empty
+	// config.
+	for _, ids := range []map[uint64]struct{}{
+		cfg.Voters.IDs(),
+		cfg.Learners,
+		cfg.LearnersNext,
+	} {
+		for id := range ids {
+			if _, ok := prs[id]; !ok {
+				return fmt.Errorf("no progress for %d", id)
+			}
+		}
+	}
+
+	// Any staged learner was staged because it could not be directly added due
+	// to a conflicting voter in the outgoing config.
+	for id := range cfg.LearnersNext {
+		if _, ok := outgoing(cfg.Voters)[id]; !ok {
+			return fmt.Errorf("%d is in LearnersNext, but not Voters[1]", id)
+		}
+		if prs[id].IsLearner {
+			return fmt.Errorf("%d is in LearnersNext, but is already marked as learner", id)
+		}
+	}
+	// Conversely Learners and Voters doesn't intersect at all.
+	for id := range cfg.Learners {
+		if _, ok := outgoing(cfg.Voters)[id]; ok {
+			return fmt.Errorf("%d is in Learners and Voters[1]", id)
+		}
+		if _, ok := incoming(cfg.Voters)[id]; ok {
+			return fmt.Errorf("%d is in Learners and Voters[0]", id)
+		}
+		if !prs[id].IsLearner {
+			return fmt.Errorf("%d is in Learners, but is not marked as learner", id)
+		}
+	}
+
+	if !joint(cfg) {
+		// We enforce that empty maps are nil instead of zero.
+		if outgoing(cfg.Voters) != nil {
+			return fmt.Errorf("Voters[1] must be nil when not joint")
+		}
+		if cfg.LearnersNext != nil {
+			return fmt.Errorf("LearnersNext must be nil when not joint")
+		}
+	}
+
+	return nil
+}
+
+// checkAndCopy copies the tracker's config and progress map (deeply enough for
+// the purposes of the Changer) and returns those copies. It returns an error
+// if checkInvariants does.
+func (c Changer) checkAndCopy() (tracker.Config, tracker.ProgressMap, error) {
+	cfg := c.Tracker.Config.Clone()
+	prs := tracker.ProgressMap{}
+
+	for id, pr := range c.Tracker.Progress {
+		// A shallow copy is enough because we only mutate the Learner field.
+		ppr := *pr
+		prs[id] = &ppr
+	}
+	return checkAndReturn(cfg, prs)
+}
+
+// checkAndReturn calls checkInvariants on the input and returns either the
+// resulting error or the input.
+func checkAndReturn(cfg tracker.Config, prs tracker.ProgressMap) (tracker.Config, tracker.ProgressMap, error) {
+	if err := checkInvariants(cfg, prs); err != nil {
+		return tracker.Config{}, tracker.ProgressMap{}, err
+	}
+	return cfg, prs, nil
+}
+
+// err returns zero values and an error.
+func (c Changer) err(err error) (tracker.Config, tracker.ProgressMap, error) {
+	return tracker.Config{}, nil, err
+}
+
+// nilAwareAdd populates a map entry, creating the map if necessary.
+func nilAwareAdd(m *map[uint64]struct{}, id uint64) {
+	if *m == nil {
+		*m = map[uint64]struct{}{}
+	}
+	(*m)[id] = struct{}{}
+}
+
+// nilAwareDelete deletes from a map, nil'ing the map itself if it is empty after.
+func nilAwareDelete(m *map[uint64]struct{}, id uint64) {
+	if *m == nil {
+		return
+	}
+	delete(*m, id)
+	if len(*m) == 0 {
+		*m = nil
+	}
+}
+
+// symdiff returns the count of the symmetric difference between the sets of
+// uint64s, i.e. len( (l - r) \union (r - l)).
+func symdiff(l, r map[uint64]struct{}) int {
+	var n int
+	pairs := [][2]quorum.MajorityConfig{
+		{l, r}, // count elems in l but not in r
+		{r, l}, // count elems in r but not in l
+	}
+	for _, p := range pairs {
+		for id := range p[0] {
+			if _, ok := p[1][id]; !ok {
+				n++
+			}
+		}
+	}
+	return n
+}
+
+func joint(cfg tracker.Config) bool {
+	return len(outgoing(cfg.Voters)) > 0
+}
+
+func incoming(voters quorum.JointConfig) quorum.MajorityConfig      { return voters[0] }
+func outgoing(voters quorum.JointConfig) quorum.MajorityConfig      { return voters[1] }
+func outgoingPtr(voters *quorum.JointConfig) *quorum.MajorityConfig { return &voters[1] }
+
+// Describe prints the type and NodeID of the configuration changes as a
+// space-delimited string.
+func Describe(ccs ...pb.ConfChange) string {
+	var buf strings.Builder
+	for _, cc := range ccs {
+		if buf.Len() > 0 {
+			buf.WriteByte(' ')
+		}
+		fmt.Fprintf(&buf, "%s(%d)", cc.Type, cc.NodeID)
+	}
+	return buf.String()
+}

raft/confchange/datadriven_test.go

@@ -0,0 +1,105 @@
+// Copyright 2019 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package confchange
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/cockroachdb/datadriven"
+	pb "go.etcd.io/etcd/raft/raftpb"
+	"go.etcd.io/etcd/raft/tracker"
+)
+
+func TestConfChangeDataDriven(t *testing.T) {
+	datadriven.Walk(t, "testdata", func(t *testing.T, path string) {
+		tr := tracker.MakeProgressTracker(10)
+		c := Changer{
+			Tracker:   tr,
+			LastIndex: 0, // incremented in this test with each cmd
+		}
+
+		// The test files use the commands
+		// - simple: run a simple conf change (i.e. no joint consensus),
+		// - enter-joint: enter a joint config, and
+		// - leave-joint: leave a joint config.
+		// The first two take a list of config changes, which have the following
+		// syntax:
+		// - vn: make n a voter,
+		// - ln: make n a learner,
+		// - rn: remove n, and
+		// - un: update n.
+		datadriven.RunTest(t, path, func(d *datadriven.TestData) string {
+			defer func() {
+				c.LastIndex++
+			}()
+			var ccs []pb.ConfChange
+			toks := strings.Split(strings.TrimSpace(d.Input), " ")
+			if toks[0] == "" {
+				toks = nil
+			}
+			for _, tok := range toks {
+				if len(tok) < 2 {
+					return fmt.Sprintf("unknown token %s", tok)
+				}
+				var cc pb.ConfChange
+				switch tok[0] {
+				case 'v':
+					cc.Type = pb.ConfChangeAddNode
+				case 'l':
+					cc.Type = pb.ConfChangeAddLearnerNode
+				case 'r':
+					cc.Type = pb.ConfChangeRemoveNode
+				case 'u':
+					cc.Type = pb.ConfChangeUpdateNode
+				default:
+					return fmt.Sprintf("unknown input: %s", tok)
+				}
+				id, err := strconv.ParseUint(tok[1:], 10, 64)
+				if err != nil {
+					return err.Error()
+				}
+				cc.NodeID = id
+				ccs = append(ccs, cc)
+			}
+
+			var cfg tracker.Config
+			var prs tracker.ProgressMap
+			var err error
+			switch d.Cmd {
+			case "simple":
+				cfg, prs, err = c.Simple(ccs...)
+			case "enter-joint":
+				cfg, prs, err = c.EnterJoint(ccs...)
+			case "leave-joint":
+				if len(ccs) > 0 {
+					err = errors.New("this command takes no input")
+				} else {
+					cfg, prs, err = c.LeaveJoint()
+				}
+			default:
+				return "unknown command"
+			}
+			if err != nil {
+				return err.Error() + "\n"
+			}
+			c.Tracker.Config, c.Tracker.Progress = cfg, prs
+			return fmt.Sprintf("%s\n%s", c.Tracker.Config, c.Tracker.Progress)
+		})
+	})
+}

raft/confchange/quick_test.go

@@ -0,0 +1,168 @@
+// Copyright 2019 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package confchange
+
+import (
+	"math/rand"
+	"reflect"
+	"testing"
+	"testing/quick"
+
+	pb "go.etcd.io/etcd/raft/raftpb"
+	"go.etcd.io/etcd/raft/tracker"
+)
+
+// TestConfChangeQuick uses quickcheck to verify that simple and joint config
+// changes arrive at the same result.
+func TestConfChangeQuick(t *testing.T) {
+	cfg := &quick.Config{
+		MaxCount: 1000,
+	}
+
+	// Log the first couple of runs to give some indication of things working
+	// as intended.
+	const infoCount = 5
+
+	runWithJoint := func(c *Changer, ccs []pb.ConfChange) error {
+		cfg, prs, err := c.EnterJoint(ccs...)
+		if err != nil {
+			return err
+		}
+		c.Tracker.Config = cfg
+		c.Tracker.Progress = prs
+		cfg, prs, err = c.LeaveJoint()
+		if err != nil {
+			return err
+		}
+		c.Tracker.Config = cfg
+		c.Tracker.Progress = prs
+		return nil
+	}
+
+	runWithSimple := func(c *Changer, ccs []pb.ConfChange) error {
+		for _, cc := range ccs {
+			cfg, prs, err := c.Simple(cc)
+			if err != nil {
+				return err
+			}
+			c.Tracker.Config, c.Tracker.Progress = cfg, prs
+		}
+		return nil
+	}
+
+	type testFunc func(*Changer, []pb.ConfChange) error
+
+	wrapper := func(invoke testFunc) func(setup initialChanges, ccs confChanges) (*Changer, error) {
+		return func(setup initialChanges, ccs confChanges) (*Changer, error) {
+			tr := tracker.MakeProgressTracker(10)
+			c := &Changer{
+				Tracker:   tr,
+				LastIndex: 10,
+			}
+
+			if err := runWithSimple(c, setup); err != nil {
+				return nil, err
+			}
+
+			err := invoke(c, ccs)
+			return c, err
+		}
+	}
+
+	var n int
+	f1 := func(setup initialChanges, ccs confChanges) *Changer {
+		c, err := wrapper(runWithSimple)(setup, ccs)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if n < infoCount {
+			t.Log("initial setup:", Describe(setup...))
+			t.Log("changes:", Describe(ccs...))
+			t.Log(c.Tracker.Config)
+			t.Log(c.Tracker.Progress)
+		}
+		n++
+		return c
+	}
+	f2 := func(setup initialChanges, ccs confChanges) *Changer {
+		c, err := wrapper(runWithJoint)(setup, ccs)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return c
+	}
+	err := quick.CheckEqual(f1, f2, cfg)
+	if err == nil {
+		return
+	}
+	cErr, ok := err.(*quick.CheckEqualError)
+	if !ok {
+		t.Fatal(err)
+	}
+
+	t.Error("setup:", Describe(cErr.In[0].([]pb.ConfChange)...))
+	t.Error("ccs:", Describe(cErr.In[1].([]pb.ConfChange)...))
+	t.Errorf("out1: %+v\nout2: %+v", cErr.Out1, cErr.Out2)
+}
+
+type confChangeTyp pb.ConfChangeType
+
+func (confChangeTyp) Generate(rand *rand.Rand, _ int) reflect.Value {
+	return reflect.ValueOf(confChangeTyp(rand.Intn(4)))
+}
+
+type confChanges []pb.ConfChange
+
+func genCC(num func() int, id func() uint64, typ func() pb.ConfChangeType) []pb.ConfChange {
+	var ccs []pb.ConfChange
+	n := num()
+	for i := 0; i < n; i++ {
+		ccs = append(ccs, pb.ConfChange{Type: typ(), NodeID: id()})
+	}
+	return ccs
+}
+
+func (confChanges) Generate(rand *rand.Rand, _ int) reflect.Value {
+	num := func() int {
+		return 1 + rand.Intn(9)
+	}
+	id := func() uint64 {
+		// Note that num() >= 1, so we're never returning 1 from this method,
+		// meaning that we'll never touch NodeID one, which is special to avoid
+		// voterless configs altogether in this test.
+		return 1 + uint64(num())
+	}
+	typ := func() pb.ConfChangeType {
+		return pb.ConfChangeType(rand.Intn(len(pb.ConfChangeType_name)))
+	}
+	return reflect.ValueOf(genCC(num, id, typ))
+}
+
+type initialChanges []pb.ConfChange
+
+func (initialChanges) Generate(rand *rand.Rand, _ int) reflect.Value {
+	num := func() int {
+		return 1 + rand.Intn(5)
+	}
+	id := func() uint64 { return uint64(num()) }
+	typ := func() pb.ConfChangeType {
+		return pb.ConfChangeAddNode
+	}
+	// NodeID one is special - it's in the initial config and will be a voter
+	// always (this is to avoid uninteresting edge cases where the simple conf
+	// changes can't easily make progress).
+	ccs := append([]pb.ConfChange{{Type: pb.ConfChangeAddNode, NodeID: 1}}, genCC(num, id, typ)...)
+	return reflect.ValueOf(ccs)
+}

raft/confchange/testdata/joint_idempotency.txt

@@ -0,0 +1,23 @@
+# Verify that operations upon entering the joint state are idempotent, i.e.
+# removing an absent node is fine, etc.
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=1
+
+enter-joint
+r1 r2 r9 v2 v3 v4 v2 v3 v4 l2 l2 r4 r4 l1 l1
+----
+voters=(3)&&(1) learners=(2) learners_next=(1)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2 learner
+3: StateProbe match=0 next=2
+
+leave-joint
+----
+voters=(3) learners=(1 2)
+1: StateProbe match=0 next=1 learner
+2: StateProbe match=0 next=2 learner
+3: StateProbe match=0 next=2

raft/confchange/testdata/joint_learners_next.txt

@@ -0,0 +1,24 @@
+# Verify that when a voter is demoted in a joint config, it will show up in
+# learners_next until the joint config is left, and only then will the progress
+# turn into that of a learner, without resetting the progress. Note that this
+# last fact is verified by `next`, which can tell us which "round" the progress
+# was originally created in.
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=1
+
+enter-joint
+v2 l1
+----
+voters=(2)&&(1) learners_next=(1)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2
+
+leave-joint
+----
+voters=(2) learners=(1)
+1: StateProbe match=0 next=1 learner
+2: StateProbe match=0 next=2

raft/confchange/testdata/joint_safety.txt

@@ -0,0 +1,81 @@
+leave-joint
+----
+can't leave a non-joint config
+
+enter-joint
+----
+can't make a zero-voter config joint
+
+enter-joint
+v1
+----
+can't make a zero-voter config joint
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=4
+
+leave-joint
+----
+can't leave a non-joint config
+
+# Can enter into joint config.
+enter-joint
+----
+voters=(1)&&(1)
+1: StateProbe match=0 next=4
+
+enter-joint
+----
+config is already joint
+
+leave-joint
+----
+voters=(1)
+1: StateProbe match=0 next=4
+
+leave-joint
+----
+can't leave a non-joint config
+
+# Can enter again, this time with some ops.
+enter-joint
+r1 v2 v3 l4
+----
+voters=(2 3)&&(1) learners=(4)
+1: StateProbe match=0 next=4
+2: StateProbe match=0 next=10
+3: StateProbe match=0 next=10
+4: StateProbe match=0 next=10 learner
+
+enter-joint
+----
+config is already joint
+
+enter-joint
+v12
+----
+config is already joint
+
+simple
+l15
+----
+can't apply simple config change in joint config
+
+leave-joint
+----
+voters=(2 3) learners=(4)
+2: StateProbe match=0 next=10
+3: StateProbe match=0 next=10
+4: StateProbe match=0 next=10 learner
+
+simple
+l9
+----
+voters=(2 3) learners=(4 9)
+2: StateProbe match=0 next=10
+3: StateProbe match=0 next=10
+4: StateProbe match=0 next=10 learner
+9: StateProbe match=0 next=15 learner

raft/confchange/testdata/simple_idempotency.txt

@@ -0,0 +1,69 @@
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=1
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=1
+
+simple
+v2
+----
+voters=(1 2)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=3
+
+simple
+l1
+----
+voters=(2) learners=(1)
+1: StateProbe match=0 next=1 learner
+2: StateProbe match=0 next=3
+
+simple
+l1
+----
+voters=(2) learners=(1)
+1: StateProbe match=0 next=1 learner
+2: StateProbe match=0 next=3
+
+simple
+r1
+----
+voters=(2)
+2: StateProbe match=0 next=3
+
+simple
+r1
+----
+voters=(2)
+2: StateProbe match=0 next=3
+
+simple
+v3
+----
+voters=(2 3)
+2: StateProbe match=0 next=3
+3: StateProbe match=0 next=8
+
+simple
+r3
+----
+voters=(2)
+2: StateProbe match=0 next=3
+
+simple
+r3
+----
+voters=(2)
+2: StateProbe match=0 next=3
+
+simple
+r4
+----
+voters=(2)
+2: StateProbe match=0 next=3

raft/confchange/testdata/simple_promote_demote.txt

@@ -0,0 +1,60 @@
+# Set up three voters for this test.
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=1
+
+simple
+v2
+----
+voters=(1 2)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2
+
+simple
+v3
+----
+voters=(1 2 3)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2
+3: StateProbe match=0 next=3
+
+# Can atomically demote and promote without a hitch.
+# This is pointless, but possible.
+simple
+l1 v1
+----
+voters=(1 2 3)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2
+3: StateProbe match=0 next=3
+
+# Can demote a voter.
+simple
+l2
+----
+voters=(1 3) learners=(2)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2 learner
+3: StateProbe match=0 next=3
+
+# Can atomically promote and demote the same voter.
+# This is pointless, but possible.
+simple
+v2 l2
+----
+voters=(1 3) learners=(2)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2 learner
+3: StateProbe match=0 next=3
+
+# Can promote a voter.
+simple
+v2
+----
+voters=(1 2 3)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2
+3: StateProbe match=0 next=3

raft/confchange/testdata/simple_safety.txt

@@ -0,0 +1,64 @@
+simple
+l1
+----
+removed all voters
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=2
+
+simple
+v2 l3
+----
+voters=(1 2) learners=(3)
+1: StateProbe match=0 next=2
+2: StateProbe match=0 next=3
+3: StateProbe match=0 next=3 learner
+
+simple
+r1 v5
+----
+more than voter changed without entering joint config
+
+simple
+r1 r2
+----
+removed all voters
+
+simple
+v3 v4
+----
+more than voter changed without entering joint config
+
+simple
+l1 v5
+----
+more than voter changed without entering joint config
+
+simple
+l1 l2
+----
+removed all voters
+
+simple
+l2 l3 l4 l5
+----
+voters=(1) learners=(2 3 4 5)
+1: StateProbe match=0 next=2
+2: StateProbe match=0 next=3 learner
+3: StateProbe match=0 next=3 learner
+4: StateProbe match=0 next=9 learner
+5: StateProbe match=0 next=9 learner
+
+simple
+r1
+----
+removed all voters
+
+simple
+r2 r3 r4 r5
+----
+voters=(1)
+1: StateProbe match=0 next=2

raft/confchange/testdata/update.txt

@@ -0,0 +1,23 @@
+# Nobody cares about ConfChangeUpdateNode, but at least use it once. It is used
+# by etcd as a convenient way to pass a blob through their conf change machinery
+# that updates information tracked outside of raft.
+
+simple
+v1
+----
+voters=(1)
+1: StateProbe match=0 next=1
+
+simple
+v2 u1
+----
+voters=(1 2)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2
+
+simple
+u1 u2 u3 u1 u2 u3
+----
+voters=(1 2)
+1: StateProbe match=0 next=1
+2: StateProbe match=0 next=2

raft/confchange/testdata/zero.txt

@@ -0,0 +1,6 @@
+# NodeID zero is ignored.
+simple
+v1 r0 v0 l0
+----
+voters=(1)
+1: StateProbe match=0 next=1

raft/raft.go

@@ -24,6 +24,7 @@ import (
 	"sync"
 	"time"
 
+	"go.etcd.io/etcd/raft/confchange"
 	"go.etcd.io/etcd/raft/quorum"
 	pb "go.etcd.io/etcd/raft/raftpb"
 	"go.etcd.io/etcd/raft/tracker"
@@ -356,15 +357,11 @@ func newRaft(c *Config) *raft {
 	}
 	for _, p := range peers {
 		// Add node to active config.
-		r.prs.InitProgress(p, 0 /* match */, 1 /* next */, false /* isLearner */)
+		r.applyConfChange(pb.ConfChange{Type: pb.ConfChangeAddNode, NodeID: p})
 	}
 	for _, p := range learners {
 		// Add learner to active config.
-		r.prs.InitProgress(p, 0 /* match */, 1 /* next */, true /* isLearner */)
-
-		if r.id == p {
-			r.isLearner = true
-		}
+		r.applyConfChange(pb.ConfChange{Type: pb.ConfChangeAddLearnerNode, NodeID: p})
 	}
 
 	if !isHardStateEqual(hs, emptyState) {
@@ -1401,55 +1398,15 @@ func (r *raft) promotable() bool {
 }
 
 func (r *raft) applyConfChange(cc pb.ConfChange) pb.ConfState {
-	addNodeOrLearnerNode := func(id uint64, isLearner bool) {
-		// NB: this method is intentionally hidden from view. All mutations of
-		// the conf state must call applyConfChange directly.
-		pr := r.prs.Progress[id]
-		if pr == nil {
-			r.prs.InitProgress(id, 0, r.raftLog.lastIndex()+1, isLearner)
-		} else {
-			if isLearner && !pr.IsLearner {
-				// Can only change Learner to Voter.
-				//
-				// TODO(tbg): why?
-				r.logger.Infof("%x ignored addLearner: do not support changing %x from raft peer to learner.", r.id, id)
-				return
-			}
-
-			if isLearner == pr.IsLearner {
-				// Ignore any redundant addNode calls (which can happen because the
-				// initial bootstrapping entries are applied twice).
-				return
-			}
-
-			// Change Learner to Voter, use origin Learner progress.
-			r.prs.RemoveAny(id)
-			r.prs.InitProgress(id, 0 /* match */, 1 /* next */, false /* isLearner */)
-			pr.IsLearner = false
-			*r.prs.Progress[id] = *pr
-		}
-
-		// When a node is first added, we should mark it as recently active.
-		// Otherwise, CheckQuorum may cause us to step down if it is invoked
-		// before the added node has had a chance to communicate with us.
-		r.prs.Progress[id].RecentActive = true
-	}
-
-	var removed int
-	if cc.NodeID != None {
-		switch cc.Type {
-		case pb.ConfChangeAddNode:
-			addNodeOrLearnerNode(cc.NodeID, false /* isLearner */)
-		case pb.ConfChangeAddLearnerNode:
-			addNodeOrLearnerNode(cc.NodeID, true /* isLearner */)
-		case pb.ConfChangeRemoveNode:
-			removed++
-			r.prs.RemoveAny(cc.NodeID)
-		case pb.ConfChangeUpdateNode:
-		default:
-			panic("unexpected conf type")
-		}
+	cfg, prs, err := confchange.Changer{
+		Tracker:   r.prs,
+		LastIndex: r.raftLog.lastIndex(),
+	}.Simple(cc)
+	if err != nil {
+		panic(err)
 	}
+	r.prs.Config = cfg
+	r.prs.Progress = prs
 
 	r.logger.Infof("%x switched to configuration %s", r.id, r.prs.Config)
 	// Now that the configuration is updated, handle any side effects.
@@ -1479,12 +1436,10 @@ func (r *raft) applyConfChange(cc pb.ConfChange) pb.ConfState {
 	if r.state != StateLeader || len(cs.Nodes) == 0 {
 		return cs
 	}
-	if removed > 0 {
+	if r.maybeCommit() {
 		// The quorum size may have been reduced (but not to zero), so see if
 		// any pending entries can be committed.
-		if r.maybeCommit() {
-			r.bcastAppend()
-		}
+		r.bcastAppend()
 	}
 	// If the the leadTransferee was removed, abort the leadership transfer.
 	if _, tOK := r.prs.Progress[r.leadTransferee]; !tOK && r.leadTransferee != 0 {

raft/raft_test.go

@@ -1140,9 +1140,13 @@ func TestCommit(t *testing.T) {
 		storage.hardState = pb.HardState{Term: tt.smTerm}
 
 		sm := newTestRaft(1, []uint64{1}, 10, 2, storage)
-		sm.prs.RemoveAny(1)
 		for j := 0; j < len(tt.matches); j++ {
-			sm.prs.InitProgress(uint64(j)+1, tt.matches[j], tt.matches[j]+1, false)
+			id := uint64(j) + 1
+			if id > 1 {
+				sm.applyConfChange(pb.ConfChange{Type: pb.ConfChangeAddNode, NodeID: id})
+			}
+			pr := sm.prs.Progress[id]
+			pr.Match, pr.Next = tt.matches[j], tt.matches[j]+1
 		}
 		sm.maybeCommit()
 		if g := sm.raftLog.committed; g != tt.w {
@@ -1927,7 +1931,7 @@ func TestNonPromotableVoterWithCheckQuorum(t *testing.T) {
 	nt := newNetwork(a, b)
 	setRandomizedElectionTimeout(b, b.electionTimeout+1)
 	// Need to remove 2 again to make it a non-promotable node since newNetwork overwritten some internal states
-	b.prs.RemoveAny(2)
+	b.applyConfChange(pb.ConfChange{Type: pb.ConfChangeRemoveNode, NodeID: 2})
 
 	if b.promotable() {
 		t.Fatalf("promotable = %v, want false", b.promotable())
@@ -3093,14 +3097,42 @@ func TestAddNode(t *testing.T) {
 // TestAddLearner tests that addLearner could update nodes correctly.
 func TestAddLearner(t *testing.T) {
 	r := newTestRaft(1, []uint64{1}, 10, 1, NewMemoryStorage())
+	// Add new learner peer.
 	r.applyConfChange(pb.ConfChange{NodeID: 2, Type: pb.ConfChangeAddLearnerNode})
+	if r.isLearner {
+		t.Fatal("expected 1 to be voter")
+	}
 	nodes := r.prs.LearnerNodes()
 	wnodes := []uint64{2}
 	if !reflect.DeepEqual(nodes, wnodes) {
 		t.Errorf("nodes = %v, want %v", nodes, wnodes)
 	}
 	if !r.prs.Progress[2].IsLearner {
-		t.Errorf("node 2 is learner %t, want %t", r.prs.Progress[2].IsLearner, true)
+		t.Fatal("expected 2 to be learner")
+	}
+
+	// Promote peer to voter.
+	r.applyConfChange(pb.ConfChange{NodeID: 2, Type: pb.ConfChangeAddNode})
+	if r.prs.Progress[2].IsLearner {
+		t.Fatal("expected 2 to be voter")
+	}
+
+	// Demote r.
+	r.applyConfChange(pb.ConfChange{NodeID: 1, Type: pb.ConfChangeAddLearnerNode})
+	if !r.prs.Progress[1].IsLearner {
+		t.Fatal("expected 1 to be learner")
+	}
+	if !r.isLearner {
+		t.Fatal("expected 1 to be learner")
+	}
+
+	// Promote r again.
+	r.applyConfChange(pb.ConfChange{NodeID: 1, Type: pb.ConfChangeAddNode})
+	if r.prs.Progress[1].IsLearner {
+		t.Fatal("expected 1 to be voter")
+	}
+	if r.isLearner {
+		t.Fatal("expected 1 to be voter")
 	}
 }
 
@@ -3148,12 +3180,13 @@ func TestRemoveNode(t *testing.T) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
 
-	// remove all nodes from cluster
+	// Removing the remaining voter will panic.
+	defer func() {
+		if r := recover(); r == nil {
+			t.Error("did not panic")
+		}
+	}()
 	r.applyConfChange(pb.ConfChange{NodeID: 1, Type: pb.ConfChangeRemoveNode})
-	w = []uint64{}
-	if g := r.prs.VoterNodes(); !reflect.DeepEqual(g, w) {
-		t.Errorf("nodes = %v, want %v", g, w)
-	}
 }
 
 // TestRemoveLearner tests that removeNode could update nodes and
@@ -3171,12 +3204,15 @@ func TestRemoveLearner(t *testing.T) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
 
-	// remove all nodes from cluster
+	// Removing the remaining voter will panic.
+	defer func() {
+		if r := recover(); r == nil {
+			t.Error("did not panic")
+		}
+	}()
 	r.applyConfChange(pb.ConfChange{NodeID: 1, Type: pb.ConfChangeRemoveNode})
-	if g := r.prs.VoterNodes(); !reflect.DeepEqual(g, w) {
-		t.Errorf("nodes = %v, want %v", g, w)
-	}
 }
+
 func TestPromotable(t *testing.T) {
 	id := uint64(1)
 	tests := []struct {
@@ -4124,12 +4160,16 @@ func newNetworkWithConfig(configFunc func(*Config), peers ...stateMachine) *netw
 			sm := newRaft(cfg)
 			npeers[id] = sm
 		case *raft:
+			// TODO(tbg): this is all pretty confused. Clean this up.
 			learners := make(map[uint64]bool, len(v.prs.Learners))
 			for i := range v.prs.Learners {
 				learners[i] = true
 			}
 			v.id = id
 			v.prs = tracker.MakeProgressTracker(v.prs.MaxInflight)
+			if len(learners) > 0 {
+				v.prs.Learners = map[uint64]struct{}{}
+			}
 			for i := 0; i < size; i++ {
 				pr := &tracker.Progress{}
 				if _, ok := learners[peerAddrs[i]]; ok {

raft/tracker/progress.go

@@ -16,6 +16,7 @@ package tracker
 
 import (
 	"fmt"
+	"sort"
 	"strings"
 )
 
@@ -235,3 +236,22 @@ func (pr *Progress) String() string {
 	}
 	return buf.String()
 }
+
+// ProgressMap is a map of *Progress.
+type ProgressMap map[uint64]*Progress
+
+// String prints the ProgressMap in sorted key order, one Progress per line.
+func (m ProgressMap) String() string {
+	ids := make([]uint64, 0, len(m))
+	for k := range m {
+		ids = append(ids, k)
+	}
+	sort.Slice(ids, func(i, j int) bool {
+		return ids[i] < ids[j]
+	})
+	var buf strings.Builder
+	for _, id := range ids {
+		fmt.Fprintf(&buf, "%d: %s\n", id, m[id])
+	}
+	return buf.String()
+}

raft/tracker/tracker.go

@@ -17,6 +17,7 @@ package tracker
 import (
 	"fmt"
 	"sort"
+	"strings"
 
 	"go.etcd.io/etcd/raft/quorum"
 )
@@ -33,12 +34,11 @@ type Config struct {
 	// simplifies the implementation since it allows peers to have clarity about
 	// its current role without taking into account joint consensus.
 	Learners map[uint64]struct{}
-	// TODO(tbg): when we actually carry out joint consensus changes and turn a
-	// voter into a learner, we cannot add the learner when entering the joint
-	// state. This is because this would violate the invariant that the inter-
-	// section of voters and learners is empty. For example, assume a Voter is
-	// removed and immediately re-added as a learner (or in other words, it is
-	// demoted).
+	// When we turn a voter into a learner during a joint consensus transition,
+	// we cannot add the learner directly when entering the joint state. This is
+	// because this would violate the invariant that the intersection of
+	// voters and learners is empty. For example, assume a Voter is removed and
+	// immediately re-added as a learner (or in other words, it is demoted):
 	//
 	// Initially, the configuration will be
 	//
@@ -51,7 +51,7 @@ type Config struct {
 	//   learners: {3}
 	//
 	// but this violates the invariant (3 is both voter and learner). Instead,
-	// we have
+	// we get
 	//
 	//   voters:   {1 2} & {1 2 3}
 	//   learners: {}
@@ -66,20 +66,40 @@ type Config struct {
 	//
 	// Note that next_learners is not used while adding a learner that is not
 	// also a voter in the joint config. In this case, the learner is added
-	// to Learners right away when entering the joint configuration, so that it
-	// is caught up as soon as possible.
-	//
-	// NextLearners        map[uint64]struct{}
+	// right away when entering the joint configuration, so that it is caught up
+	// as soon as possible.
+	LearnersNext map[uint64]struct{}
+}
+
+func (c Config) String() string {
+	var buf strings.Builder
+	fmt.Fprintf(&buf, "voters=%s", c.Voters)
+	if c.Learners != nil {
+		fmt.Fprintf(&buf, " learners=%s", quorum.MajorityConfig(c.Learners).String())
+	}
+	if c.LearnersNext != nil {
+		fmt.Fprintf(&buf, " learners_next=%s", quorum.MajorityConfig(c.LearnersNext).String())
+	}
+	return buf.String()
 }
 
-func (c *Config) String() string {
-	if len(c.Learners) == 0 {
-		return fmt.Sprintf("voters=%s", c.Voters)
+// Clone returns a copy of the Config that shares no memory with the original.
+func (c *Config) Clone() Config {
+	clone := func(m map[uint64]struct{}) map[uint64]struct{} {
+		if m == nil {
+			return nil
+		}
+		mm := make(map[uint64]struct{}, len(m))
+		for k := range m {
+			mm[k] = struct{}{}
+		}
+		return mm
+	}
+	return Config{
+		Voters:       quorum.JointConfig{clone(c.Voters[0]), clone(c.Voters[1])},
+		Learners:     clone(c.Learners),
+		LearnersNext: clone(c.LearnersNext),
 	}
-	return fmt.Sprintf(
-		"voters=%s learners=%s",
-		c.Voters, quorum.MajorityConfig(c.Learners).String(),
-	)
 }
 
 // ProgressTracker tracks the currently active configuration and the information
@@ -88,7 +108,7 @@ func (c *Config) String() string {
 type ProgressTracker struct {
 	Config
 
-	Progress map[uint64]*Progress
+	Progress ProgressMap
 
 	Votes map[uint64]bool
 
@@ -102,11 +122,10 @@ func MakeProgressTracker(maxInflight int) ProgressTracker {
 		Config: Config{
 			Voters: quorum.JointConfig{
 				quorum.MajorityConfig{},
-				// TODO(tbg): this will be mostly empty, so make it a nil pointer
-				// in the common case.
-				quorum.MajorityConfig{},
+				nil, // only populated when used
 			},
-			Learners: map[uint64]struct{}{},
+			Learners:     nil, // only populated when used
+			LearnersNext: nil, // only populated when used
 		},
 		Votes:    map[uint64]bool{},
 		Progress: map[uint64]*Progress{},
@@ -139,44 +158,6 @@ func (p *ProgressTracker) Committed() uint64 {
 	return uint64(p.Voters.CommittedIndex(matchAckIndexer(p.Progress)))
 }
 
-// RemoveAny removes this peer, which *must* be tracked as a voter or learner,
-// from the tracker.
-func (p *ProgressTracker) RemoveAny(id uint64) {
-	_, okPR := p.Progress[id]
-	_, okV1 := p.Voters[0][id]
-	_, okV2 := p.Voters[1][id]
-	_, okL := p.Learners[id]
-
-	okV := okV1 || okV2
-
-	if !okPR {
-		panic("attempting to remove unknown peer %x")
-	} else if !okV && !okL {
-		panic("attempting to remove unknown peer %x")
-	} else if okV && okL {
-		panic(fmt.Sprintf("peer %x is both voter and learner", id))
-	}
-
-	delete(p.Voters[0], id)
-	delete(p.Voters[1], id)
-	delete(p.Learners, id)
-	delete(p.Progress, id)
-}
-
-// InitProgress initializes a new progress for the given node or learner. The
-// node may not exist yet in either form or a panic will ensue.
-func (p *ProgressTracker) InitProgress(id, match, next uint64, isLearner bool) {
-	if pr := p.Progress[id]; pr != nil {
-		panic(fmt.Sprintf("peer %x already tracked as node %v", id, pr))
-	}
-	if !isLearner {
-		p.Voters[0][id] = struct{}{}
-	} else {
-		p.Learners[id] = struct{}{}
-	}
-	p.Progress[id] = &Progress{Next: next, Match: match, Inflights: NewInflights(p.MaxInflight), IsLearner: isLearner}
-}
-
 // Visit invokes the supplied closure for all tracked progresses.
 func (p *ProgressTracker) Visit(f func(id uint64, pr *Progress)) {
 	for id, pr := range p.Progress {