raft: fix restoring joint configurations

While writing interaction tests for joint configuration changes, I
realized that this wasn't working yet - restoring had no notion of
the joint configuration and was simply dropping it on the floor.

This commit introduces a helper `confchange.Restore` which takes
a `ConfState` and initializes a `Tracker` from it.

This is then used both in `(*raft).restore` as well as in `newRaft`.

raft/raftpb/confstate.go

@@ -0,0 +1,45 @@
+// 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 raftpb
+
+import (
+	"fmt"
+	"reflect"
+	"sort"
+)
+
+// Equivalent returns a nil error if the inputs describe the same configuration.
+// On mismatch, returns a descriptive error showing the differences.
+func (cs ConfState) Equivalent(cs2 ConfState) error {
+	cs1 := cs
+	orig1, orig2 := cs1, cs2
+	s := func(sl *[]uint64) {
+		*sl = append([]uint64(nil), *sl...)
+		sort.Slice(*sl, func(i, j int) bool { return (*sl)[i] < (*sl)[j] })
+	}
+
+	for _, cs := range []*ConfState{&cs1, &cs2} {
+		s(&cs.Voters)
+		s(&cs.Learners)
+		s(&cs.VotersOutgoing)
+		s(&cs.LearnersNext)
+		cs.XXX_unrecognized = nil
+	}
+
+	if !reflect.DeepEqual(cs1, cs2) {
+		return fmt.Errorf("ConfStates not equivalent after sorting:\n%+#v\n%+#v\nInputs were:\n%+#v\n%+#v", cs1, cs2, orig1, orig2)
+	}
+	return nil
+}

raft/raftpb/confstate_test.go

@@ -0,0 +1,58 @@
+// 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 raftpb
+
+import (
+	"testing"
+)
+
+func TestConfState_Equivalent(t *testing.T) {
+	type testCase struct {
+		cs, cs2 ConfState
+		ok      bool
+	}
+
+	testCases := []testCase{
+		// Reordered voters and learners.
+		{ConfState{
+			Voters:         []uint64{1, 2, 3},
+			Learners:       []uint64{5, 4, 6},
+			VotersOutgoing: []uint64{9, 8, 7},
+			LearnersNext:   []uint64{10, 20, 15},
+		}, ConfState{
+			Voters:         []uint64{1, 2, 3},
+			Learners:       []uint64{4, 5, 6},
+			VotersOutgoing: []uint64{7, 9, 8},
+			LearnersNext:   []uint64{20, 10, 15},
+		}, true},
+		// Not sensitive to nil vs empty slice.
+		{ConfState{Voters: []uint64{}}, ConfState{Voters: []uint64(nil)}, true},
+		// Non-equivalent voters.
+		{ConfState{Voters: []uint64{1, 2, 3, 4}}, ConfState{Voters: []uint64{2, 1, 3}}, false},
+		{ConfState{Voters: []uint64{1, 4, 3}}, ConfState{Voters: []uint64{2, 1, 3}}, false},
+		// Non-equivalent learners.
+		{ConfState{Voters: []uint64{1, 2, 3, 4}}, ConfState{Voters: []uint64{2, 1, 3}}, false},
+		// Sensitive to AutoLeave flag.
+		{ConfState{AutoLeave: true}, ConfState{}, false},
+	}
+
+	for _, tc := range testCases {
+		t.Run("", func(t *testing.T) {
+			if err := tc.cs.Equivalent(tc.cs2); (err == nil) != tc.ok {
+				t.Fatalf("wanted error: %t, got:\n%s", tc.ok, err)
+			}
+		})
+	}
+}