server: Written Snapshot's to WAL contains populated ConfState.

This will (among others) allow etcd-3.6 to not depend on store_v2 .snap files at all,
as WAL + db file will be self-sufficient.

server/wal/walpb/record.go

@@ -27,3 +27,15 @@ func (rec *Record) Validate(crc uint32) error {
 	rec.Reset()
 	return ErrCRCMismatch
 }
+
+// ValidateSnapshotForWrite ensures the Snapshot the newly written snapshot is valid.
+//
+// There might exist log-entries written by old etcd versions that does not conform
+// to the requirements.
+func ValidateSnapshotForWrite(e *Snapshot) error {
+	// Since etcd>=3.5.0
+	if e.ConfState == nil && e.Index > 0 {
+		return errors.New("Saved (not-initial) snapshot is missing ConfState: " + e.String())
+	}
+	return nil
+}

server/wal/walpb/record_test.go

@@ -0,0 +1,36 @@
+package walpb
+
+import (
+	"testing"
+
+	"github.com/golang/protobuf/descriptor"
+	"go.etcd.io/etcd/raft/v3/raftpb"
+)
+
+func TestSnapshotMetadataCompatibility(t *testing.T) {
+	_, snapshotMetadataMd := descriptor.ForMessage(&raftpb.SnapshotMetadata{})
+	_, snapshotMd := descriptor.ForMessage(&Snapshot{})
+	if len(snapshotMetadataMd.GetField()) != len(snapshotMd.GetField()) {
+		t.Errorf("Different number of fields in raftpb.SnapshotMetadata vs. walpb.Snapshot. " +
+			"They are supposed to be in sync.")
+	}
+}
+
+func TestValidateSnapshot(t *testing.T) {
+	tests := []struct {
+		name    string
+		snap    *Snapshot
+		wantErr bool
+	}{
+		{name: "empty", snap: &Snapshot{}, wantErr: false},
+		{name: "invalid", snap: &Snapshot{Index: 5, Term: 3}, wantErr: true},
+		{name: "valid", snap: &Snapshot{Index: 5, Term: 3, ConfState: &raftpb.ConfState{Voters: []uint64{0x00cad1}}}, wantErr: false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := ValidateSnapshotForWrite(tt.snap); (err != nil) != tt.wantErr {
+				t.Errorf("ValidateSnapshotForWrite() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}