storage: rework AdminChangeReplicas for atomic membership changes

Following the plan laid out in #39485, this adds API support for
general replication changes to `AdminChangeReplicasRequest`.

`(*DB).AtomicChangeReplicas` will now accept an arbitrary set
of additions/removals, though only on paper - the changes will
be executed individually.

The compatibility story is straightforward since this request is never
persisted. We simply populate both the deprecated and the new field (and
it isn't safe to emit "mixed" changes until all nodes run 19.2 and it's
not worth plumbing a setting around), and in 20.1 we remove the old
fields.  A maybe-snag is that as a result, there are a few months left
in this release in which folks may accidentally mix additions and
removals in a replica change without proper version gating. This wasn't
deemed very likely; to mitigate we could add in-memory state on the
request that fires a panic whenever the changeType changes.

Release note: None

pkg/internal/client/batch.go

@@ -643,10 +643,7 @@ func (b *Batch) adminTransferLease(key interface{}, target roachpb.StoreID) {
 // adminChangeReplicas is only exported on DB. It is here for symmetry with the
 // other operations.
 func (b *Batch) adminChangeReplicas(
-	key interface{},
-	changeType roachpb.ReplicaChangeType,
-	targets []roachpb.ReplicationTarget,
-	expDesc roachpb.RangeDescriptor,
+	key interface{}, expDesc roachpb.RangeDescriptor, chgs []roachpb.ReplicationChange,
 ) {
 	k, err := marshalKey(key)
 	if err != nil {
@@ -657,10 +654,10 @@ func (b *Batch) adminChangeReplicas(
 		RequestHeader: roachpb.RequestHeader{
 			Key: k,
 		},
-		ChangeType: changeType,
-		Targets:    targets,
-		ExpDesc:    expDesc,
+		ExpDesc: expDesc,
 	}
+	req.AddChanges(chgs...)
+
 	b.appendReqs(req)
 	b.initResult(1, 0, notRaw, nil)
 }

pkg/internal/client/db.go

@@ -525,12 +525,11 @@ func (db *DB) AdminTransferLease(
 func (db *DB) AdminChangeReplicas(
 	ctx context.Context,
 	key interface{},
-	changeType roachpb.ReplicaChangeType,
-	targets []roachpb.ReplicationTarget,
 	expDesc roachpb.RangeDescriptor,
+	chgs []roachpb.ReplicationChange,
 ) (*roachpb.RangeDescriptor, error) {
 	b := &Batch{}
-	b.adminChangeReplicas(key, changeType, targets, expDesc)
+	b.adminChangeReplicas(key, expDesc, chgs)
 	if err := getOneErr(db.Run(ctx, b), b); err != nil {
 		return nil, err
 	}

pkg/roachpb/api.go

@@ -1185,3 +1185,46 @@ func (e *RangeFeedEvent) MustSetValue(value interface{}) {
 		panic(fmt.Sprintf("%T excludes %T", e, value))
 	}
 }
+
+// MakeReplicationChanges returns a slice of changes of the given type with an
+// item for each target.
+func MakeReplicationChanges(
+	changeType ReplicaChangeType, targets []ReplicationTarget,
+) []ReplicationChange {
+	chgs := make([]ReplicationChange, 0, len(targets))
+	for _, target := range targets {
+		chgs = append(chgs, ReplicationChange{
+			ChangeType: changeType,
+			Target:     target,
+		})
+	}
+	return chgs
+}
+
+// AddChanges adds a batch of changes to the request in a backwards-compatible
+// way.
+func (r *AdminChangeReplicasRequest) AddChanges(chgs ...ReplicationChange) {
+	r.InternalChanges = append(r.InternalChanges, chgs...)
+
+	r.DeprecatedChangeType = chgs[0].ChangeType
+	for _, chg := range chgs {
+		r.DeprecatedTargets = append(r.DeprecatedTargets, chg.Target)
+	}
+}
+
+// Changes returns the changes requested by this AdminChangeReplicasRequest, taking
+// the deprecated method of doing so into account.
+func (r *AdminChangeReplicasRequest) Changes() []ReplicationChange {
+	if len(r.InternalChanges) > 0 {
+		return r.InternalChanges
+	}
+
+	sl := make([]ReplicationChange, len(r.DeprecatedTargets))
+	for _, target := range r.DeprecatedTargets {
+		sl = append(sl, ReplicationChange{
+			ChangeType: r.DeprecatedChangeType,
+			Target:     target,
+		})
+	}
+	return sl
+}