storage: add interfaces for copysets

Copysets significantly reduce the probability of data loss
in the presence of multi node failures. The RFC for copysets
is presented in cockroachdb#32816.

This commit adds interfaces and protos for copysets.
The implementation added later will have two independent tracks:
1. Assignment and persistance of store_id-copyset_id mapping.
2. Rebalancer changes to rebalance replicas into copysets.

Release note: None

pkg/roachpb/metadata.go

@@ -52,6 +52,11 @@ func (n StoreID) String() string {
 	return strconv.FormatInt(int64(n), 10)
 }
 
+// CopysetID is a custom type for a cockroach copyset ID.
+// Copyset is a group of stores where a range should be contained within
+// if copyset based rebalancing is enabled.
+type CopysetID int32
+
 // ReplicaID is a custom type for a range replica ID.
 type ReplicaID int32
 

pkg/storage/copysets.go

@@ -0,0 +1,38 @@
+// Copyright 2018 The Cockroach 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 storage
+
+import "context"
+
+// copysetsReader is used to read copysets in the cluster.
+type copysetsReader interface {
+	// forRF returns copysets for the given replication factor.
+	forRF(ctx context.Context, replicationFactor int32) (*Copysets, error)
+}
+
+// CopysetsMaintainer maintains and assigns copysets to stores.
+// TODO(hassan): implement CopysetsMaintainer.
+type CopysetsMaintainer struct {
+}
+
+// Ensure CopysetsMaintainer implements copysetsReader.
+var _ copysetsReader = &CopysetsMaintainer{}
+
+// forRF returns copysets for the given replication factor.
+func (cs *CopysetsMaintainer) forRF(
+	ctx context.Context, replicationFactor int32,
+) (*Copysets, error) {
+	return nil, nil
+}