[#33] core/netmap: Define read-only storage interface

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/core/netmap/storage.go

@@ -0,0 +1,35 @@
+package netmap
+
+import (
+	"errors"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/netmap"
+)
+
+// Source is an interface that wraps
+// basic network map receiving method.
+type Source interface {
+	// GetNetMap reads the diff-th past network map from the storage.
+	// Calling with zero diff returns latest network map.
+	// It returns the pointer to requested network map and any error encountered.
+	//
+	// GetNetMap must return exactly one non-nil value.
+	// GetNetMap must return ErrNotFound if the network map is not in storage.
+	//
+	// Implementations must not retain the network map pointer and modify
+	// the network map through it.
+	GetNetMap(diff uint64) (*netmap.Netmap, error)
+}
+
+// ErrNotFound is the error returned when network map was not found in storage.
+var ErrNotFound = errors.New("network map not found")
+
+// GetLatestNetworkMap requests and returns latest network map from storage.
+func GetLatestNetworkMap(src Source) (*netmap.Netmap, error) {
+	return src.GetNetMap(0)
+}
+
+// GetPreviousNetworkMap requests and returns previous from latest network map from storage.
+func GetPreviousNetworkMap(src Source) (*netmap.Netmap, error) {
+	return src.GetNetMap(1)
+}