[#234] core/netmap: Extend Source interface

Add GetNetMapByEpoch method. Add Epoch method.

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

pkg/core/netmap/storage.go

@@ -17,6 +17,21 @@ type Source interface {
 	// Implementations must not retain the network map pointer and modify
 	// the network map through it.
 	GetNetMap(diff uint64) (*netmap.Netmap, error)
+
+	// GetNetMapByEpoch reads network map by the epoch number from the storage.
+	// It returns the pointer to requested network map and any error encountered.
+	//
+	// Must return exactly one non-nil value.
+	//
+	// Implementations must not retain the network map pointer and modify
+	// the network map through it.
+	GetNetMapByEpoch(epoch uint64) (*netmap.Netmap, error)
+
+	// Epoch reads current epoch from the storage.
+	// It returns number of the current epoch and any error encountered.
+	//
+	// Must return exactly one non-default value.
+	Epoch() (uint64, error)
 }
 
 // GetLatestNetworkMap requests and returns latest network map from storage.

pkg/services/object_manager/placement/netmap.go

@@ -12,12 +12,14 @@ type netMapBuilder struct {
 }
 
 type netMapSrc struct {
+	netmap.Source
+
 	nm *netmapSDK.Netmap
 }
 
 func NewNetworkMapBuilder(nm *netmapSDK.Netmap) Builder {
 	return &netMapBuilder{
-		nmSrc: &netMapSrc{nm},
+		nmSrc: &netMapSrc{nm: nm},
 	}
 }