[#1604] policer: Properly handle maintenance nodes

Consider `REP 1 REP 1` placement (selects/filters are omitted).
The placement is `[1, 2], [1, 0]`. We are the 0-th node.
Node 1 is under maintenance, so we do not replicate object
on the node 2. In the second replication group node 1 is under maintenance,
but current caching logic considers it as "replica holder" and removes
local copy. Voilà, we have DL if the object is missing from the node 1.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/policer/check.go

@@ -128,6 +128,10 @@ func (p *Policer) processRepNodes(ctx context.Context, requirements *placementRe
 				if status == nodeHoldsObject {
 					shortage--
 				}
+				if status == nodeIsUnderMaintenance {
+					shortage--
+					uncheckedCopies++
+				}
 
 				nodes = append(nodes[:i], nodes[i+1:]...)
 				i--
@@ -174,7 +178,7 @@ func (p *Policer) processRepNodes(ctx context.Context, requirements *placementRe
 // However, additional copies should not be removed in this case,
 // because we can remove the only copy this way.
 func (p *Policer) handleMaintenance(ctx context.Context, node netmap.NodeInfo, checkedNodes nodeCache, shortage uint32, uncheckedCopies int) (uint32, int) {
-	checkedNodes.set(node, nodeHoldsObject)
+	checkedNodes.set(node, nodeIsUnderMaintenance)
 	shortage--
 	uncheckedCopies++
 

pkg/services/policer/nodecache.go

@@ -9,6 +9,7 @@ const (
 	nodeDoesNotHoldObject
 	nodeHoldsObject
 	nodeStatusUnknown
+	nodeIsUnderMaintenance
 )
 
 func (st nodeProcessStatus) Processed() bool {

pkg/services/policer/policer_test.go

@@ -170,6 +170,14 @@ func TestProcessObject(t *testing.T) {
 			placement:       [][]int{{0, 1, 2}},
 			wantReplicateTo: []int{1, 2},
 		},
+		{
+			desc:       "do not remove local copy when MAINTENANCE status is cached",
+			objType:    objectSDK.TypeRegular,
+			nodeCount:  3,
+			policy:     `REP 1 REP 1`,
+			placement:  [][]int{{1, 2}, {1, 0}},
+			headResult: map[int]error{1: new(apistatus.NodeUnderMaintenance)},
+		},
 	}
 
 	for i := range tests {