[#498] policer: Explicitly Rewind() iterator after finish

Previously, we can continue to return `EndOfListing` infinitely.
Reflect iterator reuse via Rewind() method.

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

cmd/frostfs-node/keyspaceiterator.go

@@ -26,3 +26,7 @@ func (it *keySpaceIterator) Next(ctx context.Context, batchSize uint32) ([]objec
 	it.cur = res.Cursor()
 	return res.AddressList(), nil
 }
+
+func (it *keySpaceIterator) Rewind() {
+	it.cur = nil
+}

pkg/services/policer/option.go

@@ -23,6 +23,7 @@ import (
 // when the end of the key space is reached.
 type KeySpaceIterator interface {
 	Next(context.Context, uint32) ([]objectcore.AddressWithType, error)
+	Rewind()
 }
 
 // RedundantCopyCallback is a callback to pass

pkg/services/policer/policer_test.go

@@ -244,11 +244,95 @@ func TestProcessObject(t *testing.T) {
 	}
 }
 
+func TestIteratorContract(t *testing.T) {
+	addr := oidtest.Address()
+	objs := []objectcore.AddressWithType{{
+		Address: addr,
+		Type:    object.TypeRegular,
+	}}
+
+	containerSrc := func(id cid.ID) (*container.Container, error) {
+		return nil, apistatus.ContainerNotFound{}
+	}
+	buryFn := func(ctx context.Context, a oid.Address) error {
+		return nil
+	}
+
+	pool, err := ants.NewPool(4)
+	require.NoError(t, err)
+
+	it := &predefinedIterator{
+		scenario: []nextResult{
+			{objs, nil},
+			{nil, errors.New("opaque")},
+			{nil, engine.ErrEndOfListing},
+			{nil, engine.ErrEndOfListing},
+			{nil, errors.New("opaque")},
+			{objs, engine.ErrEndOfListing},
+		},
+		finishCh: make(chan struct{}),
+	}
+
+	p := New(
+		WithKeySpaceIterator(it),
+		WithContainerSource(containerSrcFunc(containerSrc)),
+		WithBuryFunc(buryFn),
+		WithPool(pool),
+		WithNodeLoader(constNodeLoader(0)),
+	)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	go p.Run(ctx)
+
+	<-it.finishCh
+	cancel()
+	require.Equal(t, []string{
+		"Next",
+		"Next",
+		"Next",
+		"Rewind",
+		"Next",
+		"Rewind",
+		"Next",
+		"Next",
+		"Rewind",
+	}, it.calls)
+}
+
 // TODO(https://git.frostfs.info/TrueCloudLab/frostfs-sdk-go/issues/101)
 func eqAddr(a, b oid.Address) bool {
 	return a.Container().Equals(b.Container()) && a.Object().Equals(b.Object())
 }
 
+type nextResult struct {
+	objs []objectcore.AddressWithType
+	err  error
+}
+
+type predefinedIterator struct {
+	scenario []nextResult
+	finishCh chan struct{}
+	pos      int
+	calls    []string
+}
+
+func (it *predefinedIterator) Next(ctx context.Context, size uint32) ([]objectcore.AddressWithType, error) {
+	if it.pos == len(it.scenario) {
+		close(it.finishCh)
+		<-ctx.Done()
+		return nil, nil
+	}
+
+	res := it.scenario[it.pos]
+	it.pos += 1
+	it.calls = append(it.calls, "Next")
+	return res.objs, res.err
+}
+
+func (it *predefinedIterator) Rewind() {
+	it.calls = append(it.calls, "Rewind")
+}
+
 // sliceKeySpaceIterator is a KeySpaceIterator backed by a slice.
 type sliceKeySpaceIterator struct {
 	objs []objectcore.AddressWithType
@@ -257,7 +341,6 @@ type sliceKeySpaceIterator struct {
 
 func (it *sliceKeySpaceIterator) Next(_ context.Context, size uint32) ([]objectcore.AddressWithType, error) {
 	if it.cur >= len(it.objs) {
-		it.cur = 0
 		return nil, engine.ErrEndOfListing
 	}
 	end := it.cur + int(size)
@@ -269,6 +352,10 @@ func (it *sliceKeySpaceIterator) Next(_ context.Context, size uint32) ([]objectc
 	return ret, nil
 }
 
+func (it *sliceKeySpaceIterator) Rewind() {
+	it.cur = 0
+}
+
 // containerSrcFunc is a container.Source backed by a function.
 type containerSrcFunc func(cid.ID) (*container.Container, error)
 

pkg/services/policer/process.go

@@ -27,6 +27,7 @@ func (p *Policer) shardPolicyWorker(ctx context.Context) {
 		addrs, err := p.keySpaceIterator.Next(ctx, p.batchSize)
 		if err != nil {
 			if errors.Is(err, engine.ErrEndOfListing) {
+				p.keySpaceIterator.Rewind()
 				time.Sleep(time.Second) // finished whole cycle, sleep a bit
 				continue
 			}