[#902] morph: Avoid creating session in TestInvokeIterator

When the number of items to iterate over is less than 2048, there is no
need to create a session and consume resources.

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

pkg/morph/client/client.go

@@ -13,6 +13,7 @@ import (
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/metrics"
 	morphmetrics "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/morph/metrics"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/util/logger"
+	"github.com/google/uuid"
 	lru "github.com/hashicorp/golang-lru/v2"
 	"github.com/nspcc-dev/neo-go/pkg/core/native/noderoles"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
@@ -24,8 +25,10 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/rpcclient/nep17"
 	"github.com/nspcc-dev/neo-go/pkg/rpcclient/rolemgmt"
 	"github.com/nspcc-dev/neo-go/pkg/rpcclient/unwrap"
+	"github.com/nspcc-dev/neo-go/pkg/smartcontract"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/trigger"
 	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/nspcc-dev/neo-go/pkg/vm"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 	"github.com/nspcc-dev/neo-go/pkg/vm/vmstate"
 	"github.com/nspcc-dev/neo-go/pkg/wallet"
@@ -204,16 +207,27 @@ func (c *Client) Invoke(contract util.Uint160, fee fixedn.Fixed8, method string,
 	return vub, nil
 }
 
+// defaultPrefetchBatchSize is the default number of items to prefetch.
+// It is dependent on VM limits (2048 items on stack), the default works for simple items.
+// For example, to iterate over 2 field structs, the limit should be divided by 3 = 1 (struct itself) + 2 (fields).
+const defaultPrefetchBatchSize = vm.MaxStackSize - 16
+
 // TestInvokeIterator invokes contract method returning an iterator and executes cb on each element.
 // If cb returns an error, the session is closed and this error is returned as-is.
 // If the remove neo-go node does not support sessions, `unwrap.ErrNoSessionID` is returned.
-func (c *Client) TestInvokeIterator(cb func(stackitem.Item) error, contract util.Uint160, method string, args ...interface{}) error {
+// batchSize is the number of items to prefetch: if the number of items in the iterator is less than batchSize, no session will be created.
+// The default batchSize is 2000 (VM is limited by having 2048 items on stack, so if each iterator item is simple, 2000 items won't hit the limit).
+func (c *Client) TestInvokeIterator(cb func(stackitem.Item) error, batchSize int, contract util.Uint160, method string, args ...interface{}) error {
 	start := time.Now()
 	success := false
 	defer func() {
 		c.metrics.ObserveInvoke("TestInvokeIterator", contract.String(), method, success, time.Since(start))
 	}()
 
+	if batchSize <= 0 {
+		batchSize = defaultPrefetchBatchSize
+	}
+
 	c.switchLock.RLock()
 	defer c.switchLock.RUnlock()
 
@@ -221,35 +235,54 @@ func (c *Client) TestInvokeIterator(cb func(stackitem.Item) error, contract util
 		return ErrConnectionLost
 	}
 
-	val, err := c.rpcActor.Call(contract, method, args...)
+	script, err := smartcontract.CreateCallAndPrefetchIteratorScript(contract, method, batchSize, args...)
+	if err != nil {
+		return err
+	}
+
+	val, err := c.rpcActor.Run(script)
 	if err != nil {
 		return err
 	} else if val.State != HaltState {
 		return wrapFrostFSError(&notHaltStateError{state: val.State, exception: val.FaultException})
 	}
 
-	sid, r, err := unwrap.SessionIterator(val, err)
+	arr, sid, r, err := unwrap.ArrayAndSessionIterator(val, err)
 	if err != nil {
 		return err
 	}
+	for i := range arr {
+		if err := cb(arr[i]); err != nil {
+			return err
+		}
+	}
+	if (sid == uuid.UUID{}) {
+		success = true
+		return nil
+	}
 
 	defer func() {
 		_ = c.rpcActor.TerminateSession(sid)
 	}()
 
-	items, err := c.rpcActor.TraverseIterator(sid, &r, 0)
+	for {
-	for err == nil && len(items) != 0 {
+		items, err := c.rpcActor.TraverseIterator(sid, &r, batchSize)
-		for i := range items {
+		if err != nil {
-			if err = cb(items[i]); err != nil {
 			return err
 		}
-		}
-		items, err = c.rpcActor.TraverseIterator(sid, &r, 0)
-	}
 
-	success = err == nil
+		for i := range items {
+			if err := cb(items[i]); err != nil {
 				return err
 			}
+		}
+		if len(items) < batchSize {
+			break
+		}
+	}
+	success = true
+	return nil
+}
 
 // TestInvoke invokes contract method locally in neo-go node. This method should
 // be used to read data from smart-contract.

pkg/morph/client/container/containers_of.go

@@ -41,7 +41,7 @@ func (c *Client) ContainersOf(idUser *user.ID) ([]cid.ID, error) {
 	}
 
 	cnrHash := c.client.ContractAddress()
-	err := c.client.Morph().TestInvokeIterator(cb, cnrHash, containersOfMethod, rawID)
+	err := c.client.Morph().TestInvokeIterator(cb, 0, cnrHash, containersOfMethod, rawID)
 	if err != nil {
 		if errors.Is(err, unwrap.ErrNoSessionID) {
 			return c.List(idUser)