Improve function GetStateRangeScanIterator

- Close iterators if newItrCombiner fails.
- Handle nil iterators returned without errors.
- Improve error messages for debugging.

Signed-off-by: David Enyeart <enyeart@us.ibm.com>

Author: denyeart

core/ledger/kvledger/txmgmt/queryutil/query_executer_combiner.go

@@ -7,6 +7,8 @@ SPDX-License-Identifier: Apache-2.0
 package queryutil
 
 import (
+	"fmt"
+
 	"github.com/hyperledger/fabric-lib-go/common/flogging"
 	commonledger "github.com/hyperledger/fabric/common/ledger"
 	"github.com/hyperledger/fabric/core/ledger/kvledger/txmgmt/privacyenabledstate"
@@ -56,18 +58,27 @@ func (c *QECombiner) GetState(namespace string, key string) ([]byte, error) {
 // sort order of the key
 func (c *QECombiner) GetStateRangeScanIterator(namespace string, startKey string, endKey string) (commonledger.ResultsIterator, error) {
 	var itrs []statedb.ResultsIterator
-	for _, qe := range c.QueryExecuters {
+	for i, qe := range c.QueryExecuters {
 		itr, err := qe.GetStateRangeScanIterator(namespace, startKey, endKey)
 		if err != nil {
 			for _, itr := range itrs {
 				itr.Close()
 			}
-			return nil, err
+			return nil, fmt.Errorf("failed to get iterator from QueryExecuter %d: %w", i, err)
+		}
+		if itr == nil {
+			for _, itr := range itrs {
+				itr.Close()
+			}
+			return nil, fmt.Errorf("received nil iterator from QueryExecuter %d", i)
 		}
 		itrs = append(itrs, itr)
 	}
 	itrCombiner, err := newItrCombiner(namespace, itrs)
 	if err != nil {
+		for _, itr := range itrs {
+			itr.Close()
+		}
 		return nil, err
 	}
 	return itrCombiner, nil