kvnemesis: correctly handle errors from DeleteRangeUsingTombstoneOperation

Previously we were initializing the result in the
DeleteRangeUsingTombstoneOperation case using the err variable that had
been set to nil.

Now, we correctly using the Err from the result. While here, I've also
re-arranged the code so that the cases are a bit more consistent.

Epic: none
Release note: None

Author: stevendanna

pkg/kv/kvnemesis/applier.go

@@ -640,18 +640,17 @@ func applyBatchOp(
 			}
 			subO.Result.ResumeSpan = res.ResumeSpan
 		case *PutOperation:
-			err := b.Results[resultIdx].Err
-			subO.Result = resultInit(ctx, err)
-			subO.Result.ResumeSpan = b.Results[resultIdx].ResumeSpan
+			res := b.Results[resultIdx]
+			subO.Result = resultInit(ctx, res.Err)
+			subO.Result.ResumeSpan = res.ResumeSpan
 		case *ScanOperation:
 			res := b.Results[resultIdx]
-			kvs, err := res.Rows, res.Err
-			if err != nil {
-				subO.Result = resultInit(ctx, err)
+			if res.Err != nil {
+				subO.Result = resultInit(ctx, res.Err)
 			} else {
 				subO.Result.Type = ResultType_Values
-				subO.Result.Values = make([]KeyValue, len(kvs))
-				for j, kv := range kvs {
+				subO.Result.Values = make([]KeyValue, len(res.Rows))
+				for j, kv := range res.Rows {
 					subO.Result.Values[j] = KeyValue{
 						Key:   []byte(kv.Key),
 						Value: kv.Value.RawBytes,
@@ -660,26 +659,27 @@ func applyBatchOp(
 			}
 			subO.Result.ResumeSpan = res.ResumeSpan
 		case *DeleteOperation:
-			err := b.Results[resultIdx].Err
-			subO.Result = resultInit(ctx, err)
-			subO.Result.ResumeSpan = b.Results[resultIdx].ResumeSpan
+			res := b.Results[resultIdx]
+			subO.Result = resultInit(ctx, res.Err)
+			subO.Result.ResumeSpan = res.ResumeSpan
 		case *DeleteRangeOperation:
 			res := b.Results[resultIdx]
-			keys, err := res.Keys, res.Err
-			if err != nil {
-				subO.Result = resultInit(ctx, err)
+			if res.Err != nil {
+				subO.Result = resultInit(ctx, res.Err)
 			} else {
 				subO.Result.Type = ResultType_Keys
-				subO.Result.Keys = make([][]byte, len(keys))
-				for j, key := range keys {
+				subO.Result.Keys = make([][]byte, len(res.Keys))
+				for j, key := range res.Keys {
 					subO.Result.Keys[j] = key
 				}
 			}
 			subO.Result.ResumeSpan = res.ResumeSpan
 		case *DeleteRangeUsingTombstoneOperation:
-			subO.Result = resultInit(ctx, err)
-			subO.Result.ResumeSpan = b.Results[resultIdx].ResumeSpan
+			res := b.Results[resultIdx]
+			subO.Result = resultInit(ctx, res.Err)
+			subO.Result.ResumeSpan = res.ResumeSpan
 		case *MutateBatchHeaderOperation:
+			// NB: MutateBatchHeaderOperation cannot fail.
 			subO.Result = resultInit(ctx, nil)
 		case *AddSSTableOperation:
 			panic(errors.AssertionFailedf(`AddSSTable cannot be used in batches`))