batcheval: add TODO to improve Refresh{Range}Request

Also improve a comment.

Release note: None

pkg/storage/batcheval/cmd_refresh.go

@@ -53,13 +53,17 @@ func Refresh(
 	if err != nil {
 		return result.Result{}, err
 	} else if val != nil {
+		// TODO(nvanbenschoten): This is pessimistic. We only need to check
+		//   !ts.Less(h.Txn.PrevRefreshTimestamp)
+		// This could avoid failed refreshes due to requests performed after
+		// earlier refreshes.
 		if ts := val.Timestamp; !ts.Less(h.Txn.OrigTimestamp) {
 			return result.Result{}, errors.Errorf("encountered recently written key %s @%s", args.Key, ts)
 		}
 	}
 
-	// Now, check if the intent was written earlier than the command's timestamp
-	// and was not owned by this transaction.
+	// Check if an intent which is not owned by this transaction was written
+	// at or beneath the refresh timestamp.
 	if intent != nil && intent.Txn.ID != h.Txn.ID {
 		return result.Result{}, errors.Errorf("encountered recently written intent %s @%s",
 			intent.Span.Key, intent.Txn.Timestamp)

pkg/storage/batcheval/cmd_refresh_range.go

@@ -52,6 +52,10 @@ func RefreshRange(
 		Inconsistent: true,
 		Tombstones:   true,
 	}, func(kv roachpb.KeyValue) (bool, error) {
+		// TODO(nvanbenschoten): This is pessimistic. We only need to check
+		//   !ts.Less(h.Txn.PrevRefreshTimestamp)
+		// This could avoid failed refreshes due to requests performed after
+		// earlier refreshes.
 		if ts := kv.Value.Timestamp; !ts.Less(h.Txn.OrigTimestamp) {
 			return true, errors.Errorf("encountered recently written key %s @%s", kv.Key, ts)
 		}
@@ -61,8 +65,8 @@ func RefreshRange(
 		return result.Result{}, err
 	}
 
-	// Now, check intents slice for any which were written earlier than
-	// the command's timestamp, not owned by this transaction.
+	// Check if any intents which are not owned by this transaction were written
+	// at or beneath the refresh timestamp.
 	for _, i := range intents {
 		// Ignore our own intents.
 		if i.Txn.ID == h.Txn.ID {