Clean up callback interfaces.

filter/txnBuf/query_merger.go

Index: filter/txnBuf/query_merger.go
diff --git a/filter/txnBuf/query_merger.go b/filter/txnBuf/query_merger.go
index e87d91ea5daed6de1b5ebc7f5999d377b46d30ae..d0e7b37f45cd03c618e561d64b6d907a0f05e141 100644
--- a/filter/txnBuf/query_merger.go
+++ b/filter/txnBuf/query_merger.go
@@ -26,13 +26,13 @@ func queryToIter(stopChan chan struct{}, fq *ds.FinalizedQuery, d ds.RawInterfac
 	go func() {
 		defer close(c)
 
-		err := d.Run(fq, func(k *ds.Key, pm ds.PropertyMap, _ ds.CursorCB) (keepGoing bool) {
+		err := d.Run(fq, func(k *ds.Key, pm ds.PropertyMap, _ ds.CursorCB) error {
 			i := &item{key: k, data: pm}
 			select {
 			case c <- i:
-				return true
+				return nil
 			case <-stopChan:
-				return false
+				return ds.Stop
 			}
 		})
 		if err != nil {
@@ -115,7 +115,7 @@ func adjustQuery(fq *ds.FinalizedQuery) (*ds.FinalizedQuery, error) {
 //
 // See also `dsTxnBuf.Run()`.
 func runMergedQueries(fq *ds.FinalizedQuery, sizes *sizeTracker,
-	memDS, parentDS ds.RawInterface, cb func(k *ds.Key, data ds.PropertyMap) bool) error {
+	memDS, parentDS ds.RawInterface, cb func(k *ds.Key, data ds.PropertyMap) error) error {
 
 	toRun, err := adjustQuery(fq)
 	if err != nil {
@@ -234,8 +234,11 @@ func runMergedQueries(fq *ds.FinalizedQuery, sizes *sizeTracker,
 				continue
 			}
 		}
-		if !cb(toUse.key, toUse.data) {
-			break
+		if err := cb(toUse.key, toUse.data); err != nil {
+			if err == ds.Stop {
+				err = nil
+			}
+			return err
 		}
 	}