Add NaiveLimit solution

mount/mount.go

@@ -227,12 +227,6 @@ func (d *Datastore) Delete(key ds.Key) error {
 }
 
 func (d *Datastore) Query(q query.Query) (query.Results, error) {
-	if q.Limit > 0 ||
-		q.Offset > 0 {
-		// TODO this is still overly simplistic, but the only callers are
-		// `ipfs refs local` and ipfs-ds-convert.
-		return nil, errors.New("mount only supports listing all prefixed keys in random order")
-	}
 	prefix := ds.NewKey(q.Prefix)
 	dses, mounts, rests := d.lookupAll(prefix)
 
@@ -257,10 +251,12 @@ func (d *Datastore) Query(q query.Query) (query.Results, error) {
 		queries.addResults(mount, results)
 	}
 
-	return query.ResultsFromIterator(q, query.Iterator{
+	qr := query.ResultsFromIterator(q, query.Iterator{
 		Next:  queries.next,
 		Close: queries.close,
-	}), nil
+	})
+
+	return query.NaiveLimit(qr, q.Limit), nil
 }
 
 func (d *Datastore) Close() error {

mount/mount_test.go

@@ -2,12 +2,13 @@ package mount_test
 
 import (
 	"errors"
+	"github.com/ipfs/go-datastore/sync"
 	"testing"
 
 	"github.com/ipfs/go-datastore"
-	mount "github.com/ipfs/go-datastore/mount"
+	"github.com/ipfs/go-datastore/mount"
 	"github.com/ipfs/go-datastore/query"
-	dstest "github.com/ipfs/go-datastore/test"
+	"github.com/ipfs/go-datastore/test"
 )
 
 func TestPutBadNothing(t *testing.T) {
@@ -262,6 +263,10 @@ func TestQueryCross(t *testing.T) {
 	}
 	entries, err := res.Rest()
 	if err != nil {
+		err = res.Close()
+		if err != nil {
+			t.Errorf("result.Close failed %d", err)
+		}
 		t.Fatalf("Query Results.Rest fail: %v\n", err)
 	}
 	seen := 0
@@ -346,6 +351,56 @@ func TestQueryCrossWithSort(t *testing.T) {
 	}
 }
 
+func TestQueryLimitCrossWithSort(t *testing.T) {
+	mapds1 := sync.MutexWrap(datastore.NewMapDatastore())
+	mapds2 := sync.MutexWrap(datastore.NewMapDatastore())
+	mapds3 := sync.MutexWrap(datastore.NewMapDatastore())
+	m := mount.New([]mount.Mount{
+		{Prefix: datastore.NewKey("/rok"), Datastore: mapds1},
+		{Prefix: datastore.NewKey("/zoo"), Datastore: mapds2},
+		{Prefix: datastore.NewKey("/noop"), Datastore: mapds3},
+	})
+
+	m.Put(datastore.NewKey("/rok/0"), []byte("ghi"))
+	m.Put(datastore.NewKey("/zoo/0"), []byte("123"))
+	m.Put(datastore.NewKey("/rok/1"), []byte("def"))
+	m.Put(datastore.NewKey("/zoo/1"), []byte("167"))
+	m.Put(datastore.NewKey("/zoo/2"), []byte("345"))
+	m.Put(datastore.NewKey("/rok/3"), []byte("abc"))
+	m.Put(datastore.NewKey("/zoo/3"), []byte("456"))
+
+	q := query.Query{Limit: 2, Orders: []query.Order{query.OrderByKeyDescending{}}}
+	res, err := m.Query(q)
+	if err != nil {
+		t.Fatalf("Query fail: %v\n", err)
+	}
+
+	entries, err := res.Rest()
+	if err != nil {
+		t.Fatalf("Query Results.Rest fail: %v\n", err)
+	}
+
+	expect := []string{
+		"/zoo/3",
+		"/zoo/2",
+	}
+
+	if len(entries) != len(expect) {
+		t.Fatalf("expected %d entries, but got %d", len(expect), len(entries))
+	}
+
+	for i, e := range expect {
+		if e != entries[i].Key {
+			t.Errorf("expected key %s, but got %s", e, entries[i].Key)
+		}
+	}
+
+	err = res.Close()
+	if err != nil {
+		t.Errorf("result.Close failed %d", err)
+	}
+}
+
 func TestLookupPrio(t *testing.T) {
 	mapds0 := datastore.NewMapDatastore()
 	mapds1 := datastore.NewMapDatastore()

query/query.go

@@ -3,7 +3,7 @@ package query
 import (
 	"time"
 
-	goprocess "github.com/jbenet/goprocess"
+	"github.com/jbenet/goprocess"
 )
 
 /*

query/query_impl.go

 package query
 
-import "sort"
+import (
+	"sort"
+)
 
 func DerivedResults(qr Results, ch <-chan Result) Results {
 	return &results{
@@ -48,7 +50,7 @@ func NaiveLimit(qr Results, limit int) Results {
 		}
 	}()
 
-	return DerivedResults(qr, ch)
+	return ResultsWithChan(qr.Query(), ch)
 }
 
 // NaiveOffset skips a given number of results
@@ -122,6 +124,7 @@ func NaiveQueryApply(q Query, qr Results) Results {
 		qr = NaiveOffset(qr, q.Offset)
 	}
 	if q.Limit != 0 {
+		// TODO: Offset?
 		qr = NaiveLimit(qr, q.Offset)
 	}
 	return qr