[#149] selector: Add read timeout

If there are no objects for 10 second, then return nil.
It is required to prevent VU iteration hang if there
are no objects pushed to registry.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/registry/obj_selector.go

@@ -9,6 +9,8 @@ import (
 	"go.etcd.io/bbolt"
 )
 
+const nextObjectTimeout = 10 * time.Second
+
 type ObjFilter struct {
 	Status string
 	Age    int
@@ -57,7 +59,16 @@ func NewObjSelector(registry *ObjRegistry, selectionSize int, kind SelectorKind,
 //   - underlying registry context is done, nil objects will be returned on the
 //     currently blocked and every further NextObject calls.
 func (o *ObjSelector) NextObject() *ObjectInfo {
-	return <-o.objChan
+	if o.kind == SelectorOneshot {
+		return <-o.objChan
+	}
+
+	select {
+	case <-time.After(nextObjectTimeout):
+		return nil
+	case obj := <-o.objChan:
+		return obj
+	}
 }
 
 // Count returns total number of objects that match filter of the selector.