Fix Err fallout for graph/iterator.Not iterator

graph/iterator/not_iterator.go

@@ -12,6 +12,7 @@ type Not struct {
 	primaryIt graph.Iterator
 	allIt     graph.Iterator
 	result    graph.Value
+	err       error
 	runstats  graph.IteratorStats
 }
 
@@ -87,9 +88,16 @@ func (it *Not) Next() bool {
 			return graph.NextLogOut(it, curr, true)
 		}
 	}
+	if err := graph.Err(it.allIt); err != nil {
+		it.err = err
+	}
 	return graph.NextLogOut(it, nil, false)
 }
 
+func (it *Not) Err() error {
+	return it.err
+}
+
 func (it *Not) Result() graph.Value {
 	return it.result
 }

graph/iterator/not_iterator_test.go

@@ -1,6 +1,7 @@
 package iterator
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 )
@@ -42,3 +43,19 @@ func TestNotIteratorBasics(t *testing.T) {
 		}
 	}
 }
+
+func TestNotIteratorErr(t *testing.T) {
+	retErr := errors.New("unique")
+	allIt := newTestIterator(false, retErr)
+
+	toComplementIt := NewFixed(Identity)
+
+	not := NewNot(toComplementIt, allIt)
+
+	if not.Next() != false {
+		t.Errorf("Not iterator did not pass through initial 'false'")
+	}
+	if not.Err() != retErr {
+		t.Errorf("Not iterator did not pass through underlying Err")
+	}
+}