Fixes memstore transaction semantics

graph/memstore/quadstore.go

@@ -106,6 +106,26 @@ func newQuadStore() *QuadStore {
 }
 
 func (qs *QuadStore) ApplyDeltas(deltas []graph.Delta, ignoreOpts graph.IgnoreOpts) error {
+	// Precheck the whole transaction
+	for _, d := range deltas {
+		switch d.Action {
+		case graph.Add:
+			if !ignoreOpts.IgnoreDup {
+				if _, exists := qs.indexOf(d.Quad); exists {
+					return graph.ErrQuadExists
+				}
+			}
+		case graph.Delete:
+			if !ignoreOpts.IgnoreMissing {
+				if _, exists := qs.indexOf(d.Quad); !exists {
+					return graph.ErrQuadNotExist
+				}
+			}
+		default:
+			return errors.New("memstore: invalid action")
+		}
+	}
+
 	for _, d := range deltas {
 		var err error
 		switch d.Action {
@@ -120,7 +140,7 @@ func (qs *QuadStore) ApplyDeltas(deltas []graph.Delta, ignoreOpts graph.IgnoreOp
 				err = nil
 			}
 		default:
-			err = errors.New("memstore: invalid action")
+			panic("memstore: unexpected invalid action")
 		}
 		if err != nil {
 			return err

graph/memstore/quadstore_test.go

@@ -180,13 +180,17 @@ func TestLinksToOptimization(t *testing.T) {
 func TestRemoveQuad(t *testing.T) {
 	qs, w, _ := makeTestStore(simpleGraph)
 
-	w.RemoveQuad(quad.Quad{
+	err := w.RemoveQuad(quad.Quad{
 		Subject:   "E",
 		Predicate: "follows",
 		Object:    "F",
 		Label:     "",
 	})
 
+	if err != nil {
+		t.Error("Couldn't remove quad", err)
+	}
+
 	fixed := qs.FixedIterator()
 	fixed.Add(qs.ValueOf("E"))
 
@@ -204,3 +208,28 @@ func TestRemoveQuad(t *testing.T) {
 		t.Error("E should not have any followers.")
 	}
 }
+
+func TestTransaction(t *testing.T) {
+	qs, w, _ := makeTestStore(simpleGraph)
+	size := qs.Size()
+
+	tx := graph.NewTransaction()
+	tx.AddQuad(quad.Quad{
+		Subject:   "E",
+		Predicate: "follows",
+		Object:    "G",
+		Label:     ""})
+	tx.RemoveQuad(quad.Quad{
+		Subject:   "Non",
+		Predicate: "existent",
+		Object:    "quad",
+		Label:     ""})
+
+	err := w.ApplyTransaction(tx)
+	if err == nil {
+		t.Error("Able to remove a non-existent quad")
+	}
+	if size != qs.Size() {
+		t.Error("Appended a new quad in a failed transaction")
+	}
+}