Fail loudly on sibling filename collisions

objects/objects.go

@@ -140,6 +140,17 @@ func (o *objConfig) parseFromJSON(data map[string]interface{}) error {
 		delete(o.data, "ContainedObjects")
 	}
 
+	// Contained objects and states are all written into the same subdirectory
+	// (see printToFile), so their filenames must be collectively unique.
+	children := make([]*objConfig, 0, len(o.subObj)+len(o.states))
+	children = append(children, o.subObj...)
+	for _, st := range o.states {
+		children = append(children, st)
+	}
+	if err := checkFilenameCollisions(children); err != nil {
+		return fmt.Errorf("children of %q: %v", o.guid, err)
+	}
+
 	return nil
 }
 
@@ -337,6 +348,23 @@ func (o *objConfig) getAGoodFileName() string {
 	return n + "." + o.guid
 }
 
+// checkFilenameCollisions returns an error if any two objects in the slice
+// produce the same getAGoodFileName(). The name is both an object's on-disk
+// filename and its identity in the *_order arrays, so a collision among
+// siblings sharing a directory would silently overwrite the first object with
+// the second. It must be a hard error, not a silent overwrite (see issue #107).
+func checkFilenameCollisions(objs []*objConfig) error {
+	seen := map[string]string{} // filename -> guid that first produced it
+	for _, o := range objs {
+		name := o.getAGoodFileName()
+		if prevGUID, ok := seen[name]; ok {
+			return fmt.Errorf("filename collision: %q is produced by two sibling objects (GUIDs %q and %q); sibling filenames must be unique", name, prevGUID, o.guid)
+		}
+		seen[name] = o.guid
+	}
+	return nil
+}
+
 func (o *objConfig) tryGetNonEmptyStr(key string) (string, error) {
 	rawname, ok := o.data[key]
 	if !ok {
@@ -438,16 +466,23 @@ type Printer struct {
 func (p *Printer) PrintObjectStates(root string, objs []map[string]interface{}) ([]string, error) {
 	order := []string{}
 
+	ocs := make([]*objConfig, 0, len(objs))
 	for _, rootObj := range objs {
-		oc := objConfig{}
-
-		err := oc.parseFromJSON(rootObj)
-		if err != nil {
+		oc := &objConfig{}
+		if err := oc.parseFromJSON(rootObj); err != nil {
 			return nil, err
 		}
+		ocs = append(ocs, oc)
+	}
+	// Root objects all share the top-level objects directory, so their
+	// filenames must be unique or one would silently overwrite another.
+	if err := checkFilenameCollisions(ocs); err != nil {
+		return nil, fmt.Errorf("root objects: %v", err)
+	}
+
+	for _, oc := range ocs {
 		order = append(order, oc.getAGoodFileName())
-		err = oc.printToFile(root, p)
-		if err != nil {
+		if err := oc.printToFile(root, p); err != nil {
 			return nil, err
 		}
 	}

objects/objects_test.go

@@ -551,6 +551,64 @@ func TestPrintAllObjs(t *testing.T) {
 
 }
 
+// TestPrintObjectStatesCollision asserts that two sibling root objects whose
+// nicknames sanitize to the same getAGoodFileName() are a hard error rather
+// than a silent overwrite (issue #107).
+func TestPrintObjectStatesCollision(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		objs []map[string]interface{}
+	}{
+		{
+			// Objects inside containers can share a GUID in practice; two
+			// roots with the same GUID collapse to the same bare filename.
+			name: "shared guid",
+			objs: []map[string]interface{}{
+				{"GUID": "abc123"},
+				{"GUID": "abc123"},
+			},
+		},
+		{
+			// Nicknames differing only in stripped punctuation collapse to
+			// the same sanitized name.
+			name: "stripped punctuation",
+			objs: []map[string]interface{}{
+				{"GUID": "abc123", "Nickname": "My Deck ()"},
+				{"GUID": "abc123", "Nickname": "My Deck []"},
+			},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			ff := tests.NewFF()
+			p := &Printer{
+				Lua: ff,
+				Dir: ff,
+				J:   ff,
+			}
+			if _, err := p.PrintObjectStates("", tc.objs); err == nil {
+				t.Errorf("expected a filename collision error, got nil")
+			}
+		})
+	}
+}
+
+// TestContainedObjectCollision asserts that two contained (sibling) objects
+// that produce the same getAGoodFileName() cause parseFromJSON to error rather
+// than silently overwrite one another (issue #107).
+func TestContainedObjectCollision(t *testing.T) {
+	o := objConfig{}
+	err := o.parseFromJSON(map[string]interface{}{
+		"GUID": "parent1",
+		"ContainedObjects": []interface{}{
+			map[string]interface{}{"GUID": "dup", "Nickname": "Card"},
+			map[string]interface{}{"GUID": "dup", "Nickname": "Card"},
+		},
+	})
+	if err == nil {
+		t.Errorf("expected a filename collision error for contained objects, got nil")
+	}
+}
+
 func TestDBPrint(t *testing.T) {
 	ff := tests.NewFF()
 	for _, tc := range []struct {