handle some edge cases

internal/file_store.go

@@ -50,6 +50,9 @@ func (fs *FileStore) Load(ctx context.Context) (map[string]*FillerQueue, error)
 		}
 		return nil, fmt.Errorf("reading state file: %w", err)
 	}
+	if len(data) == 0 {
+		return make(map[string]*FillerQueue), nil
+	}
 
 	var s State
 	if err := json.Unmarshal(data, &s); err != nil {
@@ -74,7 +77,7 @@ func (fs *FileStore) Load(ctx context.Context) (map[string]*FillerQueue, error)
 	for symbol, persisted := range s.Queues {
 		q := new(FillerQueue)
 		for _, pf := range persisted {
-			rec, err := fs.serializer.UnmarshalRecord(ctx, pf.ReaderData)
+			rec, err := fs.serializer.UnmarshalRecord(ctx, pf.RecordData)
 			if err != nil {
 				return nil, fmt.Errorf(
 					"unmarshalling record for symbol %q: %w", symbol, err,
@@ -98,7 +101,7 @@ func (fs *FileStore) Save(ctx context.Context, queue map[string]*FillerQueue) er
 	}
 
 	for symbol, q := range queue {
-		if q == nil {
+		if q == nil || q.Len() == 0 {
 			continue
 		}
 		var persisted []persistedFiller
@@ -109,7 +112,7 @@ func (fs *FileStore) Save(ctx context.Context, queue map[string]*FillerQueue) er
 				return fmt.Errorf("marshalling record for symbol %q: %w", symbol, err)
 			}
 			persisted = append(persisted, persistedFiller{
-				ReaderData: data,
+				RecordData: data,
 				Quantity:   f.Quantity(),
 				Price:      f.Price(),
 				Filled:     f.Filled(),

internal/file_store_test.go

@@ -19,7 +19,7 @@ import (
 func TestFileStore_RoundTrip(t *testing.T) {
 	ctrl := gomock.NewController(t)
 	ser := roundTripSerializer(ctrl)
-	store, _ := newStore(t, "fake", ser)
+	store, path := newStore(t, "fake", ser)
 
 	original := map[string]*internal.FillerQueue{
 		"AAA": newQueue(
@@ -50,6 +50,73 @@ func TestFileStore_RoundTrip(t *testing.T) {
 		}
 		assertQueueEqual(t, symbol, gotQ, wantQ)
 	}
+
+	// Regression: the on-disk JSON key for the per-record blob must be
+	// "record_data" (renamed from "reader_data") so the field name stays
+	// honest about what it carries.
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read state file: %v", err)
+	}
+	if strings.Contains(string(data), `"reader_data"`) {
+		t.Errorf("saved state file still contains legacy key \"reader_data\"; want only \"record_data\"")
+	}
+	if !strings.Contains(string(data), `"record_data"`) {
+		t.Errorf("saved state file does not contain expected key \"record_data\"")
+	}
+}
+
+func TestFileStore_SaveSkipsEmptyQueue(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	ser := roundTripSerializer(ctrl)
+	store, path := newStore(t, "fake", ser)
+
+	// A non-nil FillerQueue whose underlying list is nil — i.e. a symbol
+	// that was registered but never received a Push. Save must not panic
+	// when iterating and must not emit any entry for that symbol.
+	queues := map[string]*internal.FillerQueue{
+		"EMPTY": new(internal.FillerQueue),
+		"REAL":  newQueue(newFiller(ctrl, "REAL", 5, 10, 0)),
+	}
+
+	if err := store.Save(t.Context(), queues); err != nil {
+		t.Fatalf("Save returned unexpected error: %v", err)
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read state file: %v", err)
+	}
+	body := string(data)
+	if strings.Contains(body, `"EMPTY"`) {
+		t.Errorf("saved state file contains entry for empty queue \"EMPTY\"; want it skipped")
+	}
+	if !strings.Contains(body, `"REAL"`) {
+		t.Errorf("saved state file missing expected entry for \"REAL\"")
+	}
+}
+
+func TestFileStore_LoadEmptyFileReturnsEmpty(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	ser := roundTripSerializer(ctrl)
+	store, path := newStore(t, "fake", ser)
+
+	// Pre-create the state file as a zero-byte file. Load must treat this
+	// the same as a missing file rather than failing JSON unmarshal.
+	if err := os.WriteFile(path, []byte{}, 0o644); err != nil {
+		t.Fatalf("write empty state file: %v", err)
+	}
+
+	queues, err := store.Load(t.Context())
+	if err != nil {
+		t.Fatalf("Load returned unexpected error for empty file: %v", err)
+	}
+	if queues == nil {
+		t.Fatalf("Load returned nil map; want empty map")
+	}
+	if len(queues) != 0 {
+		t.Fatalf("Load returned %d entries; want 0", len(queues))
+	}
 }
 
 func TestFileStore_LoadMissingFileReturnsEmpty(t *testing.T) {

internal/state.go

@@ -21,7 +21,7 @@ type State struct {
 // Record-specific data is kept as opaque bytes so the internal package does
 // not need to know about any broker package's concrete Record type.
 type persistedFiller struct {
-	ReaderData []byte          `json:"reader_data"`
+	RecordData []byte          `json:"record_data"`
 	Quantity   decimal.Decimal `json:"quantity"`
 	Price      decimal.Decimal `json:"price"`
 	Filled     decimal.Decimal `json:"filled"`