Run saves scan in background so app startup doesn't freeze

app/gui.py

@@ -3616,6 +3616,33 @@ class SaveSyncPlanWorker(QObject):
         self.finished.emit()
 
 
+class SaveScanWorker(QObject):
+    """Scan the three SAVEDATA locations off the GUI thread."""
+    result = Signal(int, dict)  # gen, {location: {save_id: meta}}
+    finished = Signal()
+
+    def __init__(self, gen, locations):
+        super().__init__()
+        self.gen = gen
+        self.locations = locations
+        self._cancel = threading.Event()
+
+    def cancel(self):
+        self._cancel.set()
+
+    def run(self):
+        data = {}
+        for name, root in self.locations.items():
+            if self._cancel.is_set():
+                break
+            try:
+                data[name] = {s["id"]: s for s in saves.list_saves(root)}
+            except Exception:  # noqa: BLE001
+                data[name] = {}
+        self.result.emit(self.gen, data)
+        self.finished.emit()
+
+
 class SavesPage(QWidget):
     """PSP save-data sync between PSP, PPSSPP, and a cloud folder."""
 
@@ -3628,6 +3655,8 @@ class SavesPage(QWidget):
         self._cloud_dir = ""
         self._syncing = False
         self._closing = False
+        self._gen = 0
+        self._scan_worker = None
         self._threads = []
         self._workers = []
 
@@ -3783,9 +3812,32 @@ class SavesPage(QWidget):
         QMessageBox.information(self, "Export save", f"Exported to:\n{Path(dest) / sid}")
 
     def refresh(self):
-        locs = self._locations()
-        data = {name: {s["id"]: s for s in saves.list_saves(root)} for name, root in locs.items()}
+        if self._closing:
+            return
+        self._gen += 1
+        gen = self._gen
+        if self._scan_worker is not None:
+            self._scan_worker.cancel()
+        self.status.setText("Reading saves…")
+        thread = QThread(self)
+        worker = SaveScanWorker(gen, self._locations())
+        worker.moveToThread(thread)
+        thread.started.connect(worker.run)
+        worker.result.connect(self._on_saves_scanned)
+        worker.finished.connect(thread.quit)
+        self._scan_worker = worker
+        self._threads.append(thread)
+        self._workers.append(worker)
+        thread.start()
+
+    def _on_saves_scanned(self, gen, data):
+        if gen != self._gen or self._closing:
+            return
+        self._scan_worker = None
         self._data = data
+        self._populate(data)
+
+    def _populate(self, data):
         all_ids = set()
         for d in data.values():
             all_ids.update(d)
@@ -3835,7 +3887,7 @@ class SavesPage(QWidget):
         self._populating = False
         self.table.setSortingEnabled(True)
         self.table.sortByColumn(sort_col, sort_order)
-        self.status.setText(f"{len(all_ids)} save(s) across {sum(1 for r in locs.values() if r)} location(s)")
+        self.status.setText(f"{len(all_ids)} save(s) across {sum(1 for r in self._locations().values() if r)} location(s)")
         self._update_delete_enabled()
 
     def sync_now(self, switch_tab=False):