Live mode: load sheets off the UI thread, go offline cleanly on a lost device

Opening a sheet ran its Sox requests on the UI thread, so a device that had
gone away (a closed simulator) froze the window until DASP's 30 s receive
timeout. The load now runs on a background thread and the sheet is painted
when it finishes; a load the user has clicked away from is dropped.

When the session closes, the editor goes offline with "Connection lost:
<cause>" instead of printing a traceback. A send waiting for room in the
send window now fails when the session closes rather than going out after
the CLOSE. The tree-expand error callback no longer reads the exception
after its except block has ended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app_state.py

@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.182"
+VERSION = "0.0.0.183"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True

dasp.py

@@ -598,7 +598,9 @@ class SendWindow:
             while self._is_alive and (self._full() or self._exceeds_ack_more_limit(p)):
                 self._cond.wait()
             if not self._is_alive:
-                return
+                # Waiting for room when the session closed: it must not go out after
+                # the CLOSE, and the caller must not take it as sent.
+                raise DaspException(f"Session closed: {self._session.close_cause}")
             if self._tail is None:
                 self._head = self._tail = p
             else:

device_session.py

@@ -111,6 +111,10 @@ class DeviceSession:
     def is_connected(self):
         return self.sox_client is not None and not self.sox_client.is_closed
 
+    @property
+    def close_cause(self):
+        return self.sox_client.close_cause if self.sox_client else "disconnected"
+
     # ── Schema → Slot Registry ──────────────────────────────────
 
     def _load_slot_registry(self, parts):

main.py

@@ -1109,7 +1109,6 @@ def on_tree_select(event):
 
     selected_id = selected_items[0]
     element = app_state.tree_element_map.get(selected_id)
-    components_to_render = []
 
     # Remembered so undo and tidy can repaint this same sheet from the DOM.
     app_state.current_sheet_parent = element
@@ -1118,42 +1117,16 @@ def on_tree_select(event):
     if app_state.live_mode and app_state.device_session and element is not None:
         comp_id_str = element.get("id")
         if comp_id_str is not None:
-            ds = app_state.device_session
-            ds.close_wiresheet()
-            try:
-                ds.open_wiresheet(int(comp_id_str))
-                sax_root = ds.build_dom_tree(ds.sox_client.cache_get(0))
-                ds.build_links_elements(sax_root)
-                app_state.xml_root_element = sax_root
-                sax_file.refresh_treeview_from_dom(sax_root)
-                # re-find element by comp id after DOM rebuild (tree item IDs changed)
-                element = None
-                for tid, elem in app_state.tree_element_map.items():
-                    if elem.get("id") == comp_id_str:
-                        element = elem
-                        selected_id = tid
-                        break
-                if element is not None:
-                    _restoring_selection = True
-                    tree.selection_set(selected_id)
-                    tree.see(selected_id)
-                    tree.after_idle(_clear_restoring_flag)
-                app_state.current_sheet_parent = element
-            except Exception:
-                import traceback; traceback.print_exc()
+            _load_live_sheet(app_state.device_session, comp_id_str, pending_link)
+            return
 
-        # The DOM rebuild replaced every element object, so a pending link from the
-        # old tree can no longer be found by identity.  Re-resolve it by from/to.
-        if pending_link is not None:
-            old_from = pending_link.get("from", "")
-            old_to = pending_link.get("to", "")
-            links_container = app_state.xml_root_element.find("links") if app_state.xml_root_element is not None else None
-            if links_container is not None:
-                for lnk in links_container:
-                    if lnk.get("from") == old_from and lnk.get("to") == old_to:
-                        pending_link = lnk
-                        break
+    _paint_sheet(element, selected_id, pending_link)
 
+
+def _paint_sheet(element, selected_id, pending_link):
+    """Paints the sheet of `element` (or of the tree row `selected_id` when it has none)."""
+    tree = app_state.tree
+    components_to_render = []
     if element is not None:
         components_to_render = [child for child in element if child.tag == "comp"]
     elif tree.exists(selected_id):
@@ -1170,6 +1143,98 @@ def on_tree_select(event):
         select_link_on_open_sheet(pending_link)
 
 
+# Bumped by every live sheet load, so a load the user has clicked away from is dropped
+# when it finishes rather than painted over the sheet they moved on to.
+_sheet_load_gen = 0
+# One load talks to the device at a time: each starts by unsubscribing the sheet the
+# previous one subscribed.
+_sheet_load_lock = threading.Lock()
+
+
+def _load_live_sheet(ds, comp_id_str, pending_link):
+    """Reads a folder's sheet from the device on a background thread, then paints it.
+
+    Sox requests wait for the device. A device that has gone away holds them until DASP
+    gives up (30 s), which on the UI thread froze the whole window.
+    """
+    global _sheet_load_gen
+    _sheet_load_gen += 1
+    gen = _sheet_load_gen
+
+    # Blank until the device answers: the rows on screen belong to the DOM the load
+    # is about to replace.
+    wiresheet.render_components([])
+    app_state.set_status("Loading sheet...")
+
+    def _bg():
+        with _sheet_load_lock:
+            if gen != _sheet_load_gen:
+                return      # clicked away while waiting for the previous load
+            try:
+                ds.close_wiresheet()
+                ds.open_wiresheet(int(comp_id_str))
+                sax_root = ds.build_dom_tree(ds.sox_client.cache_get(0))
+                ds.build_links_elements(sax_root)
+            except Exception as exc:
+                app_state.window.after(
+                    0, lambda err=exc: _live_sheet_failed(ds, gen, err))
+                return
+        app_state.window.after(
+            0, lambda: _live_sheet_loaded(ds, gen, comp_id_str, pending_link, sax_root))
+
+    threading.Thread(target=_bg, daemon=True).start()
+
+
+def _live_sheet_failed(ds, gen, exc):
+    if gen != _sheet_load_gen or app_state.device_session is not ds:
+        return
+    if not ds.is_connected:
+        _on_connection_lost(ds, ds.close_cause)
+        return
+    app_state.set_status(f"Could not open sheet: {exc}", warn=True)
+
+
+def _live_sheet_loaded(ds, gen, comp_id_str, pending_link, sax_root):
+    """UI thread: swaps in the freshly read DOM and paints the sheet."""
+    global _restoring_selection
+    if gen != _sheet_load_gen or app_state.device_session is not ds:
+        return
+
+    tree = app_state.tree
+    app_state.xml_root_element = sax_root
+    sax_file.refresh_treeview_from_dom(sax_root)
+    # re-find element by comp id after DOM rebuild (tree item IDs changed)
+    element = None
+    selected_id = None
+    for tid, elem in app_state.tree_element_map.items():
+        if elem.get("id") == comp_id_str:
+            element = elem
+            selected_id = tid
+            break
+    if element is not None:
+        _restoring_selection = True
+        tree.selection_set(selected_id)
+        tree.see(selected_id)
+        tree.after_idle(_clear_restoring_flag)
+    app_state.current_sheet_parent = element
+
+    # The DOM rebuild replaced every element object, so a pending link from the
+    # old tree can no longer be found by identity.  Re-resolve it by from/to.
+    if pending_link is not None:
+        old_from = pending_link.get("from", "")
+        old_to = pending_link.get("to", "")
+        links_container = sax_root.find("links")
+        if links_container is not None:
+            for lnk in links_container:
+                if lnk.get("from") == old_from and lnk.get("to") == old_to:
+                    pending_link = lnk
+                    break
+
+    app_state.clear_status()
+    if element is not None:
+        _paint_sheet(element, selected_id, pending_link)
+
+
 def delete_selected_link():
     """Removes every selected link from the DOM, as one undoable step, and repaints."""
     link_elems = []
@@ -3157,6 +3222,15 @@ def do_device_disconnect(device_down=False):
     wiresheet.render_components([])
 
 
+def _on_connection_lost(ds, cause):
+    """Main thread: the device stopped answering (closed simulator, unplugged cable) or
+    ended the session. Goes offline as Disconnect does, without asking anything of it."""
+    if app_state.device_session is not ds:
+        return      # already disconnected, or a newer connection has taken its place
+    do_device_disconnect(device_down=True)
+    app_state.set_status(f"Connection lost: {cause}", warn=True)
+
+
 def quit_app():
     """File > Exit and the window's close box: let go of the device before going.
 
@@ -3190,11 +3264,13 @@ def _start_cov_polling():
         ds = app_state.device_session
         if ds is None or not ds.is_connected:
             _cov_poll_id = None
-            if app_state.live_mode:
-                do_device_disconnect()
-                app_state.set_status("Connection lost", warn=True)
+            if ds is not None and app_state.live_mode:
+                _on_connection_lost(ds, ds.close_cause)
             return
 
+        # Scheduled first, so one bad event cannot stop the polling for good.
+        _cov_poll_id = app_state.window.after(100, poll)
+
         events = ds.poll_events()
         if events and app_state.xml_root_element is not None:
             needs_repaint = False
@@ -3210,8 +3286,6 @@ def _start_cov_polling():
                     except Exception:
                         pass
 
-        _cov_poll_id = app_state.window.after(100, poll)
-
     _cov_poll_id = app_state.window.after(100, poll)
 
 
@@ -3272,8 +3346,11 @@ def _live_tree_expand(comp_id, tree_item_id, select=True, on_loaded=None):
 
             app_state.window.after(0, _ui)
         except Exception as exc:
+            if not ds.is_connected:
+                # The COV poll sees the closed session, goes offline and says so.
+                return
             app_state.window.after(
-                0, lambda: app_state.set_status(f"Load error: {exc}", warn=True)
+                0, lambda msg=f"Load error: {exc}": app_state.set_status(msg, warn=True)
             )
 
     threading.Thread(target=_bg, daemon=True).start()

sox.py

@@ -421,6 +421,11 @@ class SoxClient:
     def is_closed(self):
         return self._session is None or self._session.is_closed
 
+    @property
+    def close_cause(self):
+        """Why the session ended, e.g. "receive timeout"."""
+        return self._session.close_cause if self._session else "closed"
+
     # ── Read Schema ─────────────────────────────────────────────
 
     def read_schema(self):