urwid: prevent memory leak

* Urwid has a cache (CanvasCache) that caches the render of each widget
  to speed up future render cycles.
* Tui is a dynamic tool, it creates a new TuiNode for each update.
* The old TuiNode and its child nodes were not being cleared out of the
  urwid cache preventing Python's garbage clearance from doing its job.
* This change explicitly clears the urwid cache for each Tui update.
* Preserving the cache post-update isn't really a performance boost for
  Tui as pretty much every node on the screen has changed anyway
  (besides the application header and footer). So this is likely of
  little consequence performance wise. The cache is still active between
  updates so things like expand/collapse operations will still benefit.

Author: oliver-sanders

changes.d/6722.fix.md

@@ -0,0 +1 @@
+Fix a slow memory leak in Tui.

cylc/flow/tui/app.py

@@ -22,13 +22,8 @@
 import re
 
 import urwid
-try:
-    from urwid.widget import SelectableIcon
-except ImportError:
-    # BACK COMPAT: urwid.wimp
-    # From: urwid 2.0
-    # To: urwid 2.2
-    from urwid.wimp import SelectableIcon
+from urwid.canvas import CanvasCache
+from urwid.widget import SelectableIcon
 
 from cylc.flow.id import Tokens
 from cylc.flow.task_state import (
@@ -541,6 +536,11 @@ def update(self, *_):
         if self.loop:
             self.loop.set_alarm_in(self.UPDATE_INTERVAL, self.update)
 
+        # NOTE: prevent a memory leak by clearing out any caches that urwid
+        # may have accumulated for the previous TuiNode instance
+        # (and its child widgets)
+        CanvasCache.clear()
+
         return True
 
     def filter_by_task_state(self, filtered_state=None):

setup.cfg

@@ -76,7 +76,7 @@ install_requires =
     pyzmq>=22
     importlib_metadata>=5.0; python_version < "3.12"
     # NOTE: exclude two urwid versions that were not compatible with Tui
-    urwid==2.*,!=2.6.2,!=2.6.3
+    urwid>=2.2,!=2.6.2,!=2.6.3,<3
     # unpinned transient dependencies used for type checking
     rx
     promise