Fix memory leak with the view store when removing items

The previous code would maintain items in the dispatch mapping if
nested children were removed between calls because it would only
remove items that are live in the view at the point of removal. This
meant that calling something like ActionRow.clear_items() would keep
all the removed items within the mapping and would not be evicted.

This attempts to fix it by maintaining a cache state snapshot and
making a diff between the two versions to know which keys are now safe
to delete since they are no longer in the live view at all.

discord/ui/view.py

@@ -207,6 +207,24 @@ class _ViewWeights:
         self.weights = [0, 0, 0, 0, 0]
 
 
+class _ViewCacheSnapshot:
+    __slots__ = ('items', 'dynamic_items')
+
+    def __init__(self) -> None:
+        self.items: Set[Tuple[int, str]] = set()
+        self.dynamic_items: Set[re.Pattern[str]] = set()
+
+    @classmethod
+    def diff(cls, older: _ViewCacheSnapshot, newer: _ViewCacheSnapshot) -> Self:
+        self = cls()
+        self.items = older.items - newer.items
+        self.dynamic_items = older.dynamic_items - newer.dynamic_items
+        return self
+
+    def __repr__(self) -> str:
+        return f'<_ViewCacheSnapshot items={self.items!r} dynamic_items={self.dynamic_items!r}>'
+
+
 class BaseView:
     __discord_ui_view__: ClassVar[bool] = False
     __discord_ui_modal__: ClassVar[bool] = False
@@ -220,6 +238,7 @@ class BaseView:
         self.__cancel_callback: Optional[Callable[[BaseView], None]] = None
         self.__timeout_expiry: Optional[float] = None
         self.__timeout_task: Optional[asyncio.Task[None]] = None
+        self.__snapshot: Optional[_ViewCacheSnapshot] = None
 
         try:
             loop = asyncio.get_running_loop()
@@ -326,6 +345,31 @@ class BaseView:
     def _add_count(self, value: int) -> None:
         self._total_children = max(0, self._total_children + value)
 
+    @property
+    def _snapshot(self) -> Optional[_ViewCacheSnapshot]:
+        return self.__snapshot
+
+    def _get_snapshot_diff(self) -> Optional[_ViewCacheSnapshot]:
+        if self.__snapshot is None:
+            self.__snapshot = self._get_snapshot()
+            return None
+
+        newer = self._get_snapshot()
+        diff = _ViewCacheSnapshot.diff(older=self.__snapshot, newer=newer)
+        # Update our snapshot to the newer version after diffing it
+        self.__snapshot = newer
+        return diff
+
+    def _get_snapshot(self) -> _ViewCacheSnapshot:
+        snapshot = _ViewCacheSnapshot()
+        for item in self.walk_children():
+            if isinstance(item, DynamicItem):
+                snapshot.dynamic_items.add(item.__discord_ui_compiled_template__)
+            elif item.is_dispatchable():
+                custom_id = item.custom_id  # type: ignore
+                snapshot.items.add((item.type.value, custom_id))
+        return snapshot
+
     @property
     def children(self) -> List[Item[Self]]:
         """List[:class:`Item`]: The list of children attached to this view."""
@@ -901,6 +945,7 @@ class ViewStore:
 
         dispatch_info = self._views.get(message_id, {})
         is_fully_dynamic = True
+        snapshot = view._get_snapshot_diff()
         for item in view.walk_children():
             if isinstance(item, DynamicItem):
                 pattern = item.__discord_ui_compiled_template__
@@ -909,6 +954,12 @@ class ViewStore:
                 dispatch_info[(item.type.value, item.custom_id)] = item  # type: ignore
                 is_fully_dynamic = False
 
+        if snapshot is not None:
+            for key in snapshot.items:
+                dispatch_info.pop(key, None)
+            for key in snapshot.dynamic_items:
+                self._dynamic_items.pop(key, None)
+
         view._cache_key = message_id
         if dispatch_info:
             self._views[message_id] = dispatch_info
@@ -922,13 +973,12 @@ class ViewStore:
             return
 
         dispatch_info = self._views.get(view._cache_key)
-        if dispatch_info:
-            for item in view.walk_children():
-                if isinstance(item, DynamicItem):
-                    pattern = item.__discord_ui_compiled_template__
-                    self._dynamic_items.pop(pattern, None)
-                elif item.is_dispatchable():
-                    dispatch_info.pop((item.type.value, item.custom_id), None)  # type: ignore
+        snapshot = view._snapshot
+        if dispatch_info and snapshot:
+            for key in snapshot.items:
+                dispatch_info.pop(key, None)
+            for key in snapshot.dynamic_items:
+                self._dynamic_items.pop(key, None)
 
         if dispatch_info is not None and len(dispatch_info) == 0:
             self._views.pop(view._cache_key, None)