Should fix https://github.com/ppy/osu/issues/28629.
First of all, to support the claim that this does fix the issue -
reproduction is rather difficult, but I believe I found a way to
maximise the chances of it reproducing by performing the following
steps:
1. Apply the following diff:
diff --git a/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableSlider.cs b/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableSlider.cs
index eacd2b3e75..4c00da031a 100644
--- a/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableSlider.cs
+++ b/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableSlider.cs
@@ -6,6 +6,7 @@
using System;
using System.Collections.Generic;
using System.Linq;
+using System.Threading;
using JetBrains.Annotations;
using osu.Framework.Allocation;
using osu.Framework.Bindables;
@@ -95,6 +96,8 @@ public DrawableSlider([CanBeNull] Slider s = null)
[BackgroundDependencyLoader]
private void load()
{
+ Thread.Sleep(100);
+
tailContainer = new Container<DrawableSliderTail> { RelativeSizeAxes = Axes.Both };
AddRangeInternal(new Drawable[]
2. Download https://osu.ppy.sh/beatmapsets/1470790#osu/3023028 and open
it in the editor.
3. Select all objects using Ctrl-A. Yes, it'll take a while, especially
so with the patch above.
4. Rotate the selection by any amount using the right toolbox.
5. Press undo. The game should crash. If it doesn't spam redo and undo
until it does.
Now to explain what the fix is.
In the issue thread I spent a considerable time hemming and hawing about
which of the dimmable pieces was null, which was a complete miss and a
failure at reading. Let's see the stack trace again:
2024-06-27 02:15:20 [error]: at osu.Game.Rulesets.Osu.Objects.Drawables.DrawableOsuHitObject.<UpdateInitialTransforms>g__applyDim|15_0(Drawable piece) in /home/runner/work/osu-auth-client/osu-auth-client/osu/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableOsuHitObject.cs:line 101
Line 101, you say? What could be null here?
bd8addfb5f/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableOsuHitObject.cs (L101)
Okay, what's `InitialLifetimeOffset`, then?
bd8addfb5f/osu.Game.Rulesets.Osu/Objects/Drawables/DrawableOsuHitObject.cs (L108)
Yes, that's right. It's `HitObject` that is null here.
Now why does *that* happen? First, let's note that all stacks where this
died went through `UpdateState()`, which means that the problematic
`applyDim()` calls had to be `ApplyCustomUpdateState` event callbacks.
Meaning that the pieces where `HitObject` was null were DHOs themselves.
Recall that parent DHOs and child DHOs are pooled separately. Therefore,
there is no guarantee that any parent and child DHOs will remain
associated with each other for the entire duration of a gameplay
session; it is quite the contrary, and nobody should rely on that.
Unfortunately for us, adding a `applyDimToDrawableHitObject` callback to
a child object's `ApplyCustomUpdateState` *implicitly creates* such an
association, because it ends up allocating a closure that captures
`this` (meaning the parent in this context).
Therefore, this now creates a situation where a child DHO can attempt to
read state from a former parent DHO which can be in an indeterminate
state, and in fact, when this crashes, the former parent DHO is most
likely not even in use - hence the null `HitObject`.
Thus, the fix is to clear the association by unsubscribing from the
event when nested objects are cleared.
My hypothesis why the reproduction scenario is like it is, is that both
the sleep and the increased pressure on the pool (by way of selecting
all objects and therefore forcing the DHOs to be materialised beyond
pool capacity) increases the likelihood of getting a crosslink.
When pool pressure is low, it is much more likely that a parent DHO
*will* get the same child DHO again on re-application, even though
that is not guaranteed.
Just as an additional detail, note that the sentry issue for this lists
the "first seen" version as 2024.312.0, which is the release that
included https://github.com/ppy/osu/pull/27401 which would be directly
responsible for this mess.
Originally noticed during review of another change:
https://github.com/ppy/osu/pull/27369#issuecomment-1966140198.
`DrawableOsuHitObject` tries to solve the initial dimming of objects
by applying transform to a list of dimmable parts. For plain drawables
this is safe, but if one of the parts is a DHO, it is not safe,
because drawable transforms can be cleared at will.
In particular, on first use of a drawable slider,
`UpdateInitialTransforms()` would fire via `LoadComplete()` on the
`DrawableSlider`, but *then*, also via `LoadComplete()`,
the `DrawableSliderTail` would update its own state and by doing so
inadvertently clear the dim transform just added by the slider.
To fix, ensure dim transforms are applied to DHOs
via `ApplyCustomUpdateState`.
