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.
These aren't used in many places, but we've since moved away from
opacity in UI elements like this, so let's just nuke it here for
legibility.
Addresses https://github.com/ppy/osu/discussions/29906.
Closes https://github.com/ppy/osu/issues/29793.
I believe that the sequence of events that makes this happens is as
follows:
- User selects a range of objects. Some of those objects are off-screen,
and thus would be presumed to be not alive - except the blueprint
container forces them to remain alive, because they're part of the
selection.
- User moves the selection to another column, which is implemented by
temporarily removing the objects from the playfield, changing their
column, and re-adding them.
This sort of pattern is supposed to kick off the
`HitObjectUsageTransferred` flow in `HitObjectUsageEventBuffer` - and
it does... for objects that are *currently visible on screen* and thus
would be alive regardless of `SetKeepAlive()`. However, this does not
hold for objects that are off-screen - nothing ensures they are kept
alive again after re-adding, and thus they inadvertently become dead.
- Thus, this doesn't kick off the `BlueprintContainer` flows associated
with transferring objects to another column, and instead fires the
removal flows, which ensure that the off-screen objects that were
being moved are instead deselected.
I tried a few other options but found no better resolution than this -
calling `SetKeepAlive()` directly would require making it public, which
seems like a bad idea. There's really no good way to generically handle
this either, because it is the ruleset that decides that its way of
implementing this operation will be a removal and re-add of objects,
so...
