Disallow zero-length sliders from specifying a non-zero number of repeats (#35220)

Author: bdach

osu.Game/Rulesets/Objects/Legacy/ConvertHitObjectParser.cs

@@ -476,12 +476,30 @@ private ConvertHitObject createHitCircle(Vector2 position, bool newCombo, int co
         private ConvertHitObject createSlider(Vector2 position, bool newCombo, int comboOffset, PathControlPoint[] controlPoints, double? length, int repeatCount,
                                               IList<IList<HitSampleInfo>> nodeSamples)
         {
+            var path = new SliderPath(controlPoints, length);
+
+            // there are known instances of beatmaps (https://osu.ppy.sh/beatmapsets/594828#osu/1258033) which contain zero-length sliders with non-zero numbers of repeats.
+            // this was exploiting a bug in stable in which the slider repeats would be generated as objects but never actually judged as a hit *or* miss during gameplay,
+            // therefore increasing the theoretical possible max combo to be gained from a slider while in practice never giving that extra combo.
+            // due to lazer ensuring that an object has its nested part fully judged, this would result in broken behaviours
+            // (either the zero-length slider giving hundreds of combo for nothing if the repeats are judged as hit, or insta-failing the player due to HP if judged as miss).
+            // to remedy this in a way that seems least damaging, detect this situation via a heuristic and reset the number of repeats to zero.
+            // this technically *does not* match stable beatmap parsing or conversion, *does not* match in-gameplay behaviour of such broken sliders,
+            // and *will* fail conversion mapping tests, but again, this is supposed to be a least-worst measure to prevent exploits.
+            // it is also applied centrally to all rulesets rather than in specific ruleset converters because this failure scenario
+            // translates across rulesets (osu! and catch are both affected).
+            if (Precision.AlmostEquals(path.Distance, 0))
+            {
+                repeatCount = 0;
+                nodeSamples = [nodeSamples[0], nodeSamples[^1]];
+            }
+
             return lastObject = new ConvertSlider
             {
                 Position = position,
                 NewCombo = firstObject || lastObject is ConvertSpinner || newCombo,
                 ComboOffset = newCombo ? comboOffset : 0,
-                Path = new SliderPath(controlPoints, length),
+                Path = path,
                 NodeSamples = nodeSamples,
                 RepeatCount = repeatCount
             };