From fe8d87ef9521f75c958ff7b7d18f447cf2f215d2 Mon Sep 17 00:00:00 2001
From: Ross Nordby <norbo@bepuentertainment.com>
Date: Sat, 13 Jan 2024 15:34:37 -0600
Subject: [PATCH] Guarded against stackalloc-induced overflow for large
 compounds.

---
 .../CompoundPairOverlapFinder.cs              | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/BepuPhysics/CollisionDetection/CollisionTasks/CompoundPairOverlapFinder.cs b/BepuPhysics/CollisionDetection/CollisionTasks/CompoundPairOverlapFinder.cs
index c019cdb1..ddf7ef78 100644
--- a/BepuPhysics/CollisionDetection/CollisionTasks/CompoundPairOverlapFinder.cs
+++ b/BepuPhysics/CollisionDetection/CollisionTasks/CompoundPairOverlapFinder.cs
@@ -26,7 +26,18 @@ public static unsafe void FindLocalOverlaps(ref Buffer<BoundsTestedPair> pairs,
             }
             overlaps = new CompoundPairOverlaps(pool, pairCount, totalCompoundChildCount);
             ref var pairsToTest = ref overlaps.pairQueries;
-            var subpairData = stackalloc SubpairData[totalCompoundChildCount];
+            //Stack overflows are very possible with larger compounds! Guard against it.
+            Buffer<SubpairData> subpairData;
+            const int stackallocThreshold = 1024;
+            if (totalCompoundChildCount <= stackallocThreshold)
+            {
+                var memory = stackalloc SubpairData[totalCompoundChildCount];
+                subpairData = new Buffer<SubpairData>(memory, totalCompoundChildCount);
+            }
+            else
+            {
+                subpairData = new Buffer<SubpairData>(totalCompoundChildCount, pool);
+            }
             int nextSubpairIndex = 0;
             for (int i = 0; i < pairCount; ++i)
             {
@@ -112,8 +123,10 @@ out GatherScatter.Get(ref maximumRadius, j),
             }
             //Doesn't matter what mesh/compound instance is used for the function; just using it as a source of the function.
             Debug.Assert(totalCompoundChildCount > 0);
-            Unsafe.AsRef<TCompoundB>(pairsToTest[0].Container).FindLocalOverlaps<CompoundPairOverlaps, ChildOverlapsCollection>(ref pairsToTest, pool, shapes, ref overlaps);
-
+            Unsafe.AsRef<TCompoundB>(pairsToTest[0].Container).FindLocalOverlaps<CompoundPairOverlaps, ChildOverlapsCollection>(ref pairsToTest, pool, shapes, ref overlaps);
+
+            if (subpairData.Length > stackallocThreshold)
+                subpairData.Dispose(pool);
         }
 
     }