Allow ValueListBuilder to work with empty scratch spans (#70917)

diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ValueListBuilder.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ValueListBuilder.cs
index 72b25bc..71ccb1b 100644 (file)
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ValueListBuilder.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ValueListBuilder.cs
@@ -84,10 +84,25 @@ namespace System.Collections.Generic
 
         private void Grow()
         {
-            T[] array = ArrayPool<T>.Shared.Rent(_span.Length * 2);
+            const int ArrayMaxLength = 0x7FFFFFC7; // same as Array.MaxLength
 
-            bool success = _span.TryCopyTo(array);
-            Debug.Assert(success);
+            // Double the size of the span.  If it's currently empty, default to size 4,
+            // although it'll be increased in Rent to the pool's minimum bucket size.
+            int nextCapacity = _span.Length != 0 ? _span.Length * 2 : 4;
+
+            // If the computed doubled capacity exceeds the possible length of an array, then we
+            // want to downgrade to either the maximum array length if that's large enough to hold
+            // an additional item, or the current length + 1 if it's larger than the max length, in
+            // which case it'll result in an OOM when calling Rent below.  In the exceedingly rare
+            // case where _span.Length is already int.MaxValue (in which case it couldn't be a managed
+            // array), just use that same value again and let it OOM in Rent as well.
+            if ((uint)nextCapacity > ArrayMaxLength)
+            {
+                nextCapacity = Math.Max(Math.Max(_span.Length + 1, ArrayMaxLength), _span.Length);
+            }
+
+            T[] array = ArrayPool<T>.Shared.Rent(nextCapacity);
+            _span.CopyTo(array);
 
             T[]? toReturn = _arrayFromPool;
             _span = _arrayFromPool = array;