CSHARP-4159: Make ThreadStaticBuffer resilient to OutOfMemoryException

Author: BorisDog

src/MongoDB.Bson/IO/ThreadStaticBuffer.cs

@@ -80,25 +80,31 @@ public static RentedBuffer RentBuffer(int size)
  throw new ArgumentOutOfRangeException(nameof(size), size, $"Valid buffer size range is [1..{MaxAllocationSize}] bytes.");
  }
 
- __isBufferRented = true;
-
  if (__threadId == default)
  {
  __threadId = Thread.CurrentThread.ManagedThreadId;
  }
 
+ // Allocate space
+ RentedBuffer rentedBuffer;
  if (size > MaxSize)
  {
- return new RentedBuffer(__threadId, new byte[size]);
+ rentedBuffer = new RentedBuffer(__threadId, new byte[size]);
  }
-
- if (__buffer == null || __buffer.Length < size)
+ else
  {
- var newSize = size <= MinSize ? MinSize : PowerOf2.RoundUpToPowerOf2(size);
- __buffer = new byte[newSize];
+ if (__buffer == null || __buffer.Length < size)
+ {
+ var newSize = size <= MinSize ? MinSize : PowerOf2.RoundUpToPowerOf2(size);
+ __buffer = new byte[newSize];
+ }
+
+ rentedBuffer = new RentedBuffer(__threadId, __buffer);
  }
 
- return new RentedBuffer(__threadId, __buffer);
+ // Set this last (in case of an exception in this method we want this to stay as-is.)
+ __isBufferRented = true;
+ return rentedBuffer;
  }
  }
 }