return out.off, out.buf
	}

	// Not enough space in the mmaped area, write to heap area instead.
	heapPos := out.off - bufLen
	heapLen := int64(len(out.heap))
	lenNeeded := heapPos + lenToWrite
	if lenNeeded > heapLen { // do we need to grow the heap storage?
		// The heap variables aren't protected by a mutex. For now, just bomb if you
		// try to use OutBuf in parallel. (Note this probably could be fixed.)