In https://chromium-review.googlesource.com/c/chromiumos/platform2/+/612963, VirtualFileProvider started remembering each file's size when it's opened.
This is needed to support seek operations on a file system implemented with FUSE which caches file sizes and uses them to perform boundary check.
However, this introduces an unavoidable race problem when a file is modified while being read.
Probably we should implement some logic to mitigate this problem.