Eio.Path.load has race condition and fails on /proc

[vog]

The current implementation of Eio.Path.load first determines the file size, then reads the file contents into a buffer of that size. This approach has two major downsides:

1. This introduces a race condition if the file is changing between stat and open/read. It can fail in two directions:
   • If the file became larger in between, Eio.Path.load will load too few data without noticing
   • If the file became smaller in between, Eio.Path.load will reach EOF earlier than expected
2. This makes reading from /proc largely impossible via Eio.Path.load, as those "files" are generated at the moment they are read, so the kernel simply reports size 0 for them.

Especially the second case caused quite some debugging time for me, which I'd like to save future Eio users from.

For example:

# Eio_main.run @@ fun env ->
  Eio.Path.load Eio.Path.(env#fs / "/proc/sys/fs/file-nr")
- : string = ""

but:

$ cat /proc/sys/fs/file-nr
16320   0       9223372036854775807

I propose to:

• change Eio.Path.load so that it uses the file size only as a hint for the initial buffer size, and to
• make Eio.Path.load path otherwise largely equivalent to Eio.Path.with_open_in path Eio.Flow.read_all

What do you think?

[vog]

Some background information:

• https://linuxvox.com/blog/any-idea-why-my-c-code-can-t-read-from-proc/

[patricoferris]

Thanks @vog!

Both of these problems are touched upon in #798 I think, though the raciness is only eluded to by @copy in:

...has the advantage of being more robust against other stat problems, at the cost of an extra syscall.

Perhaps we could update the title of this bug to be explicitly about the raciness and keep #798 about /proc?