file: parameterize file position

[MarcusSorealheis]

Motivation

Closes: #6374

Solution

1. We make the std parameter mutable by adding mut before it: mut std: StdFile.
2. We call std.seek(SeekFrom::Current(0)) to get the current position of the file.
3. The SeekFrom::Current(0) ensures that the file pointer is not modified, and the method returns the current position.
4. We use the position returned by seek to initialize the pos field of the Inner struct within the File struct.

By initializing the pos field with the correct position from the StdFile, the File struct created by from_std will maintain the same seek position as the original StdFile. I'm looking into expanding the existing test to validate no regressions introduced.

[Darksonn]

Thank you. My main concern here is how long do these calls take. We offload all other file operations to spawn_blocking, since they may take a long time. It's possible that this particular syscall never takes a long time, but I don't know ...

That said, I acknowledge that doing this is more difficult if we have to offload it to spawn_blocking. We would probably have to postpone the position lookup until the first operation on the file, since from_std is not async, so we can't do it there.

[Darksonn]

This seems to cause a compilation failure. Most likely, you can just undo the change.

[MarcusSorealheis]

My apologies. That was a mistake. I'm currently working through/experimenting offloading to spawn_blocking

[Darksonn]

Okay. Hopefully it isn't too complicated to do that. What you're doing now may be preferable if offloading is very complicated.

[MarcusSorealheis]

I will resolve it in just a moment.

[MarcusSorealheis]

I'm trying to get Clippy working on my rig.