fs::copy: Cannot copy special/device files (ex: /dev/null) into a file

[sylvestre]

I tried this code:

use std::fs;

fn main() {
    match fs::copy("/dev/null", "bar.txt") {
        Ok(_n) => (),
        Err(e) => {
            println!("{}", e);
            ()
        }
    }
}

I expected to see this happen:
I agree that the use case isn't super useful. But I would expect this to work and create an empty file.

Instead, this happened:

the source path is not an existing regular file

Don't hesitate to wontfix this bug...

[jonas-schievink]

Well, this is documented behavior:

This function will return an error in the following situations, but is not limited to just these cases:

• The from path is not a file.

[the8472]

In the light of "everything is a file" perhaps the documentation could be improved to say "regular file" .

[pickfire]

@sylvestre Not sure if #79399 is the kind of fix you are looking for, it fixes it from a documentation perspective.

I wonder if it is possible to allow copying from /dev/null, /dev/full and /dev/zero, they seemed to be useful in some cases. Should we just allow them?

[sylvestre]

@pickfire At least, the doc should be improved but yeah, I think this function should allow copy.

There are cases when /dev/null is used as a regular file like:
https://sources.debian.org/src/apparmor/2.13.5-1/debian/debhelper/postinst-apparmor/?hl=9#L9
https://sources.debian.org/src/mysql-8.0/8.0.21-1/packaging/rpm-common/mysqlrouter.init/?hl=56#L56

[the8472]

Should we just allow them?

It would be good to know why the check explicitly rejecting it has been added in the first place. It has been there since the module was introduced in 2015

And understanding the use-case would be good too. If you want to create an empty file (copying /dev/null) that can be achieved via File::with_options. Copying from /dev/zero would fill up your drive unless you dump it into something not backed by storage.

[sylvestre]

@the8472 It would be good to know why the check explicitly rejecting it has been added in the first place.

I guess the folks who implemented it didn't know that it wasn't uncommon to do
install /dev/null (for example: https://sources.debian.org/src/apparmor/2.13.5-1/debian/debhelper/postinst-apparmor/#L9 used in Debian & Ubuntu for evince, cups, libreoffice, passwd, etc)
or
cp /dev/null (for example: https://sources.debian.org/src/texlive-bin/2020.20200327.54578-5/texk/texlive/linked_scripts/texlive/mktexlsr/?hl=85#L187