*mut magic_sys::magic_set cannot be shared between threads safely

[jkoop]

Perhaps I'm doing something stupid, but this code doesn't compile:

async fn get_content_type(path: &Path) -> Result<String> {
    let cookie = magic::Cookie::open(
        magic::cookie::Flags::ERROR |
        magic::cookie::Flags::MIME_ENCODING |
        magic::cookie::Flags::MIME_TYPE
    )?.load(
        &Default::default()
    )?;  

    Ok(cookie.file(path)?)
}

Among other things, it complains:

error[E0277]: `*mut magic_sys::magic_set` cannot be shared between threads safely
    --> src/server.rs:1620:6
     |  
1620 |     )?;
     |      ^ `*mut magic_sys::magic_set` cannot be shared between threads safely
     |
     = help: within `LoadError<Open>`, the trait `Sync` is not implemented for `*mut magic_sys::magic_set`
     = help: the following other types implement trait `FromResidual<R>`:
               <Result<T, F> as FromResidual<Yeet<E>>>
               <Result<T, F> as FromResidual<Result<Infallible, E>>>

[robo9k]

Upon first look, this is a variation of robo9k/rust-magic-sys#28
The problem being that LoadError<> contains the original Cookie<> so you can recover it, but this is a raw pointer from libmagic without enough guarantees to make it Send nor Sync.

You could map this to your own error and thus discard the private !Sync field.
Currently LoadError doesn't expose much beyond std:error::Error, so if you actually need access to its internals, let me know.

[jkoop]

Thanks for the answer. It helped a lot.

For anyone who finds this discussion (including myself), what I did to fix it was I changed the ? on line 1620 to .unwrap().