Add special "void span" to discard all child spans

[andrewbanchich]

Feature Request

Add a special "void span" where all spans created within it are noops / removed.

Motivation

I am working on a way to ensure sensitive data is not accidentally logged by creating a wrapper type, which provides a map() method for users to work with the underlying secret. However, it is possible for a user to pass a function to map() which is instrumented with a span.

I would like to be able to change it to something like

fn map<U, F>(self, f: F) -> Secret<U>
    where
        F: FnOnce(T) -> U,
    {
        let void_span = void!();
        let _enter = void_span.enter();

        Self::new(f(x))
    }

to ensure nothing is logged.

Proposal

Alternatives

There are several approaches to redacting sensitive data, but it is usually necessary to have some section of the code need to work with that data directly. It would be nice to be able to minimize the scope of where that occurs, as well as ensure that no logging happens there.

[hawkw]

You can achieve the desired effect of dropping all spans and events within a scope by setting the default subscriber to NoSubscriber within that scope. This will ensure that no spans or events are recorded within that scope.

For example, you could write the following:

fn map<U, F>(self, f: F) -> Secret<U>
where
    F: FnOnce(T) -> U,
{
    use tracing::subscriber::{self, NoSubscriber};
    let _no_subscriber = subscriber::set_default(NoSubcriber::default());

    Self::new(f(x))
}

[andrewbanchich]

Fantastic! Thank you so much.

[andrewbanchich]

Related to this - is there any way to conditionally log an error! if there are any child spans produced within this?

[andrewbanchich]

I think I figured it out: I just created a new type and copied the Subscriber impl from NoSubscriber, except set enabled() to true, added an eprintln!("REDACTED") to event(), and returned false from try_close().

Does that sound right? Is using eprintln! here the right way of doing it?

[davidbarsky]

I'm guessing that this is Cedar-adjacent: I think you're better off panicking if you want a construct like this.

[andrewbanchich]

Why would you recommend panicking? I'd like to avoid panics, and just redact any logs from the this span, but also know that something attempted to log.