Custom panic handlers in the standard library

[alexcrichton]

Tracking issue for rust-lang/rfcs#1328

[sfackler]

See rust-lang/log#67 for one usage example.

[yberreby]

Is there a good reason for the global panic handler to run even if the panic is recovered from?

#![feature(std_panic, recover)]
use std::panic;

fn main() {
    panic::recover(|| {
        panic!("hello");
    }).unwrap_err();
}

http://is.gd/HP4EyO

This prints thread '<main>' panicked at 'hello', <anon>:6. It seems unexpected to me, since the panic was caught, it shouldn't trigger the global handler, should it? IMO, only uncaught panics should trigger the handler and print a message. If a message needs to be printed, it can be, explicitly, at the recover call site.

[sfackler]

There's some talk about this in the RFC PR, but the main reason is that it's very important for the handler to be able to take stack traces.

In addition, the intended use cases for recover are fairly limited, and almost all of them involve rethrowing the panic payload at a different location after catching it. The thread did panic, so it makes sense that the handler would be called IMO.

[shahn]

My biggest usecase for this is #1089, so I'd love a way to suppress the std panic handler, too. I would like to present my users with an error message, set an appropriate exit code, and be assured that desctructors ran without wrapping basically all my return values in Results when an error happens very rarely, is dependant on certain combinations of user input that can't be checked early/not very deep in the call chain.

[sfackler]

@shahn panic::set_handler(|_| ()); should do that for you.

[fables-tales]

I'm trying to do something of the form:

let orig_handler = panic::take_handler();
panic::set_handler(|_| ());
<an actual thing>
panic::set_handler(orig_handler);

and getting the error

src/example_group.rs:41:13: 41:31 error: the trait `for<'r, 'r> core::ops::Fn<(&'r std::panicking::PanicInfo<'r>,)>` is not implemented for the type `Box<for<'r, 'r> core::ops::Fn(&'r std::panicking::PanicInfo<'r>) + Send + Sync>` [E0277]
src/example_group.rs:41             panic::set_handler(orig_panic_handler);
                                    ^~~~~~~~~~~~~~~~~~
src/example_group.rs:41:13: 41:31 help: run `rustc --explain E0277` to see a detailed explanation
src/example_group.rs:41:13: 41:31 note: required by `std::panicking::set_handler`
src/example_group.rs:41:13: 41:31 error: the trait `for<'r, 'r> core::ops::FnOnce<(&'r std::panicking::PanicInfo<'r>,)>` is not implemented for the type `Box<for<'r, 'r> core::ops::Fn(&'r std::panicking::PanicInfo<'r>) + Send + Sync>` [E0277]
src/example_group.rs:41             panic::set_handler(orig_panic_handler);
                                    ^~~~~~~~~~~~~~~~~~
src/example_group.rs:41:13: 41:31 help: run `rustc --explain E0277` to see a detailed explanation
src/example_group.rs:41:13: 41:31 help: the following implementations were found:
src/example_group.rs:41:13: 41:31 help:   <Box<alloc::boxed::FnBox<A, Output=R> + 'a> as core::ops::FnOnce<A>>
src/example_group.rs:41:13: 41:31 help:   <Box<alloc::boxed::FnBox<A, Output=R> + Send + 'a> as core::ops::FnOnce<A>>
src/example_group.rs:41:13: 41:31 note: required by `std::panicking::set_handler`
error: aborting due to 2 previous errors
Could not compile `descriptor`.

I tried taking ownership of the value inside the box with

panic::set_handler(*orig_handler);

but this gives the error

src/example_group.rs:41:13: 41:31 error: the trait `core::marker::Sized` is not implemented for the type `for<'r, 'r> core::ops::Fn(&'r std::panicking::PanicInfo<'r>) + Send + Sync` [E0277]
src/example_group.rs:41             panic::set_handler(*orig_panic_handler);
                                    ^~~~~~~~~~~~~~~~~~
src/example_group.rs:41:13: 41:31 help: run `rustc --explain E0277` to see a detailed explanation
src/example_group.rs:41:13: 41:31 note: `for<'r, 'r> core::ops::Fn(&'r std::panicking::PanicInfo<'r>) + Send + Sync` does not have a constant size known at compile-time
src/example_group.rs:41:13: 41:31 note: required by `std::panicking::set_handler`
error: aborting due to previous error
Could not compile `descriptor`.

I feel like it should be easy to set_handler the result from take_handler and I might be missing a trick here. Does anyone have any advice for me? I really like this API and I'd appreciate any help ^_^

[yberreby]

@samphippen This is ugly, but it works.

#![feature(std_panic, panic_handler)]
use std::panic;

fn main() {
    let orig_handler = panic::take_handler();
    panic::set_handler(|_| ());
    // <an actual thing>
    panic::set_handler(move |info| (*orig_handler)(info));
}

http://is.gd/CzXxcb

[fables-tales]

thanks!