Find a panic location from catch_unwind()

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1

How to find panic location from catch_unwind ? Problem:

let res = catch_unwind(|| some_arbitrary_code());
if res.is_err() {
    println!("Panic occured at {file}:{line}:{col}", file = ?, line = ?, col = ?);
}

Stuff I tried that failed:

  1. Store this info in panic hook into a thread local variable and later restore it after catch_unwind(). Doesn’t actually work with case like this:
catch_unwind(|| {
    let res = thread::spawn(|| {
        // hook gets called in this thread, so uses a different thread local
        panic!()
    }).join();
    if let Err(err) = res {
        resume_unwind(err);
    }
});
  1. Store this info in panic hook into a HashMap<usize, ...>, keyed with address of the payload. This doesn’t work, because StrPanicPayload gives an address of unboxed payload to the panic hook, so it’s different in Box<_> returned by catch_unwind().

Related thread: Return value from catch_unwind is a useless "Any"

1 Like
2

It’s possible to work around the StrPanicPayload issue by trying to downcast both payloads to &str and comparing resulting addresses, but this is strongly relying on details of internal implementation.

4

No it’s not, since static strings have a single address in the binary, so if two panics have the same message, they’ll probably have the same message address.

5

It was unclear to me from the docs if this would work or not, but it does seem to. Sadly you lose the original payload. (You could try to copy/clone the common cases.)

    panic::set_hook(Box::new(|info| {
        panic::resume_unwind(Box::new(
            info.location().map(|loc| PanicData {
                file: loc.file().into(),
                line: loc.line(),
                column: loc.column(),
            })
        ))
    }));
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6

That’s a fun workaround, thanks. Original payload could be restored for known types, but not for panic_any. It looks kinda scary though. I wonder if this behaviour is guaranteed.

7

I, too, wonder where/if this is better documented, but I don't have the time to dig right now. Please update the thread if you find something :slight_smile:

8

No, that’s unfortunately an accident.

https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/.60resume_unwind.28.29.60.20from.20panic.20hook/near/361273611

1 Like
9

Dang. Really does seem underspecified too :frowning:

10

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