What happens when a panic hook panics?

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What happens if the function I passed to std::panic::set_hook panics?

I can imagine many ways of reacting to this: consider this UB, abort the program like C++ does, invoke the panic handler again for the new panic, simply abort the execution of the hook... What exactly does Rust promise here?

Context. I'm writing a web app with Rust/WASM backend and I would like to make a panic hook that sends any errors to the server for debugging. This involves a network operation, which can itself fail. So I'm trying to figure out how I can ensure some reasonable behavior in this double-failure scenario.

rust
1 Answers

It's not documented outside of the source code.

The source code for the panic entry point in std has this comment:

// If this is the third nested call (e.g., panics == 2, this is 0-indexed),
// the panic hook probably triggered the last panic, otherwise the
// double-panic check would have aborted the process. In this case abort the
// process real quickly as we don't want to try calling it again as it'll
// probably just panic again.

So the answer to your question is either "invoke the panic handler again for the new panic" or "abort the program" depending on how many times the hook already panicked.

This all assumes you aren't using #![no_std]. If you are then you're either disabling panicking altogether or you are implementing your own panic handler with #[panic_handler], in which case you get to decide what happens yourself.

user_5012500
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