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mediumthreadsconcurrencypanicerror-handlingdowncast

Handling Thread Panics

A panic unwinds only the thread it happens on. The rest of the program keeps running, which is exactly why JoinHandle::join returns a Result rather than the value directly. If the worker panicked, the calling thread finds out by getting an Err back instead of being taken down with it.

The error side is Box<dyn Any + Send>, the panic payload. It is deliberately untyped, because a panic can carry anything. In practice it carries one of two things: panic!("literal") stores a &'static str, and a formatted panic!("{}", value) stores a String. Reading the message means downcasting to each of those in turn.

This is also why blindly writing handle.join().unwrap() is worth thinking about. It works, but it converts a worker's panic into a panic in your own thread. Sometimes that is what you want. Often you would rather turn it into a value and decide what to do.

Your Task

Implement two functions.

Implement describe_panic
rust
pub fn describe_panic(
    job: Box<dyn FnOnce() + Send + 'static>,
) -> Option<String>

Run job on a spawned thread and join it.

  • Finished normally: return None.
  • Panicked: return Some(message) with the panic message.
  • Payload that is neither a &'static str nor a String: return Some("unknown panic").
rust
let ok = describe_panic(Box::new(|| {}));
assert_eq!(ok, None);
 
let bad = describe_panic(Box::new(|| panic!("boom")));
assert_eq!(bad, Some("boom".to_string()));

Implement divide_in_thread
rust
pub fn divide_in_thread(
    a: i32,
    b: i32,
) -> Result<i32, String>

Compute a / b on a spawned thread. When b is 0 the thread must panic with the message "division by zero". Join it and turn a panic into Err(message).

rust
assert_eq!(divide_in_thread(10, 2), Ok(5));
assert_eq!(
    divide_in_thread(1, 0),
    Err("division by zero".to_string()),
);

Notes
  • Your job is to panic and then recover from it, not to avoid the panic. Do not check for zero and return early.
  • The test output will show panic messages on stderr. That is expected and does not mean a test failed.

Hints
Click here to reveal hints
  • match handle.join() { Ok(value) => ..., Err(payload) => ... }.
  • payload.downcast_ref::<&'static str>() covers panic!("literal").
  • payload.downcast_ref::<String>() covers panic!("{}", x). Try both before giving up.
  • downcast_ref hands you a reference, so finish with .to_string() or .clone().
  • For the second function, join() gives a Result already, so map_err is a tidy way to convert only the error side.
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