objc2/

exception.rs

//! # `@throw` and `@try/@catch` exceptions.
//!
//! By default, if a message send (such as those generated with the
//! [`msg_send!`] and [`extern_methods!`] macros) causes an exception to be
//! thrown, `objc2` will simply let it unwind into Rust.
//!
//! While not UB, it will likely end up aborting the process, since Rust
//! cannot catch foreign exceptions like Objective-C's. However, `objc2` has
//! the `"catch-all"` Cargo feature, which, when enabled, wraps each message
//! send in a `@catch` and instead panics if an exception is caught, which
//! might lead to slightly better error messages.
//!
//! Most of the functionality in this module is only available when the
//! `"exception"` feature is enabled.
//!
//! See the following links for more information:
//! - [Exception Programming Topics for Cocoa](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Exceptions/Exceptions.html)
//! - [The Objective-C Programming Language - Exception Handling](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocExceptionHandling.html)
//! - [Exception Handling in LLVM](https://llvm.org/docs/ExceptionHandling.html)
//!
//! [`msg_send!`]: crate::msg_send

// TODO: Test this with panic=abort, and ensure that the code-size is
// reasonable in that case.

#[cfg(feature = "exception")]
use core::ffi::c_void;
use core::ffi::CStr;
use core::fmt;
#[cfg(feature = "exception")]
use core::mem;
use core::ops::Deref;
use core::panic::RefUnwindSafe;
use core::panic::UnwindSafe;
#[cfg(feature = "exception")]
use core::ptr;
use std::error::Error;

use crate::encode::{Encoding, RefEncode};
#[cfg(feature = "exception")]
use crate::ffi;
#[cfg(feature = "catch-all")]
use crate::ffi::NSUInteger;
#[cfg(feature = "catch-all")]
use crate::msg_send;
use crate::rc::{autoreleasepool_leaking, Retained};
use crate::runtime::__nsstring::nsstring_to_str;
use crate::runtime::{AnyClass, AnyObject, NSObject, NSObjectProtocol};
use crate::{extern_methods, sel, Message};

/// An Objective-C exception.
///
/// While highly recommended that any exceptions you intend to throw are
/// subclasses of `NSException`, this is not required by the runtime (similar
/// to how Rust can panic with arbitrary payloads using [`panic_any`]).
///
/// [`panic_any`]: std::panic::panic_any
#[repr(transparent)]
pub struct Exception(AnyObject);

unsafe impl RefEncode for Exception {
    const ENCODING_REF: Encoding = Encoding::Object;
}

unsafe impl Message for Exception {}

impl Deref for Exception {
    type Target = AnyObject;

    #[inline]
    fn deref(&self) -> &AnyObject {
        &self.0
    }
}

impl AsRef<AnyObject> for Exception {
    #[inline]
    fn as_ref(&self) -> &AnyObject {
        self
    }
}

impl Exception {
    fn is_nsexception(&self) -> Option<bool> {
        if self.class().responds_to(sel!(isKindOfClass:)) {
            // SAFETY: We only use `isKindOfClass:` on NSObject
            let obj: *const Exception = self;
            let obj = unsafe { obj.cast::<NSObject>().as_ref().unwrap() };
            // Get class dynamically instead of with `class!` macro
            let name = CStr::from_bytes_with_nul(b"NSException\0").unwrap();
            Some(obj.isKindOfClass(AnyClass::get(name)?))
        } else {
            Some(false)
        }
    }

    #[cfg(feature = "catch-all")]
    pub(crate) fn stack_trace(&self) -> impl fmt::Display + '_ {
        struct Helper<'a>(&'a Exception);

        impl fmt::Display for Helper<'_> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                if let Some(true) = self.0.is_nsexception() {
                    autoreleasepool_leaking(|pool| {
                        // SAFETY: The object is an `NSException`.
                        // Returns `NSArray<NSString *>`.
                        let call_stack_symbols: Option<Retained<NSObject>> =
                            unsafe { msg_send![self.0, callStackSymbols] };
                        if let Some(call_stack_symbols) = call_stack_symbols {
                            writeln!(f, "stack backtrace:")?;

                            // SAFETY: `call_stack_symbols` is an `NSArray`, and
                            // `count` returns `NSUInteger`.
                            let count: NSUInteger =
                                unsafe { msg_send![&call_stack_symbols, count] };
                            let mut i = 0;
                            while i < count {
                                // SAFETY: The index is in-bounds (so no exception will be thrown).
                                let symbol: Retained<NSObject> =
                                    unsafe { msg_send![&call_stack_symbols, objectAtIndex: i] };
                                // SAFETY: The symbol is an NSString, and is not used
                                // beyond this scope.
                                let symbol = unsafe { nsstring_to_str(&symbol, pool) };
                                writeln!(f, "{symbol}")?;
                                i += 1;
                            }
                        }
                        Ok(())
                    })
                } else {
                    Ok(())
                }
            }
        }

        Helper(self)
    }
}

impl Exception {
    extern_methods!(
        // Only safe on NSException
        // Returns NSString
        #[unsafe(method(name))]
        #[unsafe(method_family = none)]
        unsafe fn name(&self) -> Option<Retained<NSObject>>;

        // Only safe on NSException
        // Returns NSString
        #[unsafe(method(reason))]
        #[unsafe(method_family = none)]
        unsafe fn reason(&self) -> Option<Retained<NSObject>>;
    );
}

// Note: We can't implement `Send` nor `Sync` since the exception could be
// anything!

impl fmt::Debug for Exception {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "exception ")?;

        // Attempt to present a somewhat usable error message if the exception
        // is an instance of NSException.
        if let Some(true) = self.is_nsexception() {
            autoreleasepool_leaking(|pool| {
                // SAFETY: Just checked that object is an NSException
                let (name, reason) = unsafe { (self.name(), self.reason()) };

                // SAFETY:
                // - `name` and `reason` are guaranteed to be `NSString`s.
                // - We control the scope in which they are alive, so we know
                //   they are not moved outside the current autorelease pool.
                //
                // Note that these strings are immutable (`NSException` is
                // immutable, and the properties are marked as `readonly` and
                // `copy` and are copied upon creation), so we also don't have
                // to worry about the string being mutated under our feet.
                let name = name
                    .as_deref()
                    .map(|name| unsafe { nsstring_to_str(name, pool) });
                let reason = reason
                    .as_deref()
                    .map(|reason| unsafe { nsstring_to_str(reason, pool) });

                let obj: &AnyObject = self.as_ref();
                write!(f, "{obj:?} '{}'", name.unwrap_or_default())?;
                if let Some(reason) = reason {
                    write!(f, " reason: {reason}")?;
                } else {
                    write!(f, " reason: (NULL)")?;
                }
                Ok(())
            })
        } else {
            // Fall back to `AnyObject` Debug
            write!(f, "{:?}", self.0)
        }
    }
}

impl fmt::Display for Exception {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        autoreleasepool_leaking(|pool| {
            if let Some(true) = self.is_nsexception() {
                // SAFETY: Just checked that object is an NSException
                let reason = unsafe { self.reason() };

                if let Some(reason) = &reason {
                    // SAFETY: Same as above in `Debug`.
                    let reason = unsafe { nsstring_to_str(reason, pool) };
                    return write!(f, "{reason}");
                }
            }

            write!(f, "unknown exception")
        })
    }
}

impl Error for Exception {}

impl UnwindSafe for Exception {}
impl RefUnwindSafe for Exception {}

/// Throws an Objective-C exception.
///
/// This is the Objective-C equivalent of Rust's [`panic!`].
///
/// This unwinds from Objective-C, and the exception should be caught using an
/// Objective-C exception handler like [`catch`]. It _may_ be caught by
/// [`catch_unwind`], though the error message is unlikely to be great.
///
/// [`catch_unwind`]: std::panic::catch_unwind
#[inline]
#[cfg(feature = "exception")] // For consistency, not strictly required
pub fn throw(exception: Retained<Exception>) -> ! {
    // We consume the exception object since we can't make any guarantees
    // about its mutability.
    let ptr: *const AnyObject = &exception.0;
    let ptr = ptr as *mut AnyObject;
    // SAFETY: The object is valid and non-null (nil exceptions are not valid
    // in the old runtime).
    unsafe { ffi::objc_exception_throw(ptr) }
}

#[cfg(feature = "exception")]
fn try_no_ret<F: FnOnce()>(closure: F) -> Result<(), Option<Retained<Exception>>> {
    let f = {
        extern "C-unwind" fn try_objc_execute_closure<F>(closure: &mut Option<F>)
        where
            F: FnOnce(),
        {
            // This is always passed Some, so it's safe to unwrap
            let closure = closure.take().unwrap();
            closure();
        }

        let f: extern "C-unwind" fn(&mut Option<F>) = try_objc_execute_closure;
        let f: extern "C-unwind" fn(*mut c_void) = unsafe { mem::transmute(f) };
        f
    };

    // Wrap the closure in an Option so it can be taken
    let mut closure = Some(closure);
    let context: *mut Option<F> = &mut closure;
    let context = context.cast();

    let mut exception = ptr::null_mut();
    // SAFETY: The function pointer and context are valid.
    //
    // The exception catching itself is sound on the Rust side, because we
    // correctly use `extern "C-unwind"`. Objective-C does not completely
    // specify how foreign unwinds are handled, though they do have the
    // `@catch (...)` construct intended for catching C++ exceptions, so it is
    // likely that they intend to support Rust panics (and it works in
    // practice).
    //
    // See also:
    // https://github.com/rust-lang/rust/pull/128321
    // https://github.com/rust-lang/reference/pull/1226
    let success = unsafe { objc2_exception_helper::try_catch(f, context, &mut exception) };

    if success == 0 {
        Ok(())
    } else {
        // SAFETY:
        // The exception is always a valid object or NULL.
        //
        // Since we do a retain in `objc2_exception_helper/src/try_catch.m`,
        // the object has +1 retain count.
        Err(unsafe { Retained::from_raw(exception.cast()) })
    }
}

/// Tries to execute the given closure and catches an Objective-C exception
/// if one is thrown.
///
/// This is the Objective-C equivalent of Rust's [`catch_unwind`].
/// Accordingly, if your Rust code is compiled with `panic=abort`, or your
/// Objective-C code with `-fno-objc-exceptions`, this cannot catch the
/// exception.
///
/// [`catch_unwind`]: std::panic::catch_unwind
///
///
/// # Errors
///
/// Returns a `Result` that is either `Ok` if the closure succeeded without an
/// exception being thrown, or an `Err` with the exception. The exception is
/// automatically released.
///
/// The exception is `None` in the extremely exceptional case that the
/// exception object is `nil`. This should basically never happen, but is
/// technically possible on some systems with `@throw nil`, or in OOM
/// situations.
///
///
/// # Panics
///
/// This panics if the given closure panics.
///
/// That is, it completely ignores Rust unwinding and simply lets that pass
/// through unchanged.
///
/// It may also not catch all Objective-C exceptions (such as exceptions
/// thrown when handling the memory management of the exception). These are
/// mostly theoretical, and should only happen in utmost exceptional cases.
#[cfg(feature = "exception")]
pub fn catch<R>(
    closure: impl FnOnce() -> R + UnwindSafe,
) -> Result<R, Option<Retained<Exception>>> {
    let mut value = None;
    let value_ref = &mut value;
    let closure = move || {
        *value_ref = Some(closure());
    };
    let result = try_no_ret(closure);
    // If the try succeeded, value was set so it's safe to unwrap
    result.map(|()| value.unwrap_or_else(|| unreachable!()))
}

#[cfg(test)]
#[cfg(feature = "exception")]
mod tests {
    use alloc::format;
    use alloc::string::ToString;
    use core::panic::AssertUnwindSafe;
    use std::panic::catch_unwind;

    use super::*;
    use crate::msg_send;

    #[test]
    fn test_catch() {
        let mut s = "Hello".to_string();
        let result = catch(move || {
            s.push_str(", World!");
            s
        });
        assert_eq!(result.unwrap(), "Hello, World!");
    }

    #[test]
    #[cfg_attr(
        all(target_os = "macos", target_arch = "x86"),
        ignore = "`NULL` exceptions are invalid on 32-bit / w. fragile runtime"
    )]
    fn test_catch_null() {
        let s = "Hello".to_string();
        let result = catch(move || {
            if !s.is_empty() {
                unsafe { ffi::objc_exception_throw(ptr::null_mut()) }
            }
            s.len()
        });
        assert!(result.unwrap_err().is_none());
    }

    #[test]
    #[cfg_attr(
        feature = "catch-all",
        ignore = "Panics inside `catch` when catch-all is enabled"
    )]
    fn test_catch_unknown_selector() {
        let obj = AssertUnwindSafe(NSObject::new());
        let ptr = Retained::as_ptr(&obj);
        let result = catch(|| {
            let _: Retained<NSObject> = unsafe { msg_send![&*obj, copy] };
        });
        let err = result.unwrap_err().unwrap();

        assert_eq!(
            format!("{err}"),
            format!("-[NSObject copyWithZone:]: unrecognized selector sent to instance {ptr:?}"),
        );
    }

    #[test]
    fn test_throw_catch_object() {
        let obj = NSObject::new();
        // TODO: Investigate why this is required on GNUStep!
        let _obj2 = obj.clone();
        let obj: Retained<Exception> = unsafe { Retained::cast_unchecked(obj) };
        let ptr: *const Exception = &*obj;

        let result = catch(|| throw(obj));
        let obj = result.unwrap_err().unwrap();

        assert_eq!(format!("{obj:?}"), format!("exception <NSObject: {ptr:p}>"));

        assert!(ptr::eq(&*obj, ptr));
    }

    #[test]
    #[ignore = "currently aborts"]
    fn throw_catch_unwind() {
        let obj = NSObject::new();
        let obj: Retained<Exception> = unsafe { Retained::cast_unchecked(obj) };

        let result = catch_unwind(|| throw(obj));
        let _ = result.unwrap_err();
    }

    #[test]
    #[should_panic = "test"]
    #[cfg_attr(
        all(target_os = "macos", target_arch = "x86", panic = "unwind"),
        ignore = "panic won't start on 32-bit / w. fragile runtime, it'll just abort, since the runtime uses setjmp/longjump unwinding"
    )]
    fn does_not_catch_panic() {
        let _ = catch(|| panic!("test"));
    }
}