rust/unwinding.md

% Unwinding

Rust has a tiered error-handling scheme:

• If something might reasonably be absent, Option is used
• If something goes wrong and can reasonably be handled, Result is used
• If something goes wrong and cannot reasonably be handled, the thread panics
• If something catastrophic happens, the program aborts

Option and Result are overwhelmingly preferred in most situations, especially since they can be promoted into a panic or abort at the API user's discretion. However, anything and everything can panic, and you need to be ready for this. Panics cause the thread to halt normal execution and unwind its stack, calling destructors as if every function instantly returned.

As of 1.0, Rust is of two minds when it comes to panics. In the long-long-ago, Rust was much more like Erlang. Like Erlang, Rust had lightweight tasks, and tasks were intended to kill themselves with a panic when they reached an untenable state. Unlike an exception in Java or C++, a panic could not be caught at any time. Panics could only be caught by the owner of the task, at which point they had to be handled or that task would itself panic.

Unwinding was important to this story because if a task's destructors weren't called, it would cause memory and other system resources to leak. Since tasks were expected to die during normal execution, this would make Rust very poor for long-running systems!

As the Rust we know today came to be, this style of programming grew out of fashion in the push for less-and-less abstraction. Light-weight tasks were killed in the name of heavy-weight OS threads. Still, panics could only be caught by the parent thread. This means catching a panic requires spinning up an entire OS thread! This unfortunately stands in conflict to Rust's philosophy of zero-cost abstractions.

In the near future there will be a stable interface for catching panics in an arbitrary location, though we would encourage you to still only do this sparingly. In particular, Rust's current unwinding implementation is heavily optimized for the "doesn't unwind" case. If a program doesn't unwind, there should be no runtime cost for the program being ready to unwind. As a consequence, actually unwinding will be more expensive than in e.g. Java. Don't build your programs to unwind under normal circumstances. Ideally, you should only panic for programming errors or extreme problems.

Exception Safety

Being ready for unwinding is often referred to as exception safety in the broader programming world. In Rust, their are two levels of exception safety that one may concern themselves with:

• In unsafe code, we must be exception safe to the point of not violating memory safety.

• In safe code, it is good to be exception safe to the point of your program doing the right thing.

As is the case in many places in Rust, unsafe code must be ready to deal with bad safe code, and that includes code that panics. Code that transiently creates unsound states must be careful that a panic does not cause that state to be used. Generally this means ensuring that only non-panicking code is run while these states exist, or making a guard that cleans up the state in the case of a panic. This does not necessarily mean that the state a panic witnesses is a fully coherent state. We need only guarantee that it's a safe state.

Most unsafe code is leaf-like, and therefore fairly easy to make exception-safe. It controls all the code that runs, and most of that code can't panic. However it is often the case that code that works with arrays works with temporarily uninitialized data while repeatedly invoking caller-provided code. Such code needs to be careful, and consider exception-safety.

Vec::push_all

Vec::push_all is a temporary hack to get extending a Vec by a slice reliably effecient without specialization. Here's a simple implementation:

impl<T: Clone> Vec<T> {
    fn push_all(&mut self, to_push: &[T]) {
        self.reserve(to_push.len());
        unsafe {
            // can't overflow because we just reserved this
            self.set_len(self.len() + to_push.len());

            for (i, x) in to_push.iter().enumerate() {
                self.ptr().offset(i as isize).write(x.clone());
            }
        }
    }
}

We bypass push in order to avoid redundant capacity and len checks on the Vec that we definitely know has capacity. The logic is totally correct, except there's a subtle problem with our code: it's not exception-safe! set_len, offset, and write are all fine, but clone is the panic bomb we over-looked.

Clone is completely out of our control, and is totally free to panic. If it does, our function will exit early with the length of the Vec set too large. If the Vec is looked at or dropped, uninitialized memory will be read!

The fix in this case is fairly simple. If we want to guarantee that the values we did clone are dropped we can set the len in the loop. If we just want to guarantee that uninitialized memory can't be observed, we can set the len after the loop.

BinaryHeap::sift_up

Bubbling an element up a heap is a bit more complicated than extending a Vec. The pseudocode is as follows:

bubble_up(heap, index):
    while index != 0 && heap[index] < heap[parent(index)]:
        heap.swap(index, parent(index))
        index = parent(index)

A literal transcription of this code to Rust is totally fine, but has an annoying performance characteristic: the self element is swapped over and over again uselessly. We would rather have the following:

bubble_up(heap, index):
    let elem = heap[index]
    while index != 0 && element < heap[parent(index)]:
        heap[index] = heap[parent(index)]
        index = parent(index)
    heap[index] = elem

This code ensures that each element is copied as little as possible (it is in fact necessary that elem be copied twice in general). However it now exposes some exception-safety trouble! At all times, there exists two copies of one value. If we panic in this function something will be double-dropped. Unfortunately, we also don't have full control of the code: that comparison is user-defined!

Unlike Vec, the fix isn't as easy here. One option is to break the user-defined code and the unsafe code into two separate phases:

bubble_up(heap, index):
    let end_index = index;
    while end_index != 0 && heap[end_index] < heap[parent(end_index)]:
        end_index = parent(end_index)

    let elem = heap[index]
    while index != end_index:
        heap[index] = heap[parent(index)]
        index = parent(index)
    heap[index] = elem

If the user-defined code blows up, that's no problem anymore, because we haven't actually touched the state of the heap yet. Once we do start messing with the heap, we're working with only data and functions that we trust, so there's no concern of panics.

Perhaps you're not happy with this design. Surely, it's cheating! And we have to do the complex heap traversal twice! Alright, let's bite the bullet. Let's intermix untrusted and unsafe code for reals.

If Rust had try and finally like in Java, we could do the following:

bubble_up(heap, index):
    let elem = heap[index]
    try:
        while index != 0 && element < heap[parent(index)]:
            heap[index] = heap[parent(index)]
            index = parent(index)
    finally:
        heap[index] = elem

The basic idea is simple: if the comparison panics, we just toss the loose element in the logically uninitialized index and bail out. Anyone who observes the heap will see a potentially inconsistent heap, but at least it won't cause any double-drops! If the algorithm terminates normally, then this operation happens to coincide precisely with the how we finish up regardless.

Sadly, Rust has no such construct, so we're going to need to roll our own! The way to do this is to store the algorithm's state in a separate struct with a destructor for the "finally" logic. Whether we panic or not, that destructor will run and clean up after us.

struct Hole<'a, T: 'a> {
    data: &'a mut [T],
    /// `elt` is always `Some` from new until drop.
    elt: Option<T>,
    pos: usize,
}

impl<'a, T> Hole<'a, T> {
    fn new(data: &'a mut [T], pos: usize) -> Self {
        unsafe {
            let elt = ptr::read(&data[pos]);
            Hole {
                data: data,
                elt: Some(elt),
                pos: pos,
            }
        }
    }

    fn pos(&self) -> usize { self.pos }

    fn removed(&self) -> &T { self.elt.as_ref().unwrap() }

    unsafe fn get(&self, index: usize) -> &T { &self.data[index] }

    unsafe fn move_to(&mut self, index: usize) {
        let index_ptr: *const _ = &self.data[index];
        let hole_ptr = &mut self.data[self.pos];
        ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
        self.pos = index;
    }
}

impl<'a, T> Drop for Hole<'a, T> {
    fn drop(&mut self) {
        // fill the hole again
        unsafe {
            let pos = self.pos;
            ptr::write(&mut self.data[pos], self.elt.take().unwrap());
        }
    }
}

impl<T: Ord> BinaryHeap<T> {
    fn sift_up(&mut self, pos: usize) {
        unsafe {
            // Take out the value at `pos` and create a hole.
            let mut hole = Hole::new(&mut self.data, pos);

            while hole.pos() != 0 {
                let parent = parent(hole.pos());
                if hole.removed() <= hole.get(parent) { break }
                hole.move_to(parent);
            }
            // Hole will be unconditionally filled here; panic or not!
        }
    }
}

Poisoning

Although all unsafe code must ensure some minimal level of exception safety, some types may choose to explicitly poison themselves if they witness a panic. Poisoning doesn't entail anything in particular. Generally it just means preventing normal usage from proceeding. The most notable example of this is the standard library's Mutex type. A Mutex will poison itself if one of its MutexGuards (the thing it returns when a lock is obtained) is dropped during a panic. Any future attempts to lock the Mutex will return an Err.

Mutex poisons not for true safety in the sense that Rust normally cares about. It poisons as a safety-guard against blindly using the data that comes out of a Mutex that has witnessed a panic while locked. The data in such a Mutex was likely in the middle of being modified, and as such may be in an inconsistent or incomplete state. It is important to note that one cannot violate memory safety with such a type if it is correctly written. After all, it must be minimally exception safe!

However if the Mutex contained, say, a BinaryHeap that does not actually have the heap property, it's unlikely that any code that uses it will do what the author intended. As such, the program should not proceed normally. Still, if you're double-plus-sure that you can do something with the value, the Err exposes a method to get the lock anyway. It is safe, after all.

FFI

Rust's unwinding strategy is not specified to be fundamentally compatible with any other language's unwinding. As such, unwinding into Rust from another language, or unwinding into another language from Rust is Undefined Behaviour. You must absolutely catch any panics at the FFI boundary! What you do at that point is up to you, but something must be done. If you fail to do this, at best, your application will crash and burn. At worst, your application won't crash and burn, and will proceed with completely clobbered state.