rust/leaking.md

% Leaking

Ownership based resource management is intended to simplify composition. You acquire resources when you create the object, and you release the resources when it gets destroyed. Since destruction is handled for you, it means you can't forget to release the resources, and it happens as soon as possible! Surely this is perfect and all of our problems are solved.

Everything is terrible and we have new and exotic problems to try to solve.

Many people like to believe that Rust eliminates resource leaks, but this is absolutely not the case, no matter how you look at it. In the strictest sense, "leaking" is so abstract as to be unpreventable. It's quite trivial to initialize a collection at the start of a program, fill it with tons of objects with destructors, and then enter an infinite event loop that never refers to it. The collection will sit around uselessly, holding on to its precious resources until the program terminates (at which point all those resources would have been reclaimed by the OS anyway).

We may consider a more restricted form of leak: failing to drop a value that is unreachable. Rust also doesn't prevent this. In fact Rust has a function for doing this: mem::forget. This function consumes the value it is passed and then doesn't run its destructor.

In the past mem::forget was marked as unsafe as a sort of lint against using it, since failing to call a destructor is generally not a well-behaved thing to do (though useful for some special unsafe code). However this was generally determined to be an untenable stance to take: there are many ways to fail to call a destructor in safe code. The most famous example is creating a cycle of reference counted pointers using interior mutability.

It is reasonable for safe code to assume that destructor leaks do not happen, as any program that leaks destructors is probably wrong. However unsafe code cannot rely on destructors to be run to be safe. For most types this doesn't matter: if you leak the destructor then the type is by definition inaccessible, so it doesn't matter, right? For instance, if you leak a Box<u8> then you waste some memory but that's hardly going to violate memory-safety.

However where we must be careful with destructor leaks are proxy types. These are types which manage access to a distinct object, but don't actually own it. Proxy objects are quite rare. Proxy objects you'll need to care about are even rarer. However we'll focus on three interesting examples in the standard library:

• vec::Drain
• Rc
• thread::scoped::JoinGuard

Drain

drain is a collections API that moves data out of the container without consuming the container. This enables us to reuse the allocation of a Vec after claiming ownership over all of its contents. It produces an iterator (Drain) that returns the contents of the Vec by-value.

Now, consider Drain in the middle of iteration: some values have been moved out, and others haven't. This means that part of the Vec is now full of logically uninitialized data! We could backshift all the elements in the Vec every time we remove a value, but this would have pretty catastrophic performance consequences.

Instead, we would like Drain to fix the Vec's backing storage when it is dropped. It should run itself to completion, backshift any elements that weren't removed (drain supports subranges), and then fix Vec's len. It's even unwinding-safe! Easy!

Now consider the following:

let mut vec = vec![Box::new(0); 4];

{
	// start draining, vec can no longer be accessed
	let mut drainer = vec.drain(..);

	// pull out two elements and immediately drop them
	drainer.next();
	drainer.next();

	// get rid of drainer, but don't call its destructor
	mem::forget(drainer);
}

// Oops, vec[0] was dropped, we're reading a pointer into free'd memory!
println!("{}", vec[0]);

This is pretty clearly Not Good. Unfortunately, we're kind've stuck between a rock and a hard place: maintaining consistent state at every step has an enormous cost (and would negate any benefits of the API). Failing to maintain consistent state gives us Undefined Behaviour in safe code (making the API unsound).

So what can we do? Well, we can pick a trivially consistent state: set the Vec's len to be 0 when we start the iteration, and fix it up if necessary in the destructor. That way, if everything executes like normal we get the desired behaviour with minimal overhead. But if someone has the audacity to mem::forget us in the middle of the iteration, all that does is leak even more (and possibly leave the Vec in an unexpected but consistent state). Since we've accepted that mem::forget is safe, this is definitely safe. We call leaks causing more leaks a leak amplification.

Rc

Rc is an interesting case because at first glance it doesn't appear to be a proxy value at all. After all, it manages the data it points to, and dropping all the Rcs for a value will drop that value. leaking an Rc doesn't seem like it would be particularly dangerous. It will leave the refcount permanently incremented and prevent the data from being freed or dropped, but that seems just like Box, right?

Nope.

Let's consider a simplified implementation of Rc:

struct Rc<T> {
	ptr: *mut RcBox<T>,
}

struct RcBox<T> {
	data: T,
	ref_count: usize,
}

impl<T> Rc<T> {
	fn new(data: T) -> Self {
		unsafe {
			// Wouldn't it be nice if heap::allocate worked like this?
			let ptr = heap::allocate<RcBox<T>>();
			ptr::write(ptr, RcBox {
				data: data,
				ref_count: 1,
			});
			Rc { ptr: ptr }
		}
	}

	fn clone(&self) -> Self {
		unsafe {
			(*self.ptr).ref_count += 1;
		}
		Rc { ptr: self.ptr }
	}
}

impl<T> Drop for Rc<T> {
	fn drop(&mut self) {
		unsafe {
			let inner = &mut ;
			(*self.ptr).ref_count -= 1;
			if (*self.ptr).ref_count == 0 {
				// drop the data and then free it
				ptr::read(self.ptr);
				heap::deallocate(self.ptr);
			}
		}
	}
}

This code contains an implicit and subtle assumption: ref_count can fit in a usize, because there can't be more than usize::MAX Rcs in memory. However this itself assumes that the ref_count accurately reflects the number of Rcs in memory, which we know is false with mem::forget. Using mem::forget we can overflow the ref_count, and then get it down to 0 with outstanding Rcs. Then we can happily use-after-free the inner data. Bad Bad Not Good.

This can be solved by saturating the ref_count, which is sound because decreasing the refcount by n still requires n Rcs simultaneously living in memory.

thread::scoped::JoinGuard

The thread::scoped API intends to allow threads to be spawned that reference data on the stack without any synchronization over that data. Usage looked like:

let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
{
	let guards = vec![];
	for x in &mut data {
		// Move the mutable reference into the closure, and execute
		// it on a different thread. The closure has a lifetime bound
		// by the lifetime of the mutable reference `x` we store in it.
		// The guard that is returned is in turn assigned the lifetime
		// of the closure, so it also mutably borrows `data` as `x` did.
		// This means we cannot access `data` until the guard goes away.
		let guard = thread::scoped(move || {
			*x *= 2;
		});
		// store the thread's guard for later
		guards.push(guard);
	}
	// All guards are dropped here, forcing the threads to join
	// (this thread blocks here until the others terminate).
	// Once the threads join, the borrow expires and the data becomes
	// accessible again in this thread.
}
// data is definitely mutated here.

In principle, this totally works! Rust's ownership system perfectly ensures it! ...except it relies on a destructor being called to be safe.

let mut data = Box::new(0);
{
	let guard = thread::scoped(|| {
		// This is at best a data race. At worst, it's *also* a use-after-free.
		*data += 1;
	});
	// Because the guard is forgotten, expiring the loan without blocking this
	// thread.
	mem::forget(guard);
}
// So the Box is dropped here while the scoped thread may or may not be trying
// to access it.

Dang. Here the destructor running was pretty fundamental to the API, and it had to be scrapped in favour of a completely different design.