X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/5baae0ea037ed642b7fe8975fb3004b29827d5b1..d9f72d6ad261b7b8fe97fb80423780e8361abb86:/src/part12.rs diff --git a/src/part12.rs b/src/part12.rs index 40d9ad5..c2331ea 100644 --- a/src/part12.rs +++ b/src/part12.rs @@ -9,16 +9,17 @@ use std::cell::{Cell, RefCell}; //@ (There's not even an automatic derivation happening for the cases where it would be possible.) //@ This restriction propagates up to `Callbacks` itself. What could we do about this? +//@ ## `Rc` //@ The solution is to find some way of cloning `Callbacks` without cloning the environments. This can be achieved with //@ `Rc`, a *reference-counted* pointer. This is is another example of a smart pointer. You can `clone` an `Rc` as often //@ as you want, that doesn't affect the data it contains. It only creates more references to the same data. Once all the //@ references are gone, the data is deleted. //@ //@ Wait a moment, you may say here. Multiple references to the same data? That's aliasing! Indeed: -//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared borrow of the data again. +//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared reference to the data again. -//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable borrow -//@ of it's environment! So we have to go with `Fn`. We wrap that in an `Rc`, and then Rust happily derives `Clone` for us. +//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable reference +//@ to it's environment! So we have to go with `Fn`. We wrap that in an `Rc`, and then Rust happily derives `Clone` for us. #[derive(Clone)] struct Callbacks { callbacks: Vec>, @@ -60,19 +61,19 @@ pub fn main() { //@ So it would be rather sad if we were not able to write this program. Lucky enough, Rust's standard library provides a //@ solution in the form of `Cell`. This type represents a memory cell of some type `T`, providing the two basic operations //@ `get` and `set`. `get` returns a *copy* of the content of the cell, so all this works only if `T` is `Copy`. -//@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through -//@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think +//@ `set`, which overrides the content, only needs a *shared reference* to the cell. The phenomenon of a type that permits mutation through +//@ shared references (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think //@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was -//@ about replacing one piece of data by something else of the same type. This is called *exterior mutability*.
+//@ about replacing one piece of data by something else of the same type. This is called *inherited mutability*.
//@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key to why this is safe. // So, let us put our counter in a `Cell`, and replicate the example from the previous part. fn demo_cell(c: &mut Callbacks) { { let count = Cell::new(0); - // Again, we have to move ownership if the `count` into the environment closure. + // Again, we have to move ownership of the `count` into the environment closure. c.register(move |val| { - // In here, all we have is a shared borrow of our environment. But that's good enough for the `get` and `set` of the cell! + // In here, all we have is a shared reference of our environment. But that's good enough for the `get` and `set` of the cell! //@ At run-time, the `Cell` will be almost entirely compiled away, so this becomes pretty much equivalent to the version //@ we wrote in the previous part. let new_count = count.get()+1; @@ -85,8 +86,13 @@ fn demo_cell(c: &mut Callbacks) { } //@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you `clone` an `Rc`, all it has available -//@ is a shared borrow. However, it has to increment the reference count! Internally, `Rc` uses `Cell` for the count, such that it +//@ is a shared reference. However, it has to increment the reference count! Internally, `Rc` uses `Cell` for the count, such that it //@ can be updated during `clone`. +//@ +//@ Putting it all together, the story around mutation and ownership through references looks as follows: There are *unique* references, +//@ which - because of their exclusivity - are always safe to mutate through. And there are *shared* references, where the compiler cannot +//@ generally promise that mutation is safe. However, if extra circumstances guarantee that mutation *is* safe, then it can happen even +//@ through a shared reference - as we saw with `Cell`. // ## `RefCell` //@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of mutability once and for all, by adding `Cell` @@ -119,22 +125,23 @@ impl CallbacksMut { pub fn call(&mut self, val: i32) { for callback in self.callbacks.iter() { // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`. - //@ At run-time, the cell will keep track of the number of outstanding shared and mutable borrows, + //@ At run-time, the cell will keep track of the number of outstanding shared and mutable references, //@ and panic if the rules are violated.
- //@ For this check to be performed, `closure` is a *guard*: Rather than a normal borrow, `borrow_mut` returns - //@ a smart pointer (`RefMut`, in this case) that waits until is goes out of scope, and then - //@ appropriately updates the number of active borrows. + //@ For this check to be performed, `closure` is a *guard*: Rather than a normal reference, `borrow_mut` returns + //@ a smart pointer ([`RefMut`](https://doc.rust-lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is goes out of scope, and then + //@ appropriately updates the number of active references. //@ //@ Since `call` is the only place that borrows the environments of the closures, we should expect that - //@ the check will always succeed. However, this function would still typecheck with an immutable borrow of `self` (since we are - //@ relying on the interior mutability of `RefCell`). Under this condition, it could happen that a callback - //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself. - //@ This is called reentrancy. It would imply that we borrow the closure a second time, and - //@ panic at run-time. I hope this also makes it clear that there's absolutely no hope of Rust - //@ performing these checks statically, at compile-time: It would have to detect reentrancy! + //@ the check will always succeed, as is actually entirely useless. However, this is not actually true. Several different `CallbacksMut` could share + //@ a callback (as they were created with `clone`), and calling one callback here could trigger calling + //@ all callbacks of the other `CallbacksMut`, which would end up calling the initial callback again. This issue of functions accidentally recursively calling + //@ themselves is called *reentrancy*, and it can lead to subtle bugs. Here, it would mean that the closure runs twice, each time thinking it has a + //@ unique, mutable reference to its environment - so it may end up dereferencing a dangling pointer. Ouch! Lucky enough, + //@ Rust detects this at run-time and panics once we try to borrow the same environment again. I hope this also makes it + //@ clear that there's absolutely no hope of Rust performing these checks statically, at compile-time: It would have to detect reentrancy! let mut closure = callback.borrow_mut(); // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly - // dereference the smart pointer and obtain a mutable borrow of the content. + // dereference the smart pointer and obtain a mutable reference to the content. (&mut *closure)(val); } } @@ -156,8 +163,7 @@ fn demo_mut(c: &mut CallbacksMut) { c.call(1); c.clone().call(2); } -// **Exercise 12.1**: Change the type of `call` to ask only for a shared borrow. Then write some piece of code using only the available, public -// interface of `CallbacksMut` such that a reentrant call to `call` is happening, and the program aborts because the `RefCell` refuses to hand -// out a second mutable borrow to its content. +// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure +// is happening, and the program panics because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment. -//@ [index](main.html) | [previous](part11.html) | [next](part13.html) +//@ [index](main.html) | [previous](part11.html) | [raw source](workspace/src/part12.rs) | [next](part13.html)