explained generators and the need for Pin

2020-01-28 16:47:48 +01:00
parent 134b202952
commit 4249cacb91
1 changed files with 277 additions and 3 deletions
--- a/src/0_1_2_generators_pin.md
+++ b/src/0_1_2_generators_pin.md
@@ -8,7 +8,7 @@ is Generators and the `Pin` type.

 >**Relevant for:**

->- Understanding how the async/await syntax works
+>- Understanding how the async/await syntax works, and how they're implemented
 >- Why we need `Pin`
 >- Why Rusts async model is extremely efficient

@@ -175,9 +175,214 @@ impl Generator for GeneratorA {

 >The `yield` keyword was discussed first in [RFC#1823][rfc1823] and in [RFC#1832][rfc1832].

+Now that you know that the `yield` keyword in reality rewrites your code to become a state machine,
+you'll also know the basics of how `await` works. It's very similar.
+
+Now, there are some limitations in our naive state machine above. What happens when you have a 
+`borrow` across a `yield` point?
+
+We could forbid that, but **one of the major design goals** for the async/await syntax has been
+to allow this. These kinds of borrows were not possible using `Futures 1.0` so we can't let this
+limitation just slip and call it a day yet.
+
+Instead of discussing it in theory, let's look at some code. 
+
+> We'll use the optimized version of the state machines which is used in Rust today. For a more
+> in deapth explanation see [Tyler Mandry's execellent article: How Rust optimizes async/await][optimizing-await]
+
+```rust,noplaypen,ignore
+let a = 4;
+let b = move || {
+        let to_borrow = String::new("Hello");
+        let borrowed = &to_borrow;
+        println!("{}", borrowed);
+        yield a * 2;
+        println!("{} world!", borrowed);
+    };
+```
+
+Now what does our rewritten state machine look like with this example?
+
+```rust,compile_fail
+# // If you've ever wondered why the parameters are called Y and R the naming from
+# // the original rfc most likely holds the answer
+# enum GeneratorState<Y, R> {
+#     // originally called `CoResult`
+#     Yielded(Y),  // originally called `Yield(Y)`
+#     Complete(R), // originally called `Return(R)`
+# }
+# 
+# trait Generator {
+#     type Yield;
+#     type Return;
+#     fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return>;
+# }
+
+enum GeneratorA {
+    Enter,
+    Yield1 {
+        to_borrow: String,
+        borrowed: &String, // uh, what lifetime should this have?
+    },
+    Exit,
+}
+
+# impl GeneratorA {
+#     fn start() -> Self {
+#         GeneratorA::Enter
+#     }
+# }
+
+impl Generator for GeneratorA {
+    type Yield = usize;
+    type Return = ();
+    fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {
+        // lets us get ownership over current state
+        match std::mem::replace(&mut *self, GeneratorA::Exit) {
+            GeneratorA::Enter => {
+                let to_borrow = String::from("Hello");
+                let borrowed = &to_borrow;
+                *self = GeneratorA::Yield1 {to_borrow, borrowed};
+                GeneratorState::Yielded(borrowed.len())
+            }
+
+            GeneratorA::Yield1 {to_borrow, borrowed} => {
+                println!("Hello {}", borrowed);
+                *self = GeneratorA::Exit;
+                GeneratorState::Complete(())
+            }
+            GeneratorA::Exit => panic!("Can't advance an exited generator!"),
+        }
+    }
+}
+```
+
+If you try to compile this you'll get an error (just try it yourself by pressing play).
+
+What is the lifetime of `&String`. It's not the same as the lifetime of `Self`. It's not `static`.
+Turns out that it's not possible for us in Rusts syntax to describe this lifetime, which means, that
+to make this work, we'll have to let the compiler know that _we_ control this correctlt.
+
+That means turning to unsafe.
+
+Now, as you'll notice, this compiles:
+
+```rust
+pub fn test2() {
+    let mut gen = GeneratorA::start();
+
+    if let GeneratorState::Yielded(n) = gen.resume() {
+        println!("Got value {}", n);
+    }
+
+    let mut gen2 = GeneratorA::start();
+    // If you uncomment this, very bad things can happen. This is why we need `Pin`
+    // let mut gen2 = GeneratorA::start();
+    //std::mem::swap(&mut gen, &mut gen2);
+
+    if let GeneratorState::Complete(()) = gen2.resume() {
+        ()
+    };
+}
+
+use std::ptr::NonNull;
+
+// If you've ever wondered why the parameters are called Y and R the naming from
+// the original rfc most likely holds the answer
+enum GeneratorState<Y, R> {
+    // originally called `CoResult`
+    Yielded(Y),  // originally called `Yield(Y)`
+    Complete(R), // originally called `Return(R)`
+}
+
+trait Generator {
+    type Yield;
+    type Return;
+    fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return>;
+}
+
+enum GeneratorA {
+    Enter,
+    Yield1 {
+        to_borrow: String,
+        borrowed: *const String, // Normally you'll see `std::ptr::NonNull` used instead of *ptr
+    },
+    Exit,
+}
+
+impl GeneratorA {
+    fn start() -> Self {
+        GeneratorA::Enter
+    }
+}
+impl Generator for GeneratorA {
+    type Yield = usize;
+    type Return = ();
+    fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {
+        // lets us get ownership over current state
+            match self {
+            GeneratorA::Enter => {
+                let to_borrow = String::from("Hello");
+                let borrowed = &to_borrow;
+                let res = borrowed.len();
+
+                // Tricks to actually get a self reference
+                *self = GeneratorA::Yield1 {to_borrow, borrowed: std::ptr::null()};
+                match self {
+                 GeneratorA::Yield1{to_borrow, borrowed} => *borrowed = to_borrow,
+                 _ => ()  
+                };
+
+                GeneratorState::Yielded(res)
+            }
+
+            GeneratorA::Yield1 {to_borrow, borrowed} => {
+                let borrowed: &String = unsafe {&**borrowed};
+                println!("{} world", borrowed);
+                *self = GeneratorA::Exit;
+                GeneratorState::Complete(())
+            }
+            GeneratorA::Exit => panic!("Can't advance an exited generator!"),
+        }
+    }
+}
+
+```
+
+But now, let's
+
+```rust
+```
+
+However, this is also the point where we need to talk about one more concept to 
+
 ## Pin

-Pin is used to allow for self referential structs. An example:
+> Why
+>
+> 1. To understand `Generators` and `Futures`
+> 2. Knowing how to use `Pin` when implementing your own `Future`
+> 3. Understand self-referential types in Rust
+>
+> `Pin` was suggested in [RFC#2349][rfc2349]
+
+Ping consists of the `Pin` type and the `Unpin` marker. Let's start off with some general rules:
+
+1. Pin does nothing special, it only prevents the user of an API to violate some assumtions you make when writing your (most likely) unsafe code.
+2. Most standard library types implement `Unpin`
+3. `Unpin` means it's OK for this type.
+4. If you `Box` a value, that boxed value automatcally implements `Unpin`.
+5. The absolute main use case for `Pin` is to allow self referential types
+6. The implementation behind objects that doens't implement `Unpin` is always unsafe
+   1. `Pin` prevents users from your code to break the assumtions you make when writing the `unsafe` implementation
+   2. It doesn't solve the fact that you'll have to write unsafe code to actually implement it
+
+To get a 
+
+> Unsafe code does not mean it's litterally "unsafe", it only relieves the guarantees you normally get from the compiler.
+> An `unsafe` implementation can be perfectly safe to do, but you have no safety net.
+
+Let's take a look at an example:

 ```rust,editable
 use std::pin::Pin;
@@ -283,6 +488,73 @@ impl Test {

 ```

+However, to get to know the normal way of implementing such an API which is what we'll see going
+forward, we can rewrite the code above into this:
+
+```rust, editbable, compile_fail
+use std::pin::Pin;
+
+pub fn test1() {
+    let mut test1 = Test::new("test1");
+    test1.init();
+    let mut test1_pin = Pin::new(&mut test1); 
+    let mut test2 = Test::new("test2");
+    test2.init();
+    let mut test2_pin = Pin::new(&mut test2);
+
+    println!(
+        "a: {}, b: {}",
+        Test::a(test1_pin.as_ref()),
+        Test::b(test1_pin.as_ref())
+    );
+
+    // try fixing as the compiler suggests. Is there any `swap` happening?
+    // Look closely at the printout.
+    std::mem::swap(test1_pin.as_mut(), test2_pin.as_mut());
+    println!(
+        "a: {}, b: {}",
+        Test::a(test2_pin.as_ref()),
+        Test::b(test2_pin.as_ref())
+    );
+}
+
+#[derive(Debug)]
+struct Test {
+    a: String,
+    b: *const String,
+}
+
+
+impl Test {
+    fn new(txt: &str) -> Self {
+        let a = String::from(txt);
+        Test {
+            a,
+            b: std::ptr::null(),
+        }
+    }
+    fn init(&mut self) {
+        let self_ptr: *const String = &self.a;
+        self.b = self_ptr;
+    }
+
+    fn a<'a>(self: Pin<&'a Self>) -> &'a str {
+        &self.get_ref().a
+    }
+
+    fn b<'a>(self: Pin<&'a Self>) -> &'a String {
+        unsafe { &*(self.b) }
+    }
+}
+```
+
+There is one caviat here. Our struct `Test` implements `Unpin`. Now this will be the "normal case"
+since most types implement `Unpin`. However, a type which 
+
+## Putting it all together
+
+Now that we've seen how `Pin` works 
+
 pinning
 ```ignore
 // If we borrow through yield points, we end up with this error
@@ -303,3 +575,5 @@ pinning
 [greenthreads]: https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/
 [rfc1823]: https://github.com/rust-lang/rfcs/pull/1823
 [rfc1832]: https://github.com/rust-lang/rfcs/pull/1832
+[rfc2349]: https://github.com/rust-lang/rfcs/blob/master/text/2349-pin.md
+[optimizing-await]: https://tmandry.gitlab.io/blog/posts/optimizing-await-1/