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244 lines
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244 lines
13 KiB
Plaintext
---
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slug: 2018/01/captains-cookbook-part-2
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title: "Captain's Cookbook: Practical usage"
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date: 2018-01-16 13:00:00
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authors: [bspeice]
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tags: []
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---
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A look at more practical usages of Cap'N Proto
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<!-- truncate -->
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[Part 1](/2018/01/captains-cookbook-part-1) of this series took a look at a basic starting project
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with Cap'N Proto. In this section, we're going to take the (admittedly basic) schema and look at how we can add a pretty
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basic feature - sending Cap'N Proto messages between threads. It's nothing complex, but I want to make sure that there's
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some documentation surrounding practical usage of the library.
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As a quick refresher, we build a Cap'N Proto message and go through the serialization/deserialization steps
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[here](https://github.com/bspeice/capnp_cookbook_1/blob/master/src/main.rs). Our current example is going to build on
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the code we wrote there; after the deserialization step, we'll try and send the `point_reader` to a separate thread
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for verification.
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I'm going to walk through the attempts as I made them and my thinking throughout.
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If you want to skip to the final project, check out the code available [here](https://github.com/bspeice/capnp_cookbook_2)
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## Attempt 1: Move the reference
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As a first attempt, we're going to try and let Rust move the reference. Our code will look something like:
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```rust
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fn main() {
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// ...assume that we own a `buffer: Vec<u8>` containing the binary message content from
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// somewhere else
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let deserialized = capnp::serialize::read_message(
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&mut buffer.as_slice(),
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capnp::message::ReaderOptions::new()
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).unwrap();
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let point_reader = deserialized.get_root::<point_capnp::point::Reader>().unwrap();
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// By using `point_reader` inside the new thread, we're hoping that Rust can
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// safely move the reference and invalidate the original thread's usage.
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// Since the original thread doesn't use `point_reader` again, this should
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// be safe, right?
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let handle = std::thread:spawn(move || {
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assert_eq!(point_reader.get_x(), 12);
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assert_eq!(point_reader.get_y(), 14);
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});
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handle.join().unwrap()
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}
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```
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Well, the Rust compiler doesn't really like this. We get four distinct errors back:
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```
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error[E0277]: the trait bound `*const u8: std::marker::Send` is not satisfied in `[closure@src/main.rs:31:37: 36:6 point_reader:point_capnp::point::Reader<'_>]`
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--> src/main.rs:31:18
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31 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `*const u8` cannot be sent between threads safely
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error[E0277]: the trait bound `*const capnp::private::layout::WirePointer: std::marker::Send` is not satisfied in `[closure@src/main.rs:31:37: 36:6 point_reader:point_capnp::point::Reader<'_>]`
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--> src/main.rs:31:18
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31 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `*const capnp::private::layout::WirePointer` cannot be sent between threads safely
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error[E0277]: the trait bound `capnp::private::arena::ReaderArena: std::marker::Sync` is not satisfied
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--> src/main.rs:31:18
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31 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `capnp::private::arena::ReaderArena` cannot be shared between threads safely
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error[E0277]: the trait bound `*const std::vec::Vec<std::option::Option<std::boxed::Box<capnp::private::capability::ClientHook + 'static>>>: std::marker::Send` is not satisfied in `[closure@src/main.rs:31:37: 36:6 point_reader:point_capnp::point::Reader<'_>]`
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--> src/main.rs:31:18
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31 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `*const std::vec::Vec<std::option::Option<std::boxed::Box<capnp::private::capability::ClientHook + 'static>>>` cannot be sent between threads safely
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error: aborting due to 4 previous errors
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```
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Note, I've removed the help text for brevity, but suffice to say that these errors are intimidating.
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Pay attention to the text that keeps on getting repeated though: `XYZ cannot be sent between threads safely`.
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This is a bit frustrating: we own the `buffer` from which all the content was derived, and we don't have any
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unsafe accesses in our code. We guarantee that we wait for the child thread to stop first, so there's no possibility
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of the pointer becoming invalid because the original thread exits before the child thread does. So why is Rust
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preventing us from doing something that really should be legal?
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This is what is known as [fighting the borrow checker](https://doc.rust-lang.org/1.8.0/book/references-and-borrowing.html).
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Let our crusade begin.
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## Attempt 2: Put the `Reader` in a `Box`
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The [`Box`](https://doc.rust-lang.org/std/boxed/struct.Box.html) type allows us to convert a pointer we have
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(in our case the `point_reader`) into an "owned" value, which should be easier to send across threads.
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Our next attempt looks something like this:
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```rust
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fn main() {
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// ...assume that we own a `buffer: Vec<u8>` containing the binary message content
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// from somewhere else
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let deserialized = capnp::serialize::read_message(
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&mut buffer.as_slice(),
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capnp::message::ReaderOptions::new()
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).unwrap();
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let point_reader = deserialized.get_root::<point_capnp::point::Reader>().unwrap();
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let boxed_reader = Box::new(point_reader);
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// Now that the reader is `Box`ed, we've proven ownership, and Rust can
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// move the ownership to the new thread, right?
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let handle = std::thread::spawn(move || {
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assert_eq!(boxed_reader.get_x(), 12);
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assert_eq!(boxed_reader.get_y(), 14);
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});
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handle.join().unwrap();
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}
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```
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Spoiler alert: still doesn't work. Same errors still show up.
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```
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error[E0277]: the trait bound `*const u8: std::marker::Send` is not satisfied in `point_capnp::point::Reader<'_>`
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--> src/main.rs:33:18
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33 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `*const u8` cannot be sent between threads safely
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error[E0277]: the trait bound `*const capnp::private::layout::WirePointer: std::marker::Send` is not satisfied in `point_capnp::point::Reader<'_>`
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--> src/main.rs:33:18
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33 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `*const capnp::private::layout::WirePointer` cannot be sent between threads safely
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error[E0277]: the trait bound `capnp::private::arena::ReaderArena: std::marker::Sync` is not satisfied
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--> src/main.rs:33:18
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33 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `capnp::private::arena::ReaderArena` cannot be shared between threads safely
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error[E0277]: the trait bound `*const std::vec::Vec<std::option::Option<std::boxed::Box<capnp::private::capability::ClientHook + 'static>>>: std::marker::Send` is not satisfied in `point_capnp::point::Reader<'_>`
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--> src/main.rs:33:18
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33 | let handle = std::thread::spawn(move || {
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| ^^^^^^^^^^^^^^^^^^ `*const std::vec::Vec<std::option::Option<std::boxed::Box<capnp::private::capability::ClientHook + 'static>>>` cannot be sent between threads safely
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error: aborting due to 4 previous errors
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```
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Let's be a little bit smarter about the exceptions this time though. What is that
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[`std::marker::Send`](https://doc.rust-lang.org/std/marker/trait.Send.html) thing the compiler keeps telling us about?
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The documentation is pretty clear; `Send` is used to denote:
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> Types that can be transferred across thread boundaries.
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In our case, we are seeing the error messages for two reasons:
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1. Pointers (`*const u8`) are not safe to send across thread boundaries. While we're nice in our code
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making sure that we wait on the child thread to finish before closing down, the Rust compiler can't make
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that assumption, and so complains that we're not using this in a safe manner.
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2. The `point_capnp::point::Reader` type is itself not safe to send across threads because it doesn't
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implement the `Send` trait. Which is to say, the things that make up a `Reader` are themselves not thread-safe,
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so the `Reader` is also not thread-safe.
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So, how are we to actually transfer a parsed Cap'N Proto message between threads?
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## Attempt 3: The `TypedReader`
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The `TypedReader` is a new API implemented in the Cap'N Proto [Rust code](https://crates.io/crates/capnp/0.8.14).
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We're interested in it here for two reasons:
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1. It allows us to define an object where the _object_ owns the underlying data. In previous attempts,
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the current context owned the data, but the `Reader` itself had no such control.
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2. We can compose the `TypedReader` using objects that are safe to `Send` across threads, guaranteeing
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that we can transfer parsed messages across threads.
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The actual type info for the [`TypedReader`](https://github.com/capnproto/capnproto-rust/blob/f0efc35d7e9bd8f97ca4fdeb7c57fd7ea348e303/src/message.rs#L181)
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is a bit complex. And to be honest, I'm still really not sure what the whole point of the
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[`PhantomData`](https://doc.rust-lang.org/std/marker/struct.PhantomData.html) thing is either.
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My impression is that it lets us enforce type safety when we know what the underlying Cap'N Proto
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message represents. That is, technically the only thing we're storing is the untyped binary message;
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`PhantomData` just enforces the principle that the binary represents some specific object that has been parsed.
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Either way, we can carefully construct something which is safe to move between threads:
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```rust
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fn main() {
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// ...assume that we own a `buffer: Vec<u8>` containing the binary message content from somewhere else
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let deserialized = capnp::serialize::read_message(
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&mut buffer.as_slice(),
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capnp::message::ReaderOptions::new()
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).unwrap();
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let point_reader: capnp::message::TypedReader<capnp::serialize::OwnedSegments, point_capnp::point::Owned> =
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capnp::message::TypedReader::new(deserialized);
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// Because the point_reader is now working with OwnedSegments (which are owned vectors) and an Owned message
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// (which is 'static lifetime), this is now safe
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let handle = std::thread::spawn(move || {
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// The point_reader owns its data, and we use .get() to retrieve the actual point_capnp::point::Reader
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// object from it
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let point_root = point_reader.get().unwrap();
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assert_eq!(point_root.get_x(), 12);
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assert_eq!(point_root.get_y(), 14);
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});
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handle.join().unwrap();
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}
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```
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And while we've left Rust to do the dirty work of actually moving the `point_reader` into the new thread,
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we could also use things like [`mpsc` channels](https://doc.rust-lang.org/std/sync/mpsc/index.html) to achieve a similar effect.
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So now we're able to define basic Cap'N Proto messages, and send them all around our programs.
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