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+---
+layout: post
+title: "A Case Study in Borrow Checking"
+description: "...and some practical lessons learned."
+category: 
+tags: [rust]
+---
+
+I'm convinced that WebSockets are a gateway drug. The specification is reasonably easy to understand, and implementations are an opportunity to both dig into the lower-level details of networking code and experiment with new techniques. It's essentially [writing](https://www.youtube.com/watch?v=HyzD8pNlpwI) [a](https://cturt.github.io/cinoop.html) [Gameboy](https://blog.rekawek.eu/2017/02/09/coffee-gb/) [emulator](https://djhworld.github.io/post/2018/09/21/i-ported-my-gameboy-color-emulator-to-webassembly/), but for network code instead of emulation. 
+
+At least, that's how I'm approaching it. While there are [existing](https://github.com/housleyjk/ws-rs) [implementations](https://github.com/websockets-rs/rust-websocket) of the protocol for Rust, writing a WebSocket library is an opportunity for me to experiment with parser [combinators](https://github.com/Geal/nom) and [generators](https://github.com/kaitai-io/kaitai_struct), and maybe have something to show at the end of it. Recently, I've been adding support for Rust to the [Kaitai Struct](http://kaitai.io/) project so that I can generate the parser from a schema, rather than writing one by hand. But before we can generate a parser using Kaitai, we need a runtime library. This is a typical pattern in code generation; the generated code relies on a "standard library" of functionality similar to how programming languages have their own standard library.
+
+What makes this parser runtime difficult to implement in Rust is the performance concerns; we don't want to allocate new `Vec<u8>` buffers and copy data around when it's not necessary. Especially in networking code, these types of "zero-copy" operations are critical to performance. And because we're not interested in modifying the data stream, references make a lot of sense! However, that means there's a good potential to hit issues with the borrow checker; making sure all the structures being parsed use the stream correctly is difficult. As a result, I hit a lot of issues with the borrow checker, and wanted to detail what I learned about not only how to *avoid* fighting the borrow checker, but how to *work with* the borrow checker.
+
+# Design Inspiration - C++
+
+So how exactly does one go about building such a runtime? In this case, we'll start by looking at Kaitai's [C++ support](https://github.com/kaitai-io/kaitai_struct_cpp_stl_runtime) for inspiration, and see if we can adapt that to Rust. There's even an [ownership guide](http://doc.kaitai.io/lang_cpp_stl.html#_ownership_model) detailing the rules for how the C++ runtime thinks about ownership!
+
+This article will use a toy schema for illustrating lifetimes:
+
+```yaml
+meta:
+  id: toy
+  title: Toy Schema
+  endian: be
+
+seq:
+  - id: slice_size
+    type: u1
+  - id: child_structure
+    type: child
+
+types:
+  child:
+    seq:
+      - id: slice
+        size: _parent.slice_size
+      - id: grandchild_structure
+        type: grandchild
+
+  grandchild:
+    seq:
+      - id: slice
+        size: _root.slice_size
+```
+
+The parser will operate like this:
+
+1. Read a single byte (`u8` in Rust) from a stream, and store that in `slice_size`
+2. Read a child structure. First, read a byte slice whose size is the parent structure's `slice_size`, then read the grandchild
+3. Read a granchild structure by taking a byte slice whose size is the root structure's `slice_size`
+
+So let's start by generating the C++ code corresponding to our specification (edited for clarity):
+
+**toy.h**
+```cpp
+class toy_t : public kaitai::kstruct {
+
+public:
+    class child_t;
+    class grandchild_t;
+
+    toy_t(kaitai::kstream* p__io, kaitai::kstruct* p__parent = nullptr, toy_t* p__root = nullptr);
+    ~toy_t();
+
+    class child_t : public kaitai::kstruct {
+    public:
+        child_t(kaitai::kstream* p__io, toy_t* p__parent = nullptr, toy_t* p__root = nullptr);
+        ~child_t();
+
+    private:
+        std::string m_slice;
+        std::unique_ptr<grandchild_t> m_grandchild_structure;
+        toy_t* m__root;
+        toy_t* m__parent;
+    };
+
+    class grandchild_t : public kaitai::kstruct {
+    public:
+        grandchild_t(kaitai::kstream* p__io, toy_t::child_t* p__parent = nullptr, toy_t* p__root = nullptr);
+        ~grandchild_t();
+
+    private:
+        std::string m_slice;
+        toy_t* m__root;
+        toy_t::child_t* m__parent;
+    };
+
+private:
+    uint8_t m_initial_byte;
+    std::unique_ptr<child_t> m_child_structure;
+    toy_t* m__root;
+    kaitai::kstruct* m__parent;
+};
+```
+
+**toy.cpp**
+```cpp
+#include "toy.h"
+
+toy_t::toy_t(kaitai::kstream* p__io, kaitai::kstruct* p__parent, toy_t* p__root) : kaitai::kstruct(p__io) {
+    m__parent = p__parent;
+    m__root = this;
+    m_child_structure = nullptr;
+    _read();
+}
+
+void toy_t::_read() {
+    m_slice_size = m__io->read_u1();
+    m_child_structure = std::unique_ptr<child_t>(new child_t(m__io, this, m__root));
+}
+
+toy_t::child_t::child_t(kaitai::kstream* p__io, toy_t* p__parent, toy_t* p__root) : kaitai::kstruct(p__io) {
+    m__parent = p__parent;
+    m__root = p__root;
+    m_grandchild_structure = nullptr;
+    _read();
+}
+
+void toy_t::child_t::_read() {
+    m_slice = m__io->read_bytes(_parent()->slice_size());
+    m_grandchild_structure = std::unique_ptr<grandchild_t>(new grandchild_t(m__io, this, m__root));
+}
+
+toy_t::grandchild_t::grandchild_t(kaitai::kstream* p__io, toy_t::child_t* p__parent, toy_t* p__root) : kaitai::kstruct(p__io) {
+    m__parent = p__parent;
+    m__root = p__root;
+    _read();
+}
+
+void toy_t::grandchild_t::_read() {
+    m_slice = m__io->read_bytes(_root()->slice_size());
+}
+```
+
+Now, let's think about ownership as we look at the code:
+- Each parent structure (`toy_t` and `child_t`) expresses ownership of its children through `std::unique_ptr<>`.
+- Because children can refer to parents through `m__parent` and `m__root`, we have a reference cycle that will be difficult to express in Rust.
+- Everyone stores a reference to `kaitai::kstream`, but nobody owns it.
+- Structures own their data using `std::string` ([`read_bytes` implementation](https://github.com/kaitai-io/kaitai_struct_cpp_stl_runtime/blob/1ea056ad053b438e1609fe84e71b1d306777492d/kaitai/kaitaistream.cpp#L347-L361));
+  this prevents issues if the stream (`m__io`) gets destroyed, but also introduces an extra allocation and copy that Rust can avoid if we convince the borrow checker that structures won't outlive the stream.
+- The root structure (`toy_t`) stores a reference to itself; it's thus unsafe to copy or move.
+
+With all that in mind, let's talk about ownership in the Rust runtime.
+