Servo inside (introduction to Servo’s DOM binding)

Tetsuharu OHZEKI, Gecko Inside #6 (Feb 24, 2016)

About me

• Tetsuharu OHZEKI (a.k.a @saneyuki_s)

• Mozilla Committer (Level 3) as a volunteer contributor

• Firefox developer, and Servo reviewer

• My main work is Web client-side engineer at Shibuya

• JavaScript, GUI Application architecture

Rust is a systems programming language that runs blazingly fast, prevents segfaults, and guarantees

thread safety.

http://rust-lang.org/

Rust Language

• Develop by Mozilla & community

• It’s just like “Modern Redesigned C++”

• Today’s Gecko try to use Rust in some area:

• https://wiki.mozilla.org/Oxidation

• e.g. MP4 metadata parser, URL parser, style system

Servo

• Written with Rust

• By Mozilla Research

• Parallelism, Power, Correctness, Concurrent programming

• more details: https://github.com/servo/servo/wiki/Research

Servo’s key “tasks”

• Constellation

• Script

• Layout

• Paint

• Compositor

• Resource

script crate

• The implementation of “DOM” and scripting

• Binding with SpiderMonkey

• Handle a web driver’s script execution (from Constellation)

Rust Object JS Object

browser JS Engine

DOM lifecycle & JS binding• DOM needs a complex lifecycle management

• good article about this area: http://steps.dodgson.org/b/2012/12/19/dom-and-gc-or-what-happend-at-eden/

• Other browser engines have an own garbage collector for this complexity

• Cycle Collector (Gecko)

• Oilpan GC (Blink)

• Servo need not own GC in the browser side like other engines.

• All lifecycle of Servo’s DOM object are managed by SpiderMonkey GC

How to connect with JSObject & Rust Object?

• DOM Object has “Reflector” field as the first field.

• Reflector stores a pointer to “wrapper” JS Object which corresponds to it.

• JS Object has a pointer to Rust object which is corresponds to it.

• JS Object stores the pointer in its “slot”.

NodeReflector handlers parent_node ……

JSObject

Generate a glue code from WebIDL

• By /components/script/dom/bindings/codegen/BindingGen.py

• Auto generate from WebIDL with special annotation

• Methods, Property, JIT Information, Constants, etc

• “Glue” code to connect a Rust world & JS world

[Abstract]

interface Node : EventTarget {

[Constant]

readonly attribute unsigned short nodeType;

[Pure]

readonly attribute DOMString nodeName;

…

}

How to trace object graph from SpiderMonkey GC

• From GC roots, SpiderMonkey GC trace an object graph.

• Remember these simple rules:

• From a wrapper object (JSObject), GC can traces a Rust Object.

• From a Rust Object, GC can trace a wrapper JS Object.

• Some compiler plugins helps these tricks ;)

Reflector

JSObjectRust ObjectJS<T> JS<T>

ReflectorRust Object

JSObjectJS<T>

Reflector

Rust Object JSObject

GC Managed pointer• We use a special smart pointer for GC managed object

in Rust world

• JS<T> can be holden as a member of a Rust’s DOM struct

• Root<T> can be on stack with rooting explicitly.

• To avoid SpiderMonkey exact (precise) GC’s mistake.

• In practice, we use this for a return value.

Inheritance

• DOM interface requires an inheritance

• But Rust does not have a inheritance system as a language feature!

• So we generate annotation traits for safe up/down casting from WebIDL information!

• …..And write vtable by hand! 😓

vtable by hand…

Special macros and plugins• It’s hard to implement a GC hook for an object correctly.

• We use a special rust compiler plugins to check & avoid typical pitfalls.

• auto generate a tricky implementation for GC interaction.

• check unrooted object

• valid order of struct (for inheritance)

• etc…

How to start hacking Servo?

• http://servo.github.io/servo-starters/

• Servo team picks up “Good First Pull Request”

• CONTRIBUTING.md

• HACKING_QUICKSTART.md