CSC/ECE 517 Fall 2015 M1504 Implement support for missing XMLHttpRequest APIs

Introduction

Servo

Servo <ref> https://github.com/servo/servo </ref> is a web browser layout engine written in Rust<ref>https://github.com/rust-lang/rust</ref> and is currently being developed by Mozilla Research. The aim of the project is not to create a full browser but is rather to create a highly parallel environment that allows for many components be handled by fine-grained, isolated tasks.<ref>https://en.wikipedia.org/wiki/Servo_(layout_engine)</ref>

Servo is built on top of Rust to provide a secure and reliable foundation and is focused on creating a reliable and fast browser engine.

Rust

Rust is a multi-paradigm, compiled programming language that is a good language for creating highly safe systems. Rust and Servo have a symbiotic relationship as the development of servo has influenced the design of the language.

Rust is a modern, fast, memory safe and multithreaded programming language that focuses on speed and safety for developing reliable and efficient systems. It eliminates all data races by having numerous compile-time safety checks that adds no runtime overhead.<ref> http://doc.rust-lang.org/nightly/book/README.html</ref>

Servo task Architecture

Description

• Each box corresponds to a Rust task.
• The primary tasks in the browser pipeline are represented by Blue boxes.
• Dashed lines indicate supervisor relationships.
• Gray boxes indicate tasks auxiliary to the browser pipeline.
• Communication channels are represented as solid lines.
• White boxes are represented as worker tasks. Each such box represents several tasks which varies with the workload.<ref>https://github.com/servo/servo/wiki/Design</ref>

Background

XMLHttpRequest

"XMLHttpRequest is a specification which defines APIs that provides scripted client functionality for transferring data between a client and a server."<ref>https://xhr.spec.whatwg.org/</ref> XMLHttpRequest provides a way for data to be retrieved from a URL without having to retrieve the entire page. It supports protocols other than HTTP and can be used to retrieve any type of data.<ref>https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest</ref>

Some of the APIs and properties present in XMLHttpRequest specification are overrideMimeType() , responseXML and withCredentials. The following javascript code snippets briefly explain about each of these APIs.

overrideMimeType<ref>https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/XMLHttpRequest/overridemimetype-open-state-force-xml.htm</ref>

 
//override the Mime type to retrieve the response as an XML Object


 var client = new XMLHttpRequest();
 client.onreadystatechange = function() {
          if (client.readyState !== 4) return;
					try{
            var str = client.responseXML.documentElement.tagName+
                      client.responseXML.documentElement.firstChild.tagName+
                      client.responseXML.documentElement.firstChild.textContent;
					}catch(e){
						assert_unreached('Exception when reading responseXML');
					}
          assert_equals( client.responseXML.documentElement.tagName,  'test' );
          assert_equals( client.responseXML.documentElement.firstChild.tagName,  'message' );
          assert_equals( client.responseXML.documentElement.firstChild.textContent,  'Hello World！' );
          test.done();
        };
        client.open("GET", "resources/status.py?type="
                    +encodeURIComponent('text/plain;charset=Shift-JIS')
                    +'&content='+ encodeURIComponent('<test><message>Hello World！</message></test>'));
        client.overrideMimeType('application/xml;charset=UTF-8');
        client.send();

responseXML property<ref>https://msdn.microsoft.com/en-us/library/ms534370(v=vs.85).aspx</ref>

//read the response from the server as an XML Object

var oReq = new XMLHttpRequest();
oReq.open("GET", "http://localhost/books.xml", false);
oReq.send();
console.log(oReq.responseXML.xml);

withCredentials property<ref>http://arunranga.com/examples/access-control/credentialedRequest.html</ref>

//create cross site requests using cookies as credentials
 
var invocation = new XMLHttpRequest();
var url = 'http://crossoriginurl.com/resources/access-control-with-credentials/';
invocation.open('GET', url, true);
invocation.withCredentials = "true";
invocation.onreadystatechange = handler;
invocation.send(); 

The above mentioned APIs and properties for XMLHttpRequest are currently unimplemented in servo.

Project Description

Servo has implemented many API specifications that relies upon JavaScript that executes on all the browsers. XMLHttpRequest is one such specification which is used for making HTTP requests dynamically. Our project aim is to implement few of the missing XMLHttpRequest APIs. The OSS project involved implementing initial steps of the specifications. It basically involved implementing overrideMimeType method and adjusting related test expectations.

The current scope of the project involves:<ref>https://github.com/servo/servo/wiki/Implement-support-for-missing-XMLHttpRequest-APIs</ref>

• Implementing the specified behavior of the final MIME type and final charset based on the implemented override_mime_type() method in the initial steps
• Implementing responseXML API and document response type according to the XHR specifications
• Implementing withCredentials API

Requirement Analysis

Below is an overview of the various steps identified as a part of the requirement analysis:

XMLHttpRequest Interface

• We have to uncomment the responseXML API
• A webpage can make use of this API to read the document response

XMLHttpRequest DOM

1. Create helper methods for final_mime_type and final_charset
   • Implement the two methods as per the behavior mentioned in the specification
2. Implement correct behavior for the text_response method
   • Make use of the final_charset and final_mime_type helper methods to determine the correct encoding for the response and decode the response using this encoding
3. Create and implement a new method "document_response" according to the xhr specification
4. Implement the correct behavior for the responseXML API to handle exceptions and return the document response using the method implemented above <ref>https://xhr.spec.whatwg.org/#dom-xmlhttprequest-responsexml</ref>
5. Implement the withCredentials APIs (setter and getter) as per the specification
6. Add support to the send() and open() API to test the value of the withCredentials attribute where necessary

net_traits

• Add a new member to the LoadData Structure which will be used by the http_loader to conditionally exclude cookies from the XHR request based on the value of the member.

XHR Tests

• We need to pass most tests related to overrideMimeType, responseXML and withCredentials present in the XHR test-suite

Design Patterns

We will be using "State Pattern" for our final project implementation. Our implementation will store the new CHARSET and MIME type based on what user has requested. So once user has requested for override in this value which in turns ensures the system behaves as per this new value.

Proposed Test Cases

The tests have already been written for the XMLHttpRequest APIs related to our project. So the test cases that could validate our changes to the code are as follows:

• overrideMimeType() in unsent state enforces Shift-JIS encoding
• overrideMimeType() in open state enforces UTF-8 encoding
• overrideMimeType() in open state and XML MIME type with UTF-8 charset is correctly enforced
• overrideMimeType() in HEADERS RECEIVED state enforces Shift-JIS encoding
• responseXML attribute throws InvalidStateError if response type is not null or document
• The various responseXML document properties are correctly initialized
• The type of the responseXML stylesheets and implementation must be object
• If the state of XMLHttpRequest is not done then responseXML is null
• Check withCredentials defaults to false and set value is true
• Setting withCredentials when not in UNSENT, OPENED state throws InvalidStateError exception
• Setting withCredentials when synchornous flag is set throws InvalidAccessError exception

Appendix

• Steps for installing rust
• Building Servo

References

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