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CSS parse error reporting

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Josh Matthews edited this page Sep 17, 2015 · 2 revisions

Expose CSS parse errors to developer tools

Background information: Firefox supports remote developer tools - ie. communicating with an arbitrary server that implements a protocol for exposing information about web content. Servo implements a very basic developer tools server that currently supports executing JS remotely and investigating the DOM tree in the document inspector. We want to expand these capabilities by exposing CSS parsing errors.

Initial steps:

  • compile Servo and ensure that it runs on tests/html/about-mozilla.html
  • email the mozilla.dev.servo mailing list introducing your group and your progress
  • find an interesting site that doesn't crash Servo and attach the developer tools
  • define a new trait called ParseErrorReporter in components/style_traits/lib.rs with an appropriate method to report an error, and add a error_reporter member to ParserContext that uses this
  • make log_css_error in components/style/parser.rs take a &ParserContext argument and call this method
  • extract the existing code from log_css_err into a new type that implements ParseErrorReporter in components/layout/layout_task.rs and pass instances of that type to any code that ends up creating a ParserContext value
  • at this point, Servo should compile and execute almost exactly as before, but where RUST_LOG=style used to expose CSS errors, now RUST_LOG=layout::layout_task will be required instead.

Subsequent steps:

  • Add a PipelineId (from components/msg/constellation_msg.rs) member to ParserContext, to represent the source of parse errors that occur
  • Define a new message type in ConstellationControlMsg which contains all of the information necessary to report a CSS error (including the pipeline ID), and make this new error reporter communicate with the script thread by sending messages over a Sender<ConstellationControlMsg> value that can be obtained from the code in layout_task.rs.
  • Process the new message type in components/script/script_task.rs by:
    • caching each reported parse error in a vector in Document (components/script/dom/document.rs)
    • checking the devtools_wants_updates flag and sending it to the devtools server if it exists (see notify_devtools for a model in script_task.rs)
  • Retrieve any cached parse errors for a document on request in handle_get_cached_messages in components/scripts/devtools.rs
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