Background information: Major browsers supportcreating HTTP archive files which can be analyzed by other tools. Adding native support for this feature to Servo will allow us to compare our network performance more precisely against other browsers.

Tracking issue:https://github.com/servo/servo/issues/4004 (please ask questions here)

Initial steps:

• email themozilla.dev.servo mailing list (be sure to subscribe to it first!) introducing your group and asking any necessary questions
• add therust-har library as adependency of thenet_traits component
• define anenum that represents either a filename or anIpcSender that can send a vector ofhar::Log values
• using the new enum, add a new optional member toResourceChannelManager that is initialized via create_resource_threads
• modify theCoreResourceMsg::Exit message to contain an optional vector ofhar::Page values
• add acommand line flag that controls whether to store HAR data. The flag should accept a filename argument.
• using this new global option, pass an appropriate argument when callingnew_resource_threads increate_constellation
• when processing theCoreResourseMsg::Exit message, send an empty vector or write an empty vector as JSON to an appropriate file depending on the member variable that was added earlier

Subsequent steps:

• Store a vector ofhar::Page inConstellation
  • when a new pipeline is created (Constellation::new_pipeline), add an entry to this vector using the pipeline ID as the page's unique id
• Store a vector ofhar::Entry values inCoreResourceManager, wrapped insideArc andMutex types to allow other threads to manipulate the vector
  • add aArc<Mutex<Vec<har::Entry>>> member to theFetchContext structure, and initialize it from the resource manager's new field
  • inhttp_fetch, create a newhar::Request instance based on the values available from therequest variable
  • before step 5 ofhttp_fetch, create a newhar::Response value based on the values available from theresponse variable
  • create a newhar::Entry value and append it to the vector stored inFetchContext, using the request'spipeline_id field as thepageref value to associate it with the pages stored in theConstellation
  • duringConstellation::handle_shutdown as part of theCoreResourceMsg::Exit message send the vector ofhar::Page values
  • when the resource thread receives this message (CoreResourceManager::process_msg), if there is a vector present then combine the provided Page and member Entry values and process them according to the member variable (either sending them on the channel or writing them to a file)
• add a unit test to ensure that the resource thread logs the HAR information correctly:
  • create a resource manager with an IpcSender as the HAR output
  • start an HTTP server and fetch a URL from it by sending a CoreResourceMsg::Fetch message to the resource manager
  • Once the fetch is complete, send the exit message to the resource manager with a har::Page value for the single fetch (acting as the Constellation)
  • assert that the expected har::Log value is received from the resource thread