gecko-dev/devtools/server/docs/actor-e10s-handling.md

How to handle E10S in actors

In multi-process environments, most devtools actors are created and initialized in the child content process, to be able to access the resources they are exposing to the toolbox. But sometimes, these actors need to access things in the parent process too. Here's why and how.

Use case and examples

Some actors need to exchange messages between the parent and the child process (typically when some components aren't available in the child process).

E.g. the director-manager needs to ask the list of installed director scripts from the director-registry running in the parent process.

To that end, there's a parent/child setup mechanism at DebuggerServer level that can be used.

When the actor is loaded for the first time in the DebuggerServer running in the child process, it may decide to run a setup procedure to load a module in the parent process with which to communicate.

E.g. in the director-registry:

  const {DebuggerServer} = require("devtools/server/main");

  // Setup the child<->parent communication only if the actor module
  // is running in a child process.
  if (DebuggerServer.isInChildProcess) {
    setupChildProcess();
  }

  function setupChildProcess() {
    // `setupInParent`  is defined on DebuggerServerConnection,
    // your actor receive a reference to one instance in its constructor.
    conn.setupInParent({
      module: "devtools/server/actors/director-registry",
      setupParent: "setupParentProcess"
    });
    // ...
  }

The setupChildProcess helper defined and used in the previous example uses the DebuggerServerConnection.setupInParent to run a given setup function in the parent process Debugger Server, e.g. in the director-registry module.

With this, the DebuggerServer running in the parent process will require the requested module (director-registry) and call its setupParentProcess function (which should be exported on the module).

The setupParentProcess function will receive a parameter that contains a reference to the MessageManager and a prefix that should be used to send/receive messages between the child and parent processes.

See below an example implementation of a setupParent function in the parent process:

let gTrackedMessageManager = new Set();
exports.setupParentProcess = function setupParentProcess({ mm, prefix }) {
  // Prevent multiple subscriptions on the same messagemanager.
  if (gTrackedMessageManager.has(mm)) { return; }
  gTrackedMessageManager.add(mm);

  // Start listening for messages from the actor in the child process.
  mm.addMessageListener("debug:some-message-name", handleChildRequest);

  function handleChildRequest(msg) {
    switch (msg.json.method) {
      case "get":
        return doGetInParentProcess(msg.json.args[0]);
        break;
      case "list":
        return doListInParentProcess();
        break;
      default:
        console.error("Unknown method name", msg.json.method);
        throw new Error("Unknown method name");
    }
  }

  // Listen to the disconnection message to clean-up.
  DebuggerServer.once("disconnected-from-child:" + prefix, handleMessageManagerDisconnected);

  function handleMessageManagerDisconnected(evt, { mm: disconnected_mm }) {
    // filter out not subscribed message managers
    if (disconnected_mm !== mm || !gTrackedMessageManager.has(mm)) {
      return;
    }

    gTrackedMessageManager.delete(mm);

    // unregister for director-script requests handlers from the parent process (if any)
    mm.removeMessageListener("debug:director-registry-request", handleChildRequest);
  }

The DebuggerServer emits "disconnected-from-child:PREFIX" events to give the actor modules the chance to cleanup their handlers registered on the disconnected message manager.

Summary of the setup flow

In the child process:

• The DebuggerServer loads an actor module,
• the actor module checks DebuggerServer.isInChildProcess to know whether it runs in a child process or not,
• the actor module then uses the DebuggerServerConnection.setupInParent helper to start setting up a parent-process counterpart,
• the DebuggerServerConnection.setupInParent helper asks the parent process to run the required module's setup function,
• the actor module uses the DebuggerServerConnection.parentMessageManager.sendSyncMessage and DebuggerServerConnection.parentMessageManager.addMessageListener helpers to send or listen to message.

In the parent process:

• The DebuggerServer receives the DebuggerServerConnection.setupInParent request,
• tries to load the required module,
• tries to call the module[setupParent] function with the frame message manager and the prefix as parameters { mm, prefix },
• the setupParent function then uses the mm to subscribe the messagemanager events,
• the setupParent function also uses the DebuggerServer object to subscribe once to the "disconnected-from-child:PREFIX" event to unsubscribe from messagemanager events.