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.

This documentation page is deprecated. setupInParent relies on the message manager which is being deprecated. Furthermore, communications between parent and content processes should be avoided for security reasons. If possible, the client should be responsible for calling actors both on the parent and content process.

This page will be removed when all actors relying on this API are removed.

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).

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

When the actor is loaded for the first time in the DevToolsServer 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.

Example code for the actor running in the child process:

  const {DevToolsServer} = require("devtools/server/devtools-server");

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

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

The setupChildProcess helper defined and used in the previous example uses the DevToolsServerConnection.setupInParent to run a given setup function in the parent process DevTools Server.

With this, the DevToolsServer running in the parent process will require the requested module 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:

exports.setupParentProcess = function setupParentProcess({ mm, prefix }) {
  // Start listening for messages from the actor in the child process.
  setMessageManager(mm);

  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");
    }
  }

  function setMessageManager(newMM) {
    if (mm) {
      // Remove listener from old message manager
      mm.removeMessageListener("debug:some-message-name", handleChildRequest);
    }
    // Switch to the new message manager for future use
    // Note: Make sure that any other functions also use the new reference.
    mm = newMM;
    if (mm) {
      // Add listener to new message manager
      mm.addMessageListener("debug:some-message-name", handleChildRequest);
    }
  }

  return {
    onDisconnected: () => setMessageManager(null),
  };
};

The server will call the onDisconnected method returned by the parent process setup flow 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 DevToolsServer loads an actor module,
  • the actor module checks DevToolsServer.isInChildProcess to know whether it runs in a child process or not,
  • the actor module then uses the DevToolsServerConnection.setupInParent helper to start setting up a parent-process counterpart,
  • the DevToolsServerConnection.setupInParent helper asks the parent process to run the required module's setup function,
  • the actor module uses the DevToolsServerConnection.parentMessageManager.sendSyncMessage and DevToolsServerConnection.parentMessageManager.addMessageListener helpers to send or listen to message.

In the parent process:

  • The DevToolsServer receives the DevToolsServerConnection.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 message manager events,
  • the setupParent function returns an object with a onDisconnected method which the server can use to notify the module of various lifecycle events

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