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diff --git a/devtools/shared/protocol/Front.js b/devtools/shared/protocol/Front.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+var { settleAll } = require("resource://devtools/shared/DevToolsUtils.js");
+var EventEmitter = require("resource://devtools/shared/event-emitter.js");
+
+var { Pool } = require("resource://devtools/shared/protocol/Pool.js");
+var {
+  getStack,
+  callFunctionWithAsyncStack,
+} = require("resource://devtools/shared/platform/stack.js");
+
+/**
+ * Base class for client-side actor fronts.
+ *
+ * @param [DevToolsClient|null] conn
+ *   The conn must either be DevToolsClient or null. Must have
+ *   addActorPool, removeActorPool, and poolFor.
+ *   conn can be null if the subclass provides a conn property.
+ * @param [Target|null] target
+ *   If we are instantiating a target-scoped front, this is a reference to the front's
+ *   Target instance, otherwise this is null.
+ * @param [Front|null] parentFront
+ *   The parent front. This is only available if the Front being initialized is a child
+ *   of a parent front.
+ * @constructor
+ */
+class Front extends Pool {
+  constructor(conn = null, targetFront = null, parentFront = null) {
+    super(conn);
+    if (!conn) {
+      throw new Error("Front without conn");
+    }
+    this.actorID = null;
+    // The targetFront attribute represents the debuggable context. Only target-scoped
+    // fronts and their children fronts will have the targetFront attribute set.
+    this.targetFront = targetFront;
+    // The parentFront attribute points to its parent front. Only children of
+    // target-scoped fronts will have the parentFront attribute set.
+    this.parentFront = parentFront;
+    this._requests = [];
+
+    // Front listener functions registered via `watchFronts`
+    this._frontCreationListeners = null;
+    this._frontDestructionListeners = null;
+
+    // List of optional listener for each event, that is processed immediatly on packet
+    // receival, before emitting event via EventEmitter on the Front.
+    // These listeners are register via Front.before function.
+    // Map(Event Name[string] => Event Listener[function])
+    this._beforeListeners = new Map();
+
+    // This flag allows to check if the `initialize` method has resolved.
+    // Used to avoid notifying about initialized fronts in `watchFronts`.
+    this._initializeResolved = false;
+  }
+
+  /**
+   * Return the parent front.
+   */
+  getParent() {
+    return this.parentFront && this.parentFront.actorID
+      ? this.parentFront
+      : null;
+  }
+
+  destroy() {
+    // Prevent destroying twice if a `forwardCancelling` event has already been received
+    // and already called `baseFrontClassDestroy`
+    this.baseFrontClassDestroy();
+
+    // Keep `clearEvents` out of baseFrontClassDestroy as we still want TargetMixin to be
+    // able to emit `target-destroyed` after we called baseFrontClassDestroy from DevToolsClient.purgeRequests.
+    this.clearEvents();
+  }
+
+  // This method is also called from `DevToolsClient`, when a connector is destroyed
+  // and we should:
+  // - reject all pending request made to the remote process/target/thread.
+  // - avoid trying to do new request against this remote context.
+  // - unmanage this front, so that DevToolsClient.getFront no longer returns it.
+  //
+  // When a connector is destroyed a `forwardCancelling` RDP event is sent by the server.
+  // This is done in a distinct method from `destroy` in order to do all that immediately,
+  // even if `Front.destroy` is overloaded by an async method.
+  baseFrontClassDestroy() {
+    // Reject all outstanding requests, they won't make sense after
+    // the front is destroyed.
+    while (this._requests.length) {
+      const { deferred, to, type, stack } = this._requests.shift();
+      // Note: many tests are ignoring `Connection closed` promise rejections,
+      // via PromiseTestUtils.allowMatchingRejectionsGlobally.
+      // Do not update the message without updating the tests.
+      const msg =
+        "Connection closed, pending request to " +
+        to +
+        ", type " +
+        type +
+        " failed" +
+        "\n\nRequest stack:\n" +
+        stack.formattedStack;
+      deferred.reject(new Error(msg));
+    }
+
+    if (this.actorID) {
+      super.destroy();
+      this.actorID = null;
+    }
+    this._isDestroyed = true;
+
+    this.targetFront = null;
+    this.parentFront = null;
+    this._frontCreationListeners = null;
+    this._frontDestructionListeners = null;
+    this._beforeListeners = null;
+  }
+
+  async manage(front, form, ctx) {
+    if (!front.actorID) {
+      throw new Error(
+        "Can't manage front without an actor ID.\n" +
+          "Ensure server supports " +
+          front.typeName +
+          "."
+      );
+    }
+
+    if (front.parentFront && front.parentFront !== this) {
+      throw new Error(
+        `${this.actorID} (${this.typeName}) can't manage ${front.actorID}
+        (${front.typeName}) since it has a different parentFront ${
+          front.parentFront
+            ? front.parentFront.actorID + "(" + front.parentFront.typeName + ")"
+            : "<no parentFront>"
+        }`
+      );
+    }
+
+    super.manage(front);
+
+    if (typeof front.initialize == "function") {
+      await front.initialize();
+    }
+    front._initializeResolved = true;
+
+    // Ensure calling form() *before* notifying about this front being just created.
+    // We exprect the front to be fully initialized, especially via its form attributes.
+    // But do that *after* calling manage() so that the front is already registered
+    // in Pools and can be fetched by its ID, in case a child actor, created in form()
+    // tries to get a reference to its parent via the actor ID.
+    if (form) {
+      front.form(form, ctx);
+    }
+
+    // Call listeners registered via `watchFronts` method
+    // (ignore if this front has been destroyed)
+    if (this._frontCreationListeners) {
+      this._frontCreationListeners.emit(front.typeName, front);
+    }
+  }
+
+  async unmanage(front) {
+    super.unmanage(front);
+
+    // Call listeners registered via `watchFronts` method
+    if (this._frontDestructionListeners) {
+      this._frontDestructionListeners.emit(front.typeName, front);
+    }
+  }
+
+  /*
+   * Listen for the creation and/or destruction of fronts matching one of the provided types.
+   *
+   * @param {String} typeName
+   *        Actor type to watch.
+   * @param {Function} onAvailable (optional)
+   *        Callback fired when a front has been just created or was already available.
+   *        The function is called with one arguments, the front.
+   * @param {Function} onDestroy (optional)
+   *        Callback fired in case of front destruction.
+   *        The function is called with the same argument than onAvailable.
+   */
+  watchFronts(typeName, onAvailable, onDestroy) {
+    if (this.isDestroyed()) {
+      // The front was already destroyed, bail out.
+      console.error(
+        `Tried to call watchFronts for the '${typeName}' type on an ` +
+          `already destroyed front '${this.typeName}'.`
+      );
+      return;
+    }
+
+    if (onAvailable) {
+      // First fire the callback on fronts with the correct type and which have
+      // been initialized. If initialize() is still in progress, the front will
+      // be emitted via _frontCreationListeners shortly after.
+      for (const front of this.poolChildren()) {
+        if (front.typeName == typeName && front._initializeResolved) {
+          onAvailable(front);
+        }
+      }
+
+      if (!this._frontCreationListeners) {
+        this._frontCreationListeners = new EventEmitter();
+      }
+      // Then register the callback for fronts instantiated in the future
+      this._frontCreationListeners.on(typeName, onAvailable);
+    }
+
+    if (onDestroy) {
+      if (!this._frontDestructionListeners) {
+        this._frontDestructionListeners = new EventEmitter();
+      }
+      this._frontDestructionListeners.on(typeName, onDestroy);
+    }
+  }
+
+  /**
+   * Stop listening for the creation and/or destruction of a given type of fronts.
+   * See `watchFronts()` for documentation of the arguments.
+   */
+  unwatchFronts(typeName, onAvailable, onDestroy) {
+    if (this.isDestroyed()) {
+      // The front was already destroyed, bail out.
+      console.error(
+        `Tried to call unwatchFronts for the '${typeName}' type on an ` +
+          `already destroyed front '${this.typeName}'.`
+      );
+      return;
+    }
+
+    if (onAvailable && this._frontCreationListeners) {
+      this._frontCreationListeners.off(typeName, onAvailable);
+    }
+    if (onDestroy && this._frontDestructionListeners) {
+      this._frontDestructionListeners.off(typeName, onDestroy);
+    }
+  }
+
+  /**
+   * Register an event listener that will be called immediately on packer receival.
+   * The given callback is going to be called before emitting the event via EventEmitter
+   * API on the Front. Event emitting will be delayed if the callback is async.
+   * Only one such listener can be registered per type of event.
+   *
+   * @param String type
+   *   Event emitted by the actor to intercept.
+   * @param Function callback
+   *   Function that will process the event.
+   */
+  before(type, callback) {
+    if (this._beforeListeners.has(type)) {
+      throw new Error(
+        `Can't register multiple before listeners for "${type}".`
+      );
+    }
+    this._beforeListeners.set(type, callback);
+  }
+
+  toString() {
+    return "[Front for " + this.typeName + "/" + this.actorID + "]";
+  }
+
+  /**
+   * Update the actor from its representation.
+   * Subclasses should override this.
+   */
+  form(form) {}
+
+  /**
+   * Send a packet on the connection.
+   */
+  send(packet) {
+    if (packet.to) {
+      this.conn._transport.send(packet);
+    } else {
+      packet.to = this.actorID;
+      // The connection might be closed during the promise resolution
+      if (this.conn && this.conn._transport) {
+        this.conn._transport.send(packet);
+      }
+    }
+  }
+
+  /**
+   * Send a two-way request on the connection.
+   */
+  request(packet) {
+    const deferred = Promise.withResolvers();
+    // Save packet basics for debugging
+    const { to, type } = packet;
+    this._requests.push({
+      deferred,
+      to: to || this.actorID,
+      type,
+      stack: getStack(),
+    });
+    this.send(packet);
+    return deferred.promise;
+  }
+
+  /**
+   * Handler for incoming packets from the client's actor.
+   */
+  onPacket(packet) {
+    if (this.isDestroyed()) {
+      // If the Front was already destroyed, all the requests have been purged
+      // and rejected with detailed error messages in baseFrontClassDestroy.
+      return;
+    }
+
+    // Pick off event packets
+    const type = packet.type || undefined;
+    if (this._clientSpec.events && this._clientSpec.events.has(type)) {
+      const event = this._clientSpec.events.get(packet.type);
+      let args;
+      try {
+        args = event.request.read(packet, this);
+      } catch (ex) {
+        console.error("Error reading event: " + packet.type);
+        console.exception(ex);
+        throw ex;
+      }
+      // Check for "pre event" callback to be processed before emitting events on fronts
+      // Use event.name instead of packet.type to use specific event name instead of RDP
+      // packet's type.
+      const beforeEvent = this._beforeListeners.get(event.name);
+      if (beforeEvent) {
+        const result = beforeEvent.apply(this, args);
+        // Check to see if the beforeEvent returned a promise -- if so,
+        // wait for their resolution before emitting. Otherwise, emit synchronously.
+        if (result && typeof result.then == "function") {
+          result.then(() => {
+            super.emit(event.name, ...args);
+            ChromeUtils.addProfilerMarker(
+              "DevTools:RDP Front",
+              null,
+              `${this.typeName}.${event.name}`
+            );
+          });
+          return;
+        }
+      }
+
+      super.emit(event.name, ...args);
+      ChromeUtils.addProfilerMarker(
+        "DevTools:RDP Front",
+        null,
+        `${this.typeName}.${event.name}`
+      );
+      return;
+    }
+
+    // Remaining packets must be responses.
+    if (this._requests.length === 0) {
+      const msg =
+        "Unexpected packet " + this.actorID + ", " + JSON.stringify(packet);
+      const err = Error(msg);
+      console.error(err);
+      throw err;
+    }
+
+    const { deferred, stack } = this._requests.shift();
+    callFunctionWithAsyncStack(
+      () => {
+        if (packet.error) {
+          let message;
+          if (packet.error && packet.message) {
+            message =
+              "Protocol error (" + packet.error + "): " + packet.message;
+          } else {
+            message = packet.error;
+          }
+          message += " from: " + this.actorID;
+          if (packet.fileName) {
+            const { fileName, columnNumber, lineNumber } = packet;
+            message += ` (${fileName}:${lineNumber}:${columnNumber})`;
+          }
+          const packetError = new Error(message);
+          deferred.reject(packetError);
+        } else {
+          deferred.resolve(packet);
+        }
+      },
+      stack,
+      "DevTools RDP"
+    );
+  }
+
+  hasRequests() {
+    return !!this._requests.length;
+  }
+
+  /**
+   * Wait for all current requests from this front to settle.  This is especially useful
+   * for tests and other utility environments that may not have events or mechanisms to
+   * await the completion of requests without this utility.
+   *
+   * @return Promise
+   *         Resolved when all requests have settled.
+   */
+  waitForRequestsToSettle() {
+    return settleAll(this._requests.map(({ deferred }) => deferred.promise));
+  }
+}
+
+exports.Front = Front;