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diff --git a/dom/console/ConsoleAPIStorage.sys.mjs b/dom/console/ConsoleAPIStorage.sys.mjs
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+++ b/dom/console/ConsoleAPIStorage.sys.mjs
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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/. */
+
+const STORAGE_MAX_EVENTS = 1000;
+
+var _consoleStorage = new Map();
+
+// NOTE: these listeners used to just be added as observers and notified via
+// Services.obs.notifyObservers. However, that has enough overhead to be a
+// problem for this. Using an explicit global array is much cheaper, and
+// should be equivalent.
+var _logEventListeners = [];
+
+const CONSOLEAPISTORAGE_CID = Components.ID(
+  "{96cf7855-dfa9-4c6d-8276-f9705b4890f2}"
+);
+
+/**
+ * The ConsoleAPIStorage is meant to cache window.console API calls for later
+ * reuse by other components when needed. For example, the Web Console code can
+ * display the cached messages when it opens for the active tab.
+ *
+ * ConsoleAPI messages are stored as they come from the ConsoleAPI code, with
+ * all their properties. They are kept around until the inner window object that
+ * created the messages is destroyed. Messages are indexed by the inner window
+ * ID.
+ *
+ * Usage:
+ *    let ConsoleAPIStorage = Cc["@mozilla.org/consoleAPI-storage;1"]
+ *                              .getService(Ci.nsIConsoleAPIStorage);
+ *
+ *    // Get the cached events array for the window you want (use the inner
+ *    // window ID).
+ *    let events = ConsoleAPIStorage.getEvents(innerWindowID);
+ *    events.forEach(function(event) { ... });
+ *
+ *    // Clear the events for the given inner window ID.
+ *    ConsoleAPIStorage.clearEvents(innerWindowID);
+ */
+export function ConsoleAPIStorageService() {
+  this.init();
+}
+
+ConsoleAPIStorageService.prototype = {
+  classID: CONSOLEAPISTORAGE_CID,
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIConsoleAPIStorage",
+    "nsIObserver",
+  ]),
+
+  observe: function CS_observe(aSubject, aTopic, aData) {
+    if (aTopic == "xpcom-shutdown") {
+      Services.obs.removeObserver(this, "xpcom-shutdown");
+      Services.obs.removeObserver(this, "inner-window-destroyed");
+      Services.obs.removeObserver(this, "memory-pressure");
+    } else if (aTopic == "inner-window-destroyed") {
+      let innerWindowID = aSubject.QueryInterface(Ci.nsISupportsPRUint64).data;
+      this.clearEvents(innerWindowID + "");
+    } else if (aTopic == "memory-pressure") {
+      this.clearEvents();
+    }
+  },
+
+  /** @private */
+  init: function CS_init() {
+    Services.obs.addObserver(this, "xpcom-shutdown");
+    Services.obs.addObserver(this, "inner-window-destroyed");
+    Services.obs.addObserver(this, "memory-pressure");
+  },
+
+  /**
+   * Get the events array by inner window ID or all events from all windows.
+   *
+   * @param string [aId]
+   *        Optional, the inner window ID for which you want to get the array of
+   *        cached events.
+   * @returns array
+   *          The array of cached events for the given window. If no |aId| is
+   *          given this function returns all of the cached events, from any
+   *          window.
+   */
+  getEvents: function CS_getEvents(aId) {
+    if (aId != null) {
+      return (_consoleStorage.get(aId) || []).slice(0);
+    }
+
+    let result = [];
+
+    for (let [, events] of _consoleStorage) {
+      result.push.apply(result, events);
+    }
+
+    return result.sort(function (a, b) {
+      return a.timeStamp - b.timeStamp;
+    });
+  },
+
+  /**
+   * Adds a listener to be notified of log events.
+   *
+   * @param jsval [aListener]
+   *        A JS listener which will be notified with the message object when
+   *        a log event occurs.
+   * @param nsIPrincipal [aPrincipal]
+   *        The principal of the listener - used to determine if we need to
+   *        clone the message before forwarding it.
+   */
+  addLogEventListener: function CS_addLogEventListener(aListener, aPrincipal) {
+    // If our listener has a less-privileged principal than us, then they won't
+    // be able to access the log event object which was populated for our
+    // scope. Accordingly we need to clone it for these listeners.
+    //
+    // XXX: AFAICT these listeners which we need to clone messages for are all
+    // tests. Alternative solutions are welcome.
+    const clone = !aPrincipal.subsumes(
+      Cc["@mozilla.org/systemprincipal;1"].createInstance(Ci.nsIPrincipal)
+    );
+    _logEventListeners.push({
+      callback: aListener,
+      clone,
+    });
+  },
+
+  /**
+   * Removes a listener added with `addLogEventListener`.
+   *
+   * @param jsval [aListener]
+   *        A JS listener which was added with `addLogEventListener`.
+   */
+  removeLogEventListener: function CS_removeLogEventListener(aListener) {
+    const index = _logEventListeners.findIndex(l => l.callback === aListener);
+    if (index != -1) {
+      _logEventListeners.splice(index, 1);
+    } else {
+      console.error(
+        "Attempted to remove a log event listener that does not exist."
+      );
+    }
+  },
+
+  /**
+   * Record an event associated with the given window ID.
+   *
+   * @param string aId
+   *        The ID of the inner window for which the event occurred or "jsm" for
+   *        messages logged from JavaScript modules..
+   * @param object aEvent
+   *        A JavaScript object you want to store.
+   */
+  recordEvent: function CS_recordEvent(aId, aEvent) {
+    if (!_consoleStorage.has(aId)) {
+      _consoleStorage.set(aId, []);
+    }
+
+    let storage = _consoleStorage.get(aId);
+
+    storage.push(aEvent);
+
+    // truncate
+    if (storage.length > STORAGE_MAX_EVENTS) {
+      storage.shift();
+    }
+
+    for (let { callback, clone } of _logEventListeners) {
+      try {
+        if (clone) {
+          callback(Cu.cloneInto(aEvent, callback));
+        } else {
+          callback(aEvent);
+        }
+      } catch (e) {
+        // A failing listener should not prevent from calling other listeners.
+        console.error(e);
+      }
+    }
+  },
+
+  /**
+   * Clear storage data for the given window.
+   *
+   * @param string [aId]
+   *        Optional, the inner window ID for which you want to clear the
+   *        messages. If this is not specified all of the cached messages are
+   *        cleared, from all window objects.
+   */
+  clearEvents: function CS_clearEvents(aId) {
+    if (aId != null) {
+      _consoleStorage.delete(aId);
+    } else {
+      _consoleStorage.clear();
+      Services.obs.notifyObservers(null, "console-storage-reset");
+    }
+  },
+};