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diff --git a/comm/mailnews/base/src/FolderLookupService.jsm b/comm/mailnews/base/src/FolderLookupService.jsm
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+++ b/comm/mailnews/base/src/FolderLookupService.jsm
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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/. */
+
+/**
+ * This module implements the folder lookup service (nsIFolderLookupService).
+ */
+
+"use strict";
+
+var EXPORTED_SYMBOLS = ["FolderLookupService"];
+
+// This ensures that the service is only created once.
+var gCreated = false;
+
+/**
+ * FolderLookupService maintains an index of folders and provides
+ * lookup by folder URI.
+ *
+ * @class
+ */
+function FolderLookupService() {
+  if (gCreated) {
+    throw Components.Exception("", Cr.NS_ERROR_ALREADY_INITIALIZED);
+  }
+  this._map = new Map();
+  gCreated = true;
+}
+
+FolderLookupService.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["nsIFolderLookupService"]),
+
+  /**
+   * Fetch the folder corresponding to the given URI.
+   * Will only return folders which already exist and have a parent. If this
+   * not the case then null is returned.
+   *
+   * @param {string} uri - URI of folder to get.
+   * @returns {nsIMsgFolder|null}
+   */
+  getFolderForURL(uri) {
+    let folder = this._getExisting(uri);
+
+    if (folder && !this._isValidFolder(folder)) {
+      folder = null; // no dangling folders!
+    }
+    return folder;
+  },
+
+  /**
+   * Fetch the folder corresponding to the given URI, creating it if it does
+   * not exist. If the folder is created, it will be a "dangling" folder,
+   * without a parent and not part of a normal folder hierarchy.
+   * A lot of code relies on this behaviour, but for new code this
+   * call should probably be avoided.
+   *
+   * @param {string} uri - URI of folder to get.
+   * @returns {nsIMsgFolder}
+   */
+  getOrCreateFolderForURL(uri) {
+    let folder = this._getExisting(uri);
+    if (folder) {
+      return folder;
+    }
+
+    // Create new folder.
+
+    // Check that uri has an active scheme, in case this folder is from
+    // an extension that is currently disabled or hasn't started up yet.
+    let schemeMatch = uri.match(/^([-+.\w]+):/);
+    if (!schemeMatch) {
+      return null;
+    }
+    let scheme = schemeMatch[1];
+    let contractID = "@mozilla.org/mail/folder-factory;1?name=" + scheme;
+    if (!(contractID in Cc)) {
+      console.error(
+        "getOrCreateFolderForURL: factory not registered for " + uri
+      );
+      return null;
+    }
+
+    let factory = Components.manager.getClassObject(
+      Cc[contractID],
+      Ci.nsIFactory
+    );
+    if (!factory) {
+      console.error(
+        "getOrCreateFolderForURL: failed to get factory for " + uri
+      );
+      return null;
+    }
+
+    folder = factory.createInstance(Ci.nsIMsgFolder);
+    if (folder) {
+      folder.Init(uri);
+      // Add the new folder to our map. Store a weak reference instead, so that
+      // the folder can be closed when necessary.
+      let weakRef = folder
+        .QueryInterface(Ci.nsISupportsWeakReference)
+        .GetWeakReference();
+      this._map.set(uri, weakRef);
+    }
+
+    return folder;
+  },
+
+  /**
+   * Set pretty name again from original name on all folders,
+   * typically used when locale changes.
+   */
+  setPrettyNameFromOriginalAllFolders() {
+    for (const val of this._map.values()) {
+      try {
+        let folder = val.QueryReferent(Ci.nsIMsgFolder);
+        folder.setPrettyNameFromOriginal();
+      } catch (e) {}
+    }
+  },
+
+  // "private" stuff starts here.
+
+  /**
+   * Internal helper to find a folder (which may or may not be dangling).
+   *
+   * @param {string} uri - URI of folder to look up.
+   *
+   * @returns {nsIMsgFolder|null} - The folder, if in the index, else null.
+   */
+  _getExisting(uri) {
+    let folder = null;
+    // already created?
+    if (this._map.has(uri)) {
+      try {
+        folder = this._map.get(uri).QueryReferent(Ci.nsIMsgFolder);
+      } catch (e) {
+        // The object was deleted, so we can drop it.
+        this._map.delete(uri);
+      }
+    }
+    return folder;
+  },
+
+  /**
+   * Internal helper function to test if a folder is dangling or parented.
+   * Because we can return folders that don't exist, and we may be working
+   * with a deleted folder but we're still holding on to the reference. For
+   * valid folders, one of two scenarios is true: either the folder has a parent
+   * (the deletion code clears the parent to indicate its nonvalidity), or the
+   * folder is a root folder of some server. Getting the root folder may throw
+   * an exception if we attempted to create a server that doesn't exist, so we
+   * need to guard for that error.
+   *
+   * @returns {boolean} - true if folder valid (and parented).
+   */
+  _isValidFolder(folder) {
+    try {
+      return folder.parent != null || folder.rootFolder == folder;
+    } catch (e) {
+      return false;
+    }
+  },
+};