async.diff

diff --git a/toolkit/mozapps/extensions/internal/XPIProviderUtils.js b/toolkit/mozapps/extensions/internal/XPIProviderUtils.js
--- a/toolkit/mozapps/extensions/internal/XPIProviderUtils.js
+++ b/toolkit/mozapps/extensions/internal/XPIProviderUtils.js
@@ -614,24 +614,21 @@ this.XPIDatabase = {
       else {
         this.rebuildUnreadableDB(e, aRebuildOnError);
       }
     }
     finally {
       if (fstream)
         fstream.close();
     }
-    // If an async load was also in progress, resolve that promise with our DB;
-    // otherwise create a resolved promise
+    // If an async load was also in progress, record in telemetry.
     if (this._dbPromise) {
       AddonManagerPrivate.recordSimpleMeasure("XPIDB_overlapped_load", 1);
-      this._dbPromise.resolve(this.addonDB);
     }
-    else
-      this._dbPromise = Promise.resolve(this.addonDB);
+    this._dbPromise = Promise.resolve(this.addonDB);
   },
 
   /**
    * Parse loaded data, reconstructing the database if the loaded data is not valid
    * @param aRebuildOnError
    *        If true, synchronously reconstruct the database from installed add-ons
    */
   parseDB: function(aData, aRebuildOnError) {
@@ -760,48 +757,48 @@ this.XPIDatabase = {
       return this._dbPromise;
     }
 
     logger.debug("Starting async load of XPI database " + this.jsonFile.path);
     AddonManagerPrivate.recordSimpleMeasure("XPIDB_async_load", XPIProvider.runPhase);
     let readOptions = {
       outExecutionDuration: 0
     };
-    return this._dbPromise = OS.File.read(this.jsonFile.path, null, readOptions).then(
-      byteArray => {
+    return this._dbPromise = new Promise((resolve, reject) => {
+      OS.File.read(this.jsonFile.path, null, readOptions).then(byteArray => {
         logger.debug("Async JSON file read took " + readOptions.outExecutionDuration + " MS");
         AddonManagerPrivate.recordSimpleMeasure("XPIDB_asyncRead_MS",
           readOptions.outExecutionDuration);
         if (this._addonDB) {
           logger.debug("Synchronous load completed while waiting for async load");
           return this.addonDB;
         }
         logger.debug("Finished async read of XPI database, parsing...");
         let decodeTimer = AddonManagerPrivate.simpleTimer("XPIDB_decode_MS");
         let decoder = new TextDecoder();
         let data = decoder.decode(byteArray);
         decodeTimer.done();
         this.parseDB(data, true);
-        return this.addonDB;
+        return resolve(this.addonDB);
       })
-    .then(null,
-      error => {
+      .then(null, error => {
         if (this._addonDB) {
           logger.debug("Synchronous load completed while waiting for async load");
-          return this.addonDB;
+          return reject(this.addonDB);
         }
         if (error.becauseNoSuchFile) {
           this.upgradeDB(true);
         }
         else {
           // it's there but unreadable
           this.rebuildUnreadableDB(error, true);
         }
-        return this.addonDB;
+        return resolve(this.addonDB);
       });
+    });
   },
 
   /**
    * Rebuild the database from addon install directories. If this.migrateData
    * is available, uses migrated information for settings on the addons found
    * during rebuild
    * @param aRebuildOnError
    *         A boolean indicating whether add-on information should be loaded