GUACAMOLE-128: Do not allow overlapping clipboard read attempts.

guacamole/src/main/webapp/app/clipboard/services/clipboardService.js

@@ -44,6 +44,14 @@ angular.module('clipboard').factory('clipboardService', ['$injector',
      */
     var CLIPBOARD_READ_DELAY = 100;
 
+    /**
+     * The promise associated with the current pending clipboard read attempt.
+     * If no clipboard read is active, this will be null.
+     *
+     * @type Promise.<ClipboardData>
+     */
+    var pendingRead = null;
+
     /**
      * Reference to the window.document object.
      *
@@ -398,8 +406,16 @@ angular.module('clipboard').factory('clipboardService', ['$injector',
      */
     service.getLocalClipboard = function getLocalClipboard() {
 
+        // If the clipboard is already being read, do not overlap the read
+        // attempts; instead share the result across all requests
+        if (pendingRead)
+            return pendingRead;
+
         var deferred = $q.defer();
 
+        // Mark read attempt as in progress
+        pendingRead = deferred.promise;
+
         // Wait for the next event queue run before attempting to read
         // clipboard data (in case the copy/cut has not yet completed)
         $window.setTimeout(function deferredClipboardRead() {
@@ -467,6 +483,9 @@ angular.module('clipboard').factory('clipboardService', ['$injector',
                 originalElement.focus();
                 popSelection();
 
+                // No read is pending any longer
+                pendingRead = null;
+
             });
 
             // Ensure clipboard element is blurred (and that the "focus" event