[IMP] web: allow js code to abort xhr

We have a need to be able to abort a pending xmlhttprequest.  This is
necessary for the longpoll system so it can restart properly.

With this commit, we simply propagate the abort method from the jquery
deferred to the deferred returned by the session rpc method.

Note that this is not documented on purpose. The reason is that we do
not want to commit on maintaining this feature.

It is kind of awkward to have a deferred which can be aborted, but once
it is chained, the abort feature is lost. So, for now, this will stay a
lower level feature used exclusively for our needs.

addons/web/static/src/js/core/session.js

@@ -335,7 +335,15 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
 
         delete options.shadow;
 
-        return self.check_session_id().then(function () {
+        var deferred = $.Deferred();
+        var aborted = false;
+        deferred.abort = function () {
+            aborted = true;
+        };
+        self.check_session_id().then(function () {
+            if (aborted) {
+                return deferred.reject('communication', $.Event(), 'abort', 'abort');
+            }
             // TODO: remove
             if (! _.isString(url)) {
                 _.extend(options, url);
@@ -363,10 +371,11 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
                 options.session_id = self.session_id || '';
             }
             var p = fct(url, "call", params, options);
+            deferred.abort = p.abort.bind(p); // Allow to abort the rpc call
             p = p.then(function (result) {
                 if (! shadow)
                     self.trigger('response');
-                return result;
+                return deferred.resolve(result);
             }, function (type, error, textStatus, errorThrown) {
                 if (type === "server") {
                     if (! shadow)
@@ -374,7 +383,7 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
                     if (error.code === 100) {
                         self.uid = false;
                     }
-                    return $.Deferred().reject(error, $.Event());
+                    return deferred.reject(error, $.Event());
                 } else {
                     if (! shadow)
                         self.trigger('response_failed');
@@ -383,17 +392,18 @@ var Session = core.Class.extend(mixins.EventDispatcherMixin, {
                         message: "XmlHttpRequestError " + errorThrown,
                         data: {type: "xhr"+textStatus, debug: error.responseText, objects: [error, errorThrown] }
                     };
-                    return $.Deferred().reject(nerror, $.Event());
+                    return deferred.reject(nerror, $.Event());
                 }
             });
-            return p.fail(function () { // Allow deferred user to disable rpc_error call in fail
-                p.fail(function (error, event) {
+            return deferred.fail(function () { // Allow deferred user to disable rpc_error call in fail
+                deferred.fail(function (error, event) {
                     if (!event.isDefaultPrevented()) {
                         self.trigger('error', error, event);
                     }
                 });
             });
         });
+        return deferred;
     },
     url: function (path, params) {
         params = _.extend(params || {});