[FIX] web: do not create new line if onchange has warning in o2m

In a one2many field, when the user clicks on the 'Add a line' button, we
need to create a new record.  This means that we call the default_get
route, then call the onchanges.  The onchange can return a 'warning'
key, which is displayed to the user.  But in the specific case of a
one2many, we don't want to actually create the record.  So, we reject the
deferred to prevent that from happening.

addons/web/static/src/js/views/basic/basic_model.js

@@ -770,6 +770,7 @@ var BasicModel = AbstractModel.extend({
             var onchangeDef;
             if (onChangeFields.length) {
                 onchangeDef = self._applyOnChange(record, onChangeFields).then(function (result) {
+                    delete record._warning;
                     return _.keys(changes).concat(Object.keys(result && result.value || {}));
                 });
             } else {
@@ -884,6 +885,7 @@ var BasicModel = AbstractModel.extend({
                         title: result.warning.title,
                         type: 'dialog',
                     });
+                    record._warning = true;
                 }
                 var defs = [];
                 _.each(result.value, function (val, name) {
@@ -2045,7 +2047,11 @@ var BasicModel = AbstractModel.extend({
                             }
                         }
                         if (shouldApplyOnchange) {
-                            return self._applyOnChange(record, fields_key);
+                            return self._applyOnChange(record, fields_key).then(function () {
+                                if (record._warning) {
+                                    return $.Deferred().reject();
+                                }
+                            });
                         } else {
                             return $.when();
                         }

addons/web/static/test/views/form_tests.js

@@ -1868,6 +1868,50 @@ QUnit.module('Views', {
         form.destroy();
     });
 
+    QUnit.test('do nothing if add a line in one2many result in a onchange with a warning', function (assert) {
+        assert.expect(2);
+
+        this.data.partner.onchanges = { foo: true };
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form string="Partners">' +
+                    '<field name="p">' +
+                        '<tree editable="top">' +
+                            '<field name="foo"/>' +
+                        '</tree>' +
+                    '</field>' +
+                '</form>',
+            res_id: 2,
+            mockRPC: function (route, args) {
+                if (args.method === 'onchange') {
+                    return $.when({
+                        value: {},
+                        warning: {
+                            title: "Warning",
+                            message: "You must first select a partner"
+                        }
+                    });
+                }
+                return this._super.apply(this, arguments);
+            },
+            intercepts: {
+                warning: function () {
+                    assert.step("should have triggered a warning");
+                },
+            },
+        });
+
+        // go to edit mode, click to add a record in the o2m
+        form.$buttons.find('.o_form_button_edit').click();
+        form.$('.o_form_field_x2many_list_row_add a').click();
+        assert.strictEqual(form.$('tr.o_data_row').length, 0,
+            "should not have added a line");
+        form.destroy();
+    });
+
     QUnit.test('attrs are properly transmitted to new records', function (assert) {
         assert.expect(2);