forms: feed transient_formdata when computing field values for postconditions (#59399)

tests/form_pages/test_live.py

@@ -1218,6 +1218,7 @@ def test_dynamic_item_field_from_custom_view_on_cards(pub):
     # use custom view as source
     ds = {'type': 'carddef:%s:%s' % (carddef.url_name, custom_view.slug)}
     formdef.fields = [
+        fields.PageField(id='2', label='1st page', type='page'),
         fields.ItemField(id='0', type='item', label='item', varname='blah', items=['foo', 'bar', 'baz']),
         fields.ItemField(id='1', label='string', type='item', data_source=ds, display_disabled_items=True),
     ]
@@ -1237,12 +1238,13 @@ def test_dynamic_item_field_from_custom_view_on_cards(pub):
 
     resp.form['f1'] = resp.form['f1'].options[0][0]
     resp = resp.form.submit('submit')  # -> validation page
+    assert 'Technical error' not in resp.text
     resp = resp.form.submit('submit')  # -> submit
     assert formdef.data_class().select()[0].data['1'] in baz_ids
     assert formdef.data_class().select()[0].data['1_structured']['item'] == 'baz'
 
     # same in autocomplete mode
-    formdef.fields[1].display_mode = 'autocomplete'
+    formdef.fields[2].display_mode = 'autocomplete'
     formdef.store()
     app = get_app(pub)
     resp = app.get('/test/')

wcs/forms/root.py

@@ -1092,7 +1092,8 @@ class FormPage(Directory, FormTemplateMixin):
                 # variables.
                 form_data = copy.copy(session.get_by_magictoken(magictoken, {}))
                 try:
-                    data = self.formdef.get_data(form, raise_on_error=True)
+                    with get_publisher().substitutions.temporary_feed(transient_formdata, force_mode='lazy'):
+                        data = self.formdef.get_data(form, raise_on_error=True)
                 except SetValueError:
                     return self.page(
                         page,