Discovered with and applies to openstack-keystone-2013.1.1-1.el6ost.

When there are existing Trusts with Roles specified, the list-trusts request fails with 500 Internal Server Error.

When there are Trusts but non of them has the Role(s) specified, or after they were deleted, listing of Trusts works correctly.

1) Create Role/User/Project and add that role to the user on that project
> keystone tenant-create/user-create ...
> keystone role-create --name testRole
> keystone user-role-add --user yourUser --role testRole --tenant yourProject

2) Create Trust with a Role(s) specified with request like this:
> localhost:5000 POST /v3/OS-TRUST/trusts
> {"trust":
>  {"impersonation":false,
>   "project_id":"<your-project-id>",
>   "trustor_user_id":"<your-user-id>",
>   "trustee_user_id":"<other-user-id>",
>   "roles":[{"name":"testRole"}]
>   }}

3) List Trusts
> localhost:5000 GET /v3/OS-TRUST/trusts
which ends with 500 error instead of response with list of Trusts:
> reply: 'HTTP/1.1 500 Internal Server Error\r\n'
> header: Vary: X-Auth-Token
> header: Content-Type: application/json
> header: Content-Length: 148
> header: Date: Wed, 22 May 2013 15:30:33 GMT
> Reply body:
> {'error': {'code': 500,
>            'message': "An unexpected error prevented the server from
>                        fulfilling your request. 'id'",
>            'title': 'Internal Server Error'}}

In the keystone.log there is following backtrace after such request:
> 2013-05-22 17:30:33    ERROR [root] 'id'
> Traceback (most recent call last):
>  File "/usr/lib/python2.6/site-packages/keystone/common/wsgi.py", line 236, in __call__
>    result = method(context, **params)
>  File "/usr/lib/python2.6/site-packages/keystone/common/controller.py", line 104, in wrapper
>    return f(self, context, **kwargs)
>  File "/usr/lib/python2.6/site-packages/keystone/trust/controllers.py", line 181, in list_trusts
>    self._fill_in_roles(context, trust, global_roles)
>  File "/usr/lib/python2.6/site-packages/keystone/trust/controllers.py", line 76, in _fill_in_roles
>    if x['id'] == trust_role['id']]
> KeyError: 'id'