Description of problem:
Customer can't query some resources due to an issue between LDAP field and OpenStack:
The investigation started with this error from customer (I'll post the log with the correct values on the next comment as private):
~~~
String length exceeded. The length of string 'AA-BBBBBB-CCCC-DDDDDDDDDDD-EEEEEEE-FFFFFFFFFFFFF-GGGGGGGGGGGGG-HH' exceeds the limit of column local_id(CHAR(64)). (HTTP 400) (Request-ID: req-aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee)
~~~

From an upstream bug[1] we had the following solution:
~~~
The workaround for this issue is to not use objectGUID as the user or group ID. However, that workaround might not be applicable in all situations. For example, the default value for user_id_attribute is 'cn', but if that value spans more than 64 characters, keystone can't work with it.
~~~

But for security reasons, customer can't change the field mapped.

So the other solution for this could be changing the value of max_param_size on keystone configuration[2]:
~~~
[DEFAULT]
[...]
# Limit the sizes of user & project ID/names. (integer value)
#max_param_size = 64
~~~

But this parameter is not reflecting on the actual limit size, I actually couldn't find where this is being used on the code;
Using the default value (64) I was able to create a user with more than 64 characters (I don't know how to change the ID, but since the parameter is supposed to limit both ID and name, testing it just with the name should work):
~~~
[root@controller-0 ~]# grep max_param_size /var/lib/config-data/keystone/etc/keystone/keystone.conf 
#max_param_size = 64
# Similar to `[DEFAULT] max_param_size`, but provides an exception for token
[root@controller-0 ~]# 

=====

(overcloud) [stack@undercloud-0 ~]$ openstack user create aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggg # 64 characters
No password was supplied, authentication will fail when a user does not have a password.
+---------------------+------------------------------------------------------------------+
| Field               | Value                                                            |
+---------------------+------------------------------------------------------------------+
| domain_id           | default                                                          |
| enabled             | True                                                             |
| id                  | 599b18e330bb4b1886f996cc35b169c1                                 |
| name                | aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggg |
| options             | {}                                                               |
| password_expires_at | None                                                             |
+---------------------+------------------------------------------------------------------+
(overcloud) [stack@undercloud-0 ~]$ 

(overcloud) [stack@undercloud-0 ~]$ openstack user create aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggghh # 66 characters
No password was supplied, authentication will fail when a user does not have a password.
+---------------------+--------------------------------------------------------------------+
| Field               | Value                                                              |
+---------------------+--------------------------------------------------------------------+
| domain_id           | default                                                            |
| enabled             | True                                                               |
| id                  | c22eab2648624fad82d6fd925962445d                                   |
| name                | aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggghh |
| options             | {}                                                                 |
| password_expires_at | None                                                               |
+---------------------+--------------------------------------------------------------------+


(overcloud) [stack@undercloud-0 ~]$ openstack user list
+----------------------------------+--------------------------------------------------------------------+
| ID                               | Name                                                               |
+----------------------------------+--------------------------------------------------------------------+
| 0a4036ae92084c098575141da51f9d98 | neutron                                                            |
| 44864fdba2f2413fb05a427da7da8535 | gnocchi                                                            |
| 599b18e330bb4b1886f996cc35b169c1 | aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggg   |
| 5df8a2b9040648d1be5df64c6c3a8926 | nova                                                               |
| 6b630377adea4941bd61a16f72fcb25f | ceilometer                                                         |
| 7873d0a3da6b464688112a34874e6125 | swift                                                              |
| 78c417ac4eaf4ce88f4ec47507477307 | panko                                                              |
| 9858f21827c3415aa8f654ae8dd574a4 | placement                                                          |
| a179350bbbcd4581b48a6198be00d007 | admin                                                              |
| a6f6f94a922f4888b50a8fd1e2b5baf1 | heat                                                               |
| aa117d550d14449bbe8366401998ccde | aodh                                                               |
| ae5b2042d8b245fdbf4f679cd0a73429 | cinder                                                             |
| b838d04566974fa584040a08aa066d3c | heat_stack_domain_admin                                            |
| c22eab2648624fad82d6fd925962445d | aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeffffffffffgggghh |
| e477e3e4aa8a4a9899c9083b23aee855 | glance                                                             |
| f003b1c512fa45b388811a0970b6f276 | heat-cfn                                                           |
+----------------------------------+--------------------------------------------------------------------+
(overcloud) [stack@undercloud-0 ~]$ 
~~~

Looking at the code (For the customer issue), I believe this is being hardcoded[3]:
~~~
class IDMapping(sql.ModelBase, sql.ModelDictMixin):
    __tablename__ = 'id_mapping'
    [...]
    local_id = sql.Column(sql.String(64), nullable=False)
~~~

My question is, do we have any solution for that? Could we increase this value safely?
Also, why the user name and the id are being treated differently? Since I can create a user exceeding the name length limit, but not the id.


[1] https://bugs.launchpad.net/keystone/+bug/1889936/comments/1
[2] https://docs.openstack.org/keystone/queens/configuration/samples/keystone-conf.html
[3] https://github.com/openstack/keystone/blob/stable/queens/keystone/identity/mapping_backends/sql.py#L24


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