`Unexpected Exception occurred at X: 'group_base'`

One user is now actively using our Indico installation based on

$ git log --no-decorate --oneline -1
c6b5a3042a Fix showing nonbookable period outside admin UI

Just now, the logger sent the log below:

```
Unexpected Exception occurred at platsch.molgen.mpg.de: 'group_base'

2022-06-30 18:10:35,693 cddddabd26354a15 2 indico.flask - ERROR errors.py:110 -- 'group_base'

Traceback (most recent call last):
File "/project/indico/home/.local/lib/python3.9/site-packages/flask/app.py", line 1523, in full_dispatch_request
rv = self.dispatch_request()
File "/project/indico/home/.local/lib/python3.9/site-packages/flask/app.py", line 1509, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/project/indico/home/.local/lib/python3.9/site-packages/indico/web/flask/util.py", line 79, in wrapper
return obj().process()
File "/project/indico/home/.local/lib/python3.9/site-packages/indico/web/rh.py", line 281, in process
res = self._do_process()
File "/project/indico/home/.local/lib/python3.9/site-packages/indico/web/rh.py", line 252, in _do_process
rv = self._process()
File "/project/indico/home/.local/lib/python3.9/site-packages/webargs/core.py", line 452, in wrapper
return func(*args, **kwargs)
File "/project/indico/home/.local/lib/python3.9/site-packages/indico/modules/groups/controllers.py", line 158, in _process
groups = GroupProxy.search(name, exact=exact)
File "/project/indico/home/.local/lib/python3.9/site-packages/indico/modules/groups/core.py", line 115, in search
result |= {GroupProxy(g.name, g.provider.name, _group=g)
File "/project/indico/home/.local/lib/python3.9/site-packages/indico/modules/groups/core.py", line 115, in
result |= {GroupProxy(g.name, g.provider.name, _group=g)
File "/project/indico/home/.local/lib/python3.9/site-packages/flask_multipass/core.py", line 437, in search_groups
yield from provider.search_groups(name, exact=exact)
File "/project/indico/home/.local/lib/python3.9/site-packages/flask_multipass/providers/ldap/providers.py", line 232, in search_groups
for group_dn, group_data in self._search_groups(search_filter):
File "/project/indico/home/.local/lib/python3.9/site-packages/flask_multipass/providers/ldap/providers.py", line 182, in _search_groups
return search(self.ldap_settings['group_base'], search_filter, attributes=[self.ldap_settings['gid']])
KeyError: 'group_base'

{'data': {'get': {'name': 'Organisators'},
'headers': {'Accept': 'application/json, text/plain, */*',
'Accept-Encoding': 'gzip, deflate, br',
'Accept-Language': 'fr-FR,fr;q=0.9',
'Cookie': 'XXX',
'Host': 'events.molgen.mpg.de',
'Referer': 'https://events.molgen.mpg.de/event/1/manage/protection',
'User-Agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X '
'10_15_7) AppleWebKit/605.1.15 (KHTML, '
'like Gecko) Version/15.5 Safari/605.1.15',
'X-Csrf-Token': '93165c3d-f87f-4e44-8215-66dfab63006e',
'X-Requested-With': 'XMLHttpRequest'},
'json': None,
'post': {},
'url': {}},
'endpoint': 'groups.group_search',
'id': 'cddddabd26354a15',
'ip': 'XXX'
'method': 'GET',
'referrer': 'https://events.molgen.mpg.de/event/1/manage/protection',
'rh': 'RHGroupSearch',
'time': '2022-06-30T18:10:35.819807',
'url': 'https://events.molgen.mpg.de/groups/api/search?name=Organisators',
'user': {'email': '[x@molgen.mpg.de](mailto:x@molgen.mpg.de)',
'id': 2,
'name': 'XXX'},
'user_agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) '
'AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.5 '
'Safari/605.1.15'}
```

We do not maintain a group field in LDAP, and configured it like below:

```
LOCAL_IDENTITIES = False

_ldap_config = {
'uri': 'ldaps://ldap.molgen.mpg.de',
'bind_dn': 'cn=indico,dc=ldap,dc=apps,dc=molgen,dc=mpg,dc=DE',
'bind_password': 'X',
'timeout': 30,
'verify_cert': True,
'page_size': 1500,

'uid': 'uid',
'user_base': 'dc=user,dc=apps,dc=molgen,dc=mpg,dc=de',
'user_filter': '(objectClass=molgenUser)',
}

AUTH_PROVIDERS = {
'ldap': {
'type': 'ldap',
'title': 'LDAP',
'ldap': _ldap_config,
'default': True
}
}

IDENTITY_PROVIDERS = {
'ldap': {
'type': 'ldap',
'title': 'LDAP',
'ldap': _ldap_config,
'mapping': {
'first_name': 'givenName',
'last_name': 'sn',
'email': 'mail',
},
'trusted_email': True,
'synced_fields': {'first_name', 'last_name'}
}
}
```

Start with indico/modules/groups/controllers.py and flask_multipass/providers/ldap/providers.py, especially the groups.group_search path and _search_groups. Review the supplied LDAP configuration alongside the failing traceback. Done means the reported group search no longer raises KeyError when group_base is not configured, with behavior covered by the relevant tests if available.

Written by the indexing model from the issue text.