[SSSD] Design document - Do not always override home directory with subdomain_homedir value in server mode

[Pavel Reichl]

Hello,

please see design page for simple - RFE: Do not always override home 
directory with subdomain_homedir value in server mode.

https://fedorahosted.org/sssd/wiki/DesignDocs/use_AD_homedir

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= Do not always override home directory with subdomain_homedir value in 
server mode =

Related ticket(s):
  * https://fedorahosted.org/sssd/ticket/2583

=== Problem statement ===
Prior to sssd 1.12, we didn't have the ability to read home directory 
values from AD in AD-IPA trust setups at all. Instead, we always used 
the `subdomain_homedir` value. We can read custom LDAP values now, but 
in order to stay backwards-compatible, we kept using the 
`subdomain_homedir` value.

=== Use cases ===
Users from AD with POSIX attributes want to use individually set value 
for home directory.

=== Overview of the solution ===
`subdomain_homedir` for SSSD in server mode should support '%o' template 
expansion (The original home directory retrieved from the identity 
provider). In case when `subdomain_homedir` would be expanded to an 
empty string ('subdomain_homedir=%o' and AD user without POSIX 
attributes) SSSD should not error out but `fallback_homedir` should be 
utilized instead.

=== Implementation details ===
* Extend set of attributes returned by `get_object_from_cache()` by 
SYSDB_HOMEDIR attribute.
* Update `apply_subdomain_homedir()`
  * to parse AD home dirrctory from ldb_msg
  * do not call `store_homedir_of_user()` if value of expanded home 
directory is an empty string.
* Extend interface of `get_subdomain_homedir_of_user()` to accept AD 
home directory as parameter.

=== Configuration changes ===
No configuration changes are proposed.

=== How To Test ===
1. On SSSD in server mode
  * For AD user with posix attributes set home directory attribute
   * in sssd.conf set `subdomain_homedir` option to '%o'
   * delete cache and restart SSSD
   * call `getent passwd user` and check that home directory reflects 
value from AD
  * For AD user `without` posix attributes
   * in sssd.conf set `subdomain_homedir` option to %o and 
`fallback_homedir` to /home/%u
   * delete cache and restart SSSD
   * call `getent passwd user` and check that home directory reflects 
`fallback_homedir`
2. On SSSD acting as IPA client
  * Check that results are the same as on SSSD in server mode and that 
local `fallback_homedir` is ignored

=== Authors ===
preichl(a)redhat.com