[SSSD] [sssd PR#475][opened] LDAP: Only add a sdap_domain instance for the current domain when instantiating a new ad_id_ctx

[jhrozek]

   URL: https://github.com/SSSD/sssd/pull/475
Author: jhrozek
 Title: #475: LDAP: Only add a sdap_domain instance for the current domain when
instantiating a new ad_id_ctx
Action: opened

PR body:
"""
NOTE: This fix doesn't address the segfault, only the condition that led
      to it. I would prefer to track the segfault and the search base issues
      separately

NOTE2: I didn't have much time to test this PR yet. I'm mostly submitting
       it to get feedback

Please see the full commit message below. I'm really confused about this
issue mostly because it seems we've had this bug for quite some time but did
not see it. I would be glad if somebody helps me understand if iterating
over all domains and adding all domains that are not yet present in the
ad_id_ctx->ad_options->sdap_id_ctx->sdom list is correct or not

The commit message follows:

Resolves: https://pagure.io/SSSD/sssd/issue/3594

Previously, sdap_domain_subdom_add() was called when a new ad_id_ctx was
being instantiated in the AD subdomains provider. The
sdap_domain_subdom_add() call iterates over all known subdomains and adds a
sdap_domain instance for every domain that is not present in an existing
sdap_domain list.

This is problematic for the AD subdomains provider e.g in this scenario
found by downstream ticket #3594:
   - there is a domain child1.sssdad.com the sssd is joined to
   - the subdomain provider auto-discovers ssdad_tree.com and
     sssdad.com, in this order (which is important). The list of
     sss_domain_info objects is updated in this order, too
   - for each domain, ad_subdom_ad_ctx_new() is called. This function
     creates a new ad_id_ctx and calls sdap_domain_subdom_add() to
     add an sdap_domain object into the sdap_id_ctx. The
     sdap_domain_subdom_add() call adds both domains to the list
     -- for the sssdad_tree subdomain is is ok, because subsequent
        calls only use the first sdap_domain object which is
        ssdad_tree.com (remember, order is important)
     -- for the sssdad.com domain, ssdad_tree.com is added first,
        which then causes all searches in the sssdad.com to have a
        search base from ssdad_tree.com

Because the sdap_domain instance in sdap_id_ctx should not be a list, but a
single domain, this patch adds a utility function that creates an
sdap_domain instance for a single subdomain instance.
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/475/head:pr475
git checkout pr475