trusts: Detect missing Samba instance

In the event of invocation of trust related commands, IPA server needs to
contact local Samba instance. This is not possible on servers that
merely act as AD trust agents, since they do not have Samba instance
running.

Properly detect the absence of the Samba instance and output
user-friendly
message which includes list of servers that are capable of running
the command, if such exist.

List of commands affected:
* ipa trust-add
* ipa trust-fetch-domains
* all of the trustdomain commands available via CLI

https://fedorahosted.org/freeipa/ticket/5165

Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

ipalib/plugins/trust.py

@@ -199,6 +199,73 @@ def make_trust_dn(env, trust_type, dn):
         return DN(dn, container_dn)
     return dn
 
+def find_adtrust_masters(ldap, api):
+    """
+    Returns a list of names of IPA servers with ADTRUST component configured.
+    """
+
+    try:
+        entries, truncated = ldap.find_entries(
+                "cn=ADTRUST",
+                base_dn=api.env.container_masters + api.env.basedn
+        )
+    except errors.NotFound:
+        entries = []
+
+    return [entry.dn[1].value for entry in entries]
+
+def verify_samba_component_presence(ldap, api):
+    """
+    Verifies that Samba is installed and configured on this particular master.
+    If Samba is not available, provide a heplful hint with the list of masters
+    capable of running the commands.
+    """
+
+    adtrust_present = api.Command['adtrust_is_enabled']()['result']
+
+    hint = _(
+        ' Alternatively, following servers are capable of running this '
+        'command: %(masters)s'
+        )
+
+    def raise_missing_component_error(message):
+        masters_with_adtrust = find_adtrust_masters(ldap, api)
+
+        # If there are any masters capable of running Samba requiring commands
+        # let's advertise them directly
+        if masters_with_adtrust:
+            message += hint % dict(masters=', '.join(masters_with_adtrust))
+
+        raise errors.NotFound(
+            name=_('AD Trust setup'),
+            reason=message,
+        )
+
+    # We're ok in this case, bail out
+    if adtrust_present and _bindings_installed:
+        return
+
+    # First check for packages missing
+    elif not _bindings_installed:
+        error_message=_(
+            'Cannot perform the selected command without Samba 4 support '
+            'installed. Make sure you have installed server-trust-ad '
+            'sub-package of IPA.'
+        )
+
+        raise_missing_component_error(error_message)
+
+    # Packages present, but ADTRUST instance is not configured
+    elif not adtrust_present:
+        error_message=_(
+            'Cannot perform the selected command without Samba 4 instance '
+            'configured on this machine. Make sure you have run '
+            'ipa-adtrust-install on this server.'
+        )
+
+        raise_missing_component_error(error_message)
+
+
 def generate_creds(trustinstance, style, **options):
     """
     Generate string representing credentials using trust instance
@@ -554,6 +621,10 @@ sides.
     has_output_params = LDAPCreate.has_output_params + trust_output_params
 
     def execute(self, *keys, **options):
+        ldap = self.obj.backend
+
+        verify_samba_component_presence(ldap, self.api)
+
         full_join = self.validate_options(*keys, **options)
         old_range, range_name, dom_sid = self.validate_range(*keys, **options)
         result = self.execute_ad(full_join, *keys, **options)
@@ -569,7 +640,6 @@ sides.
             created_range_type = old_range['result']['iparangetype'][0]
 
         trust_filter = "cn=%s" % result['value']
-        ldap = self.obj.backend
         (trusts, truncated) = ldap.find_entries(
                          base_dn=DN(self.api.env.container_trusts, self.api.env.basedn),
                          filter=trust_filter)
@@ -642,16 +712,6 @@ sides.
     def validate_options(self, *keys, **options):
         trusted_realm_domain = keys[-1]
 
-        if not _bindings_installed:
-            raise errors.NotFound(
-                name=_('AD Trust setup'),
-                reason=_(
-                    'Cannot perform join operation without Samba 4 support '
-                    'installed. Make sure you have installed server-trust-ad '
-                    'sub-package of IPA'
-                )
-            )
-
         if not _murmur_installed and 'base_id' not in options:
             raise errors.ValidationError(
                 name=_('missing base_id'),
@@ -1406,6 +1466,9 @@ class trustdomain_del(LDAPDelete):
     msg_summary = _('Removed information about the trusted domain "%(value)s"')
 
     def execute(self, *keys, **options):
+        ldap = self.api.Backend.ldap2
+        verify_samba_component_presence(ldap, self.api)
+
         # Note that pre-/post- callback handling for LDAPDelete is causing pre_callback
         # to always receive empty keys. We need to catch the case when root domain is being deleted
 
@@ -1478,15 +1541,9 @@ class trust_fetch_domains(LDAPRetrieve):
     )
 
     def execute(self, *keys, **options):
-        if not _bindings_installed:
-            raise errors.NotFound(
-                name=_('AD Trust setup'),
-                reason=_(
-                    'Cannot perform join operation without Samba 4 support '
-                    'installed. Make sure you have installed server-trust-ad '
-                    'sub-package of IPA'
-                )
-            )
+        ldap = self.api.Backend.ldap2
+        verify_samba_component_presence(ldap, self.api)
+
         trust = self.api.Command.trust_show(keys[0], raw=True)['result']
 
         result = dict()
@@ -1532,6 +1589,7 @@ class trustdomain_enable(LDAPQuery):
 
     def execute(self, *keys, **options):
         ldap = self.api.Backend.ldap2
+        verify_samba_component_presence(ldap, self.api)
 
         if keys[0].lower() == keys[1].lower():
             raise errors.ValidationError(name='domain',
@@ -1572,6 +1630,7 @@ class trustdomain_disable(LDAPQuery):
 
     def execute(self, *keys, **options):
         ldap = self.api.Backend.ldap2
+        verify_samba_component_presence(ldap, self.api)
 
         if keys[0].lower() == keys[1].lower():
             raise errors.ValidationError(name='domain',