Add required_if to AnsibleModule

There is a common pattern in modules where some parameters are required
only if another parameter is present AND set to a particular value. For
instance, if a cloud server state is "present" it's important to
indicate the image to be used, but if it's "absent", the image that was
used to launch it is not necessary. Provide a check that takes as an
input a list of 3-element tuples containing parameter to depend on, the
value it should be set to, and a list of parameters which are required
if the required parameter is set to the required value.

lib/ansible/module_utils/basic.py

@@ -337,7 +337,8 @@ class AnsibleModule(object):
 
     def __init__(self, argument_spec, bypass_checks=False, no_log=False,
         check_invalid_arguments=True, mutually_exclusive=None, required_together=None,
-        required_one_of=None, add_file_common_args=False, supports_check_mode=False):
+        required_one_of=None, add_file_common_args=False, supports_check_mode=False,
+        required_if=None):
 
         '''
         common code for quickly building an ansible module in Python
@@ -385,6 +386,7 @@ class AnsibleModule(object):
             self._check_argument_types()
             self._check_required_together(required_together)
             self._check_required_one_of(required_one_of)
+            self._check_required_if(required_if)
 
         self._set_defaults(pre=False)
         if not self.no_log:
@@ -958,6 +960,20 @@ class AnsibleModule(object):
         if len(missing) > 0:
             self.fail_json(msg="missing required arguments: %s" % ",".join(missing))
 
+    def _check_required_if(self, spec):
+        ''' ensure that parameters which conditionally required are present '''
+        if spec is None:
+            return
+        for (key, val, requirements) in spec:
+            missing = []
+            if key in self.params and self.params[key] == val:
+                for check in requirements:
+                    count = self._count_terms(check)
+                    if count == 0:
+                        missing.append(check)
+            if len(missing) > 0:
+                self.fail_json(msg="%s is %s but the following are missing: %s" % (key, val, ','.join(missing))
+
     def _check_argument_values(self):
         ''' ensure all arguments have the requested values, and there are no stray arguments '''
         for (k,v) in self.argument_spec.iteritems():