alternatives: Fix bug with priority default

If neigther the priority nor the subcommands where specified the module decided to update the priority with the default value anyway. This resulted in bug ansible-collections#4803 and ansible-collections#4804

plugins/modules/system/alternatives.py

@@ -40,9 +40,8 @@
     type: path
   priority:
     description:
-      - The priority of the alternative.
+      - The priority of the alternative. If no priority is given for creation 50 is used as a fallback.
     type: int
-    default: 50
   state:
     description:
       - C(present) - install the alternative (if not already installed), but do
@@ -171,9 +170,10 @@ def run(self):
         if self.mode_present:
             # Check if we need to (re)install
             subcommands_parameter = self.module.params['subcommands']
+            priority_parameter = self.module.params['priority']
             if (
                 self.path not in self.current_alternatives or
-                self.current_alternatives[self.path].get('priority') != self.priority or
+                (priority_parameter and self.current_alternatives[self.path].get('priority') != priority_parameter) or
                 (subcommands_parameter is not None and (
                     not all(s in subcommands_parameter for s in self.current_alternatives[self.path].get('subcommands')) or
                     not all(s in self.current_alternatives[self.path].get('subcommands') for s in subcommands_parameter)
@@ -273,7 +273,7 @@ def link(self):
 
     @property
     def priority(self):
-        return self.module.params.get('priority')
+        return self.module.params.get('priority') or self.current_alternatives.get(self.path, {}).get('priority') or 50
 
     @property
     def subcommands(self):
@@ -373,7 +373,7 @@ def main():
             name=dict(type='str', required=True),
             path=dict(type='path', required=True),
             link=dict(type='path'),
-            priority=dict(type='int', default=50),
+            priority=dict(type='int'),
             state=dict(
                 type='str',
                 choices=AlternativeState.to_list(),

tests/integration/targets/alternatives/tasks/test.yml

@@ -48,4 +48,4 @@
   when: ansible_os_family == 'RedHat' and not with_alternatives and item == 1
 
 - name: check that alternative has been updated
-  command: "grep -Pzq '/bin/dummy{{ item }}\\n50' '{{ alternatives_dir }}/dummy'"
+  command: "grep -Pzq '/bin/dummy{{ item }}\\n' '{{ alternatives_dir }}/dummy'"

tests/integration/targets/alternatives/tasks/tests_set_priority.yml

@@ -31,4 +31,16 @@
   register: alternative
 
 - name: check that alternative priority has been updated
-  command: "grep -Pzq '/bin/dummy{{ item }}\\n{{ 70 + item|int }}' '{{ alternatives_dir }}/dummy'"
+  command: "grep -Pzq '/bin/dummy{{ item }}\\n{{ 70 + item|int }}' '{{ alternatives_dir }}/dummy'"
+
+- name: no change without priority
+  alternatives:
+    name: dummy
+    path: '/usr/bin/dummy{{ item }}'
+    link: /usr/bin/dummy
+  register: alternative
+
+- name: check no change was triggerd without priority
+  assert:
+    that:
+      - 'alternative is not changed'