Show diff when file: state=link changes the symlink target (#22243)

* Show diff when file: state=link changes the symlink target

* Integration test for symlink target change

* Also check soft link idempotency

lib/ansible/modules/files/file.py

@@ -392,6 +392,8 @@ def main():
         elif prev_state == 'link':
             b_old_src = os.readlink(b_path)
             if b_old_src != b_src:
+                diff['before']['src'] = to_native(b_old_src, errors='strict')
+                diff['after']['src'] = src
                 changed = True
         elif prev_state == 'hard':
             if not (state == 'hard' and os.stat(b_path).st_ino == os.stat(b_src).st_ino):

test/integration/targets/file/tasks/main.yml

@@ -97,6 +97,26 @@
     that:
       - "file5_result.changed == true"
 
+- name: change soft link to relative
+  file: src={{output_file|basename}} dest={{output_dir}}/soft.txt state=link
+  register: file5a_result
+
+- name: verify that the file was marked as changed
+  assert:
+    that:
+      - "file5a_result.changed == true"
+      - "file5a_result.diff.before.src == output_file|expanduser"
+      - "file5a_result.diff.after.src == output_file|basename"
+
+- name: soft link idempotency check
+  file: src={{output_file|basename}} dest={{output_dir}}/soft.txt state=link
+  register: file5b_result
+
+- name: verify that the file was not marked as changed
+  assert:
+    that:
+      - "file5b_result.changed == false"
+
 - name: create hard link to file
   file: src={{output_file}} dest={{output_dir}}/hard.txt state=hard
   register: file6_result