Backport cron module regression fix (#71244)

Change:
- Backport of #71207

Test Plan:
- CI

Tickets:
- Fixes #71213
- Fixes #71217
- Fixes #71216
- Fixes #71226
- Refs #71207

Signed-off-by: Rick Elrod <rick@elrod.me>

changelogs/fragments/fix-cron-file-regression.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - cron - cron file should not be empty after adding var (https://github.com/ansible/ansible/pull/71207)

lib/ansible/modules/system/cron.py

@@ -258,6 +258,7 @@ class CronTab(object):
             try:
                 f = open(self.b_cron_file, 'rb')
                 self.n_existing = to_native(f.read(), errors='surrogate_or_strict')
+                self.lines = self.n_existing.splitlines()
                 f.close()
             except IOError:
                 # cron file does not exist

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

@@ -100,6 +100,32 @@
 - assert:
     that: remove_cron_file is not changed
 
+- name: Non regression test - cron file should not be empty after adding var (#71207)
+  when: ansible_distribution != 'Alpine'
+  block:
+  - name: Cron file creation
+    cron:
+      cron_file: cron_filename
+      name: "simple cron job"
+      job: 'echo "_o/"'
+      user: root
+
+  - name: Add var to the cron file
+    cron:
+      cron_file: cron_filename
+      env: yes
+      name: FOO
+      value: bar
+      user: root
+
+  - name: "Ensure cron_file still contains job string"
+    replace:
+      path: /etc/cron.d/cron_filename
+      regexp: "_o/"
+      replace: "OK"
+    register: find_chars
+    failed_when: (find_chars is not changed) or (find_chars is failed)
+
 - name: Allow non-ascii chars in job (#69492)
   block:
   - name: Cron file creation