apt: better error msg when update_cache fails (#37410) (#40143)

(cherry picked from commit a6d31b6)

lib/ansible/modules/packaging/os/apt.py

@@ -979,14 +979,15 @@ def main():
             tdelta = datetime.timedelta(seconds=p['cache_valid_time'])
             if not mtimestamp + tdelta >= now:
                 # Retry to update the cache up to 3 times
+                err = ''
                 for retry in range(3):
                     try:
                         cache.update()
                         break
-                    except apt.cache.FetchFailedException:
-                        pass
+                    except apt.cache.FetchFailedException as e:
+                        err = to_native(e)
                 else:
-                    module.fail_json(msg='Failed to update apt cache.')
+                    module.fail_json(msg='Failed to update apt cache: %s' % err)
                 cache.open(progress=None)
                 updated_cache = True
                 mtimestamp, updated_cache_time = get_updated_cache_time()

test/integration/targets/apt/tasks/apt.yml

@@ -197,6 +197,29 @@
     - libcaca-dev
     - libslang2-dev
 
+# https://github.com/ansible/ansible/issues/23155
+- name: create a repo file
+  copy:
+    dest: /etc/apt/sources.list.d/non-existing.list
+    content: deb http://ppa.launchpad.net/non-existing trusty main
+
+- name: test for sane error message
+  apt:
+    update_cache: yes
+  register: apt_result
+  ignore_errors: yes
+
+- name: verify sane error message
+  assert:
+    that:
+      - "'E:Failed to fetch' in apt_result['msg']"
+      - "'403' in apt_result['msg']"
+
+- name: Clean up
+  file:
+    name: /etc/apt/sources.list.d/non-existing.list
+    state: absent
+
 - name: autoclean during install
   apt: pkg=hello state=present autoclean=yes