Ensure the yum cache has at most one writer

If the yum cache path exists and already mounted by someone,
do not attempt writing to it, use the overlay mode instead.

This still leaves a window of opportunity for another workers to
RW mount the cache after the ansible check has reported a stale
fact about there was no other mounts found. But this is unlikely
to happen.

Also, if it has to be retried in the rescue block, do not use the yum
cache for the maximum data safety and clean (a scratch) cache state
reasons.

This drastically reduces chances to have multiple writers for the
cache.

Closes-bug: #1860804

Change-Id: I19491a162e5bf6d6517fd343d675aff12bdc9719
Signed-off-by: Bogdan Dobrelya <bdobreli@redhat.com>

tasks/yum_update_buildah.yml

@@ -80,12 +80,18 @@
         setype: svirt_sandbox_file_t
       when: not yum_cache_stat.stat.exists|default()
 
+    - name: Check if the cache path has been already mounted
+      command: findmnt {{ yum_cache }}
+      register: findmnt_result
+      failed_when: false
+
     - name: Use the pre-populated non-empty cache as an overlay fs
       set_fact:
         cache_volume: "{{ yum_cache }}:{{ cache_path }}:O"
       when:
         - yum_cache_stat.stat.exists|default()
         - yum_cache_contents.stdout
+        - findmnt_result.rc != 0
 
     - name: Define the cache populating mode otherwise
       set_fact:
@@ -114,15 +120,12 @@
       register: result
 
   rescue:
-    - name: Run yum_update.sh (retry)
+    - name: Run yum_update.sh (retry without yum cache)
       command: >
         buildah --debug run
           --volume {{ yum_update.path }}:/tmp/yum_update.sh
           --volume {{ yum_repos_dir_path }}:/etc/yum.repos.d
           --volume /etc/pki:/etc/pki
-          {% if cache_volume is defined and cache_volume %}
-          --volume {{ cache_volume }}
-          {% endif %}
           {% for repo in file_repos.stdout_lines %}
             {% if repo is exists %}
               --volume {{ repo }}:{{ repo }}