Check for new Gerrit on disk caches when upgrading Gerrit

Update the Gerrit upgrade job to check for new on disk h2 cache files.
We discovered well after the fact that Gerrit 3.5 added new (large)
cache files to disk that would've been good to be aware of prior to the
upgrade. This change will check for new files and produce a message if
they exist.

Change-Id: I4b52f95dd4b23636c0360c9960d84bbed1a5b2d4

playbooks/zuul/upgrade-review.yaml

@@ -34,6 +34,8 @@
         cmd: docker-compose down
         chdir: /etc/gerrit-compose/
 
+    # This allows us to check that our config file isn't modified by newer
+    # Gerrit versions.
     - name: Backup config files
       block:
         - name: Find .config files
@@ -49,6 +51,14 @@
             remote_src: true
           loop: "{{ _config_files.files | map(attribute='path') | list }}"
 
+    # Record h2 cache files. We will use this to highlight any new caches
+    # under the new Gerrit version.
+    - name: Record Gerrit old cache files
+      find:
+        paths: /home/gerrit2/review_site/cache
+        patterns: '*.h2.db'
+      register: _old_cache_files
+
 - name: Perform gerrit upgrade
   import_playbook: ../service-review.yaml
   vars:
@@ -64,8 +74,28 @@
       loop: "{{ _config_files.files | map(attribute='path') | list }}"
       register: _diff_output
 
-    - name: Check diffs
+    - name: Check config diffs
       fail:
         msg: 'Difference detected in file {{ item.item }} '
       when: item.rc != 0
       loop: '{{ _diff_output.results }}'
+
+    - name: Record Gerrit new cache files
+      find:
+        paths: /home/gerrit2/review_site/cache
+        patterns: '*.h2.db'
+      register: _new_cache_files
+
+    - name: Manipulate find data for caches
+      set_fact:
+        _old_cache_paths: "{{ _old_cache_files.files | map(attribute='path') | list }}"
+        _new_cache_paths: "{{ _new_cache_files.files | map(attribute='path') | list }}"
+
+    - name: Find delta between cache listings
+      set_fact:
+        _gerrit_cache_difference: "{{ _old_cache_paths | symmetric_difference(_new_cache_paths) }}"
+
+    - name: Check for new cache files
+      debug:
+        msg: "The new Gerrit version produces new on disk caches: {{ _gerrit_cache_difference }}"
+      when: _gerrit_cache_difference | length > 0