Prevent group wiki writes on read-only DBs

With Geo, not having this prevents all group wikis from working,
due to the DB load balancer always being enabled, this gets marked
as a primary, and we attempt to stick to the primary, writeable DB,
which does not exist in the Geo case.

Changelog: fixed
EE: true

ee/app/models/group_wiki.rb

@@ -12,6 +12,8 @@ class GroupWiki < Wiki
   end
 
   def track_wiki_repository(shard)
+    return unless ::Gitlab::Database.read_write?
+
     storage_record = container.group_wiki_repository || container.build_group_wiki_repository
     storage_record.update!(shard_name: shard, disk_path: storage.disk_path)
   end

ee/spec/models/group_wiki_spec.rb

@@ -47,6 +47,16 @@ RSpec.describe GroupWiki do
             shard_name: shard
           )
         end
+
+        context 'on a read-only instance' do
+          before do
+            allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+          end
+
+          it 'does not attempt to create a new entry' do
+            expect { subject.track_wiki_repository(shard) }.not_to change(wiki_container, :group_wiki_repository)
+          end
+        end
       end
 
       context 'when a tracking entry exists' do
@@ -64,6 +74,23 @@ RSpec.describe GroupWiki do
             shard_name: shard
           )
         end
+
+        context 'on a read-only instance' do
+          before do
+            allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+          end
+
+          it 'does not update the storage location' do
+            allow(subject.storage).to receive(:disk_path).and_return('fancy/new/path')
+
+            subject.track_wiki_repository(shard)
+
+            expect(wiki_container.group_wiki_repository).not_to have_attributes(
+              disk_path: 'fancy/new/path',
+              shard_name: shard
+            )
+          end
+        end
       end
     end