Force clearing of deprecated cache key

.

lib/gitlab/diff/highlight_cache.rb

@@ -54,6 +54,15 @@ module Gitlab
 
       private
 
+      # We create a Gitlab::Diff::DeprecatedHighlightCache here in order to
+      #   expire deprecated cache entries while we make the transition. This can
+      #   be removed when :hset_redis_diff_caching is fully launched.
+      # See https://gitlab.com/gitlab-org/gitlab/issues/38008
+      #
+      def deprecated_cache
+        @deprecated_cache ||= Gitlab::Diff::DeprecatedHighlightCache.new(@diff_collection)
+      end
+
       def uncached_files
         diff_files = @diff_collection.diff_files
 
@@ -85,6 +94,10 @@ module Gitlab
             redis.expire(key, EXPIRATION)
           end
         end
+
+        # Clean up any deprecated hash entries
+        #
+        deprecated_cache.clear
       end
 
       def file_paths

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -98,6 +98,16 @@ describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
       expect { cache.send(:write_to_redis_hash, diff_hash) }
         .to change { Gitlab::Redis::Cache.with { |r| r.hgetall(cache_key) } }
     end
+
+    # Note that this spec and the code it confirms can be removed when
+    #   :hset_redis_diff_caching is fully launched.
+    #
+    it 'attempts to clear deprecated cache entries' do
+      expect_any_instance_of(Gitlab::Diff::DeprecatedHighlightCache)
+        .to receive(:clear).and_call_original
+
+      cache.send(:write_to_redis_hash, diff_hash)
+    end
   end
 
   describe '#clear' do