Gracefully fall back to DEL

If UNLINK is not supported by the Redis server, we should
fall back to DEL. UNLINK was introduced in Redis 4.

lib/gitlab/set_cache.rb

@@ -17,12 +17,7 @@ module Gitlab
     def expire(*keys)
       with do |redis|
         keys = keys.map { |key| cache_key(key) }
-
-        if Feature.enabled?(:repository_set_cache_unlink, default_enabled: true)
-          redis.unlink(*keys)
-        else
-          redis.delete(*keys)
-        end
+        unlink_or_delete(redis, *keys)
       end
     end
 
@@ -59,5 +54,15 @@ module Gitlab
     def with(&blk)
       Gitlab::Redis::Cache.with(&blk) # rubocop:disable CodeReuse/ActiveRecord
     end
+
+    def unlink_or_delete(redis, *keys)
+      if Feature.enabled?(:repository_set_cache_unlink, default_enabled: true)
+        redis.unlink(*keys)
+      else
+        redis.del(*keys)
+      end
+    rescue ::Redis::CommandError
+      redis.del(*keys)
+    end
   end
 end

spec/lib/gitlab/repository_set_cache_spec.rb

@@ -87,6 +87,17 @@ describe Gitlab::RepositorySetCache, :clean_gitlab_redis_cache do
         expect(cache.read(:bar)).to be_empty
       end
     end
+
+    context "unlink isn't supported" do
+      before do
+        allow_any_instance_of(Redis).to receive(:unlink) { raise ::Redis::CommandError }
+      end
+
+      it 'still deletes the given key' do
+        expect(cache.expire(:foo)).to eq(1)
+        expect(cache.read(:foo)).to be_empty
+      end
+    end
   end
 
   describe '#exist?' do