Swap to UNLINK for Redis set cache

This uses UNLINK in Redis 4 to delete sets. This unlinks the key
from the value but actually deletes the value asynchronously.
With large sets this should give us a reasonable performance boost.

This commit also implements multi-key deletes for RepositorySetCache
methods in the Repository model, meaning only one UNLINK call will be
issued for all sets being expired.

changelogs/unreleased/unlink-cache-deletions.yml

+---
+title: Swap to UNLINK for Redis set cache
+merge_request: 27116
+author:
+type: performance

lib/gitlab/repository_cache_adapter.rb

@@ -237,7 +237,7 @@ module Gitlab
     end
 
     def expire_redis_set_method_caches(methods)
-      methods.each { |name| redis_set_cache.expire(name) }
+      redis_set_cache.expire(*methods)
     end
 
     def expire_redis_hash_method_caches(methods)

lib/gitlab/set_cache.rb

@@ -14,8 +14,16 @@ module Gitlab
       "#{key}:set"
     end
 
-    def expire(key)
-      with { |redis| redis.del(cache_key(key)) }
+    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
+      end
     end
 
     def exist?(key)

spec/lib/gitlab/repository_cache_adapter_spec.rb

@@ -211,8 +211,7 @@ describe Gitlab::RepositoryCacheAdapter do
     it 'expires the caches of the given methods' do
       expect(cache).to receive(:expire).with(:rendered_readme)
       expect(cache).to receive(:expire).with(:branch_names)
-      expect(redis_set_cache).to receive(:expire).with(:rendered_readme)
-      expect(redis_set_cache).to receive(:expire).with(:branch_names)
+      expect(redis_set_cache).to receive(:expire).with(:rendered_readme, :branch_names)
       expect(redis_hash_cache).to receive(:delete).with(:rendered_readme)
       expect(redis_hash_cache).to receive(:delete).with(:branch_names)
 

spec/lib/gitlab/repository_set_cache_spec.rb

@@ -51,15 +51,44 @@ describe Gitlab::RepositorySetCache, :clean_gitlab_redis_cache do
   end
 
   describe '#expire' do
-    it 'expires the given key from the cache' do
+    subject { cache.expire(*keys) }
+
+    before do
       cache.write(:foo, ['value'])
+      cache.write(:bar, ['value2'])
+    end
 
+    it 'actually wrote the values' do
       expect(cache.read(:foo)).to contain_exactly('value')
-      expect(cache.expire(:foo)).to eq(1)
+      expect(cache.read(:bar)).to contain_exactly('value2')
+    end
+
+    context 'single key' do
+      let(:keys) { %w(foo) }
+
+      it { is_expected.to eq(1) }
+
+      it 'deletes the given key from the cache' do
+        subject
+
         expect(cache.read(:foo)).to be_empty
       end
     end
 
+    context 'multiple keys' do
+      let(:keys) { %w(foo bar) }
+
+      it { is_expected.to eq(2) }
+
+      it 'deletes the given keys from the cache' do
+        subject
+
+        expect(cache.read(:foo)).to be_empty
+        expect(cache.read(:bar)).to be_empty
+      end
+    end
+  end
+
   describe '#exist?' do
     it 'checks whether the key exists' do
       expect(cache.exist?(:foo)).to be(false)