FEATURE: Reintroduce better thread reply counter cache (#21197)

This was reverted in 38cebd3ed509524ad635adb107163d0496d0c550.
The issue was that I was using Discourse.redis.delete_prefixed
which does a slow redis KEYS lookup, which is not advised in
production. This commit removes that, and also ensures the periodical
thread count update only happens if threading is enabled.

I changed to use a redis INCR/DECR for reply count
cache. This avoids a round trip to redis to GET the current
count, and also avoids multi-process issues, where
if there's two processes trying to increment at the
same time, they may both receive the same value, add one
to it, then both write the same value back.
Then, it's only n+1 instead of n+2.

This also prevents almost all chat scheduled jobs from
running if chat is disabled, the only one remaining is
the message retention job.

plugins/chat/spec/models/chat/thread_spec.rb

@@ -1,6 +1,11 @@
 # frozen_string_literal: true
 
 RSpec.describe Chat::Thread do
+  before do
+    SiteSetting.chat_enabled = true
+    SiteSetting.enable_experimental_chat_threaded_discussions = true
+  end
+
   describe ".ensure_consistency!" do
     fab!(:channel) { Fabricate(:category_channel) }
     fab!(:thread_1) { Fabricate(:chat_thread, channel: channel) }
@@ -40,6 +45,69 @@ RSpec.describe Chat::Thread do
         described_class.ensure_consistency!
         expect(thread_1.reload.replies_count).to eq(0)
       end
+
+      it "clears the affected replies_count caches" do
+        thread_1.set_replies_count_cache(100)
+        expect(thread_1.replies_count_cache).to eq(100)
+        expect(thread_1.replies_count_cache_updated_at).not_to eq(nil)
+
+        described_class.ensure_consistency!
+        expect(Discourse.redis.get(Chat::Thread.replies_count_cache_redis_key(thread_1.id))).to eq(
+          nil,
+        )
+        expect(
+          Discourse.redis.get(Chat::Thread.replies_count_cache_updated_at_redis_key(thread_1.id)),
+        ).to eq(nil)
+      end
+
+      it "does not attempt to clear caches if no replies_count caches are updated" do
+        described_class.ensure_consistency!
+        Chat::Thread.expects(:clear_caches!).never
+        described_class.ensure_consistency!
+      end
+
+      it "does nothing if threads are disabled" do
+        SiteSetting.enable_experimental_chat_threaded_discussions = false
+        Chat::Thread.expects(:update_counts).never
+        described_class.ensure_consistency!
+      end
+    end
+  end
+
+  describe ".clear_caches" do
+    fab!(:channel) { Fabricate(:category_channel) }
+    fab!(:thread_1) { Fabricate(:chat_thread, channel: channel) }
+    fab!(:thread_2) { Fabricate(:chat_thread, channel: channel) }
+
+    before do
+      thread_1.set_replies_count_cache(100)
+      thread_2.set_replies_count_cache(100)
+    end
+
+    it "clears multiple keys" do
+      Chat::Thread.clear_caches!([thread_1.id, thread_2.id])
+      expect(Discourse.redis.get(Chat::Thread.replies_count_cache_redis_key(thread_1.id))).to eq(
+        nil,
+      )
+      expect(
+        Discourse.redis.get(Chat::Thread.replies_count_cache_updated_at_redis_key(thread_1.id)),
+      ).to eq(nil)
+      expect(Discourse.redis.get(Chat::Thread.replies_count_cache_redis_key(thread_2.id))).to eq(
+        nil,
+      )
+      expect(
+        Discourse.redis.get(Chat::Thread.replies_count_cache_updated_at_redis_key(thread_2.id)),
+      ).to eq(nil)
+    end
+
+    it "wraps the ids into an array if only an integer is provided" do
+      Chat::Thread.clear_caches!(thread_1.id)
+      expect(Discourse.redis.get(Chat::Thread.replies_count_cache_redis_key(thread_1.id))).to eq(
+        nil,
+      )
+      expect(
+        Discourse.redis.get(Chat::Thread.replies_count_cache_updated_at_redis_key(thread_1.id)),
+      ).to eq(nil)
     end
   end