PERF: Reduce number of EXPIRE calls from CachedCounting (#15958)

Previously we were calling `EXPIRE` every time we incremented a given key. Instead, we can call EXPIRE once when the key is first populated. A LUA script is used to make this as efficient as possible.

Consumers of this Concern use daily keys. Since we're now calling EXPIRE only at the beginning of the day, rather than throughout the day, the expire time has been increased from 3 to 4 days.

app/models/concerns/cached_counting.rb

@@ -3,6 +3,18 @@
 module CachedCounting
   extend ActiveSupport::Concern
 
+  EXPIRE_CACHE_AFTER = 4.days.to_i
+
+  LUA_INCR_AND_EXPIRE = DiscourseRedis::EvalHelper.new <<~LUA
+    local result = redis.call("INCR", KEYS[1])
+
+    if result == 1 then
+      redis.call("EXPIRE", KEYS[1], ARGV[1])
+    end
+
+    return result
+  LUA
+
   included do
     class << self
       attr_accessor :autoflush, :autoflush_seconds, :last_flush
@@ -19,13 +31,16 @@ module CachedCounting
 
   class_methods do
     def perform_increment!(key, opts = nil)
-      val = Discourse.redis.incr(key).to_i
+      val = DiscourseRedis.ignore_readonly do
+        LUA_INCR_AND_EXPIRE.eval(
+          Discourse.redis.without_namespace,
+          [Discourse.redis.namespace_key(key)],
+          [EXPIRE_CACHE_AFTER]
+        ).to_i
+      end
 
-      # readonly mode it is going to be 0, skip
-      return if val == 0
-
-      # 3.days, see: https://github.com/rails/rails/issues/21296
-      Discourse.redis.expire(key, 259200)
+      # readonly mode it is going to be nil, skip
+      return if val.nil?
 
       autoflush = (opts && opts[:autoflush]) || self.autoflush
       if autoflush > 0 && val >= autoflush
@@ -45,7 +60,7 @@ module CachedCounting
     # this may seem a bit fancy but in so it allows
     # for concurrent calls without double counting
     def get_and_reset(key)
-      Discourse.redis.set(key, '0', ex: 259200, get: true).to_i
+      Discourse.redis.set(key, '0', ex: EXPIRE_CACHE_AFTER, get: true).to_i
     end
 
     def request_id(query_params, retries = 0)

spec/models/application_request_spec.rb

@@ -29,14 +29,16 @@ describe ApplicationRequest do
       inc(:http_total)
       inc(:http_total)
 
-      Discourse.redis.without_namespace.stubs(:incr).raises(Redis::CommandError.new("READONLY"))
+      Discourse.redis.without_namespace.stubs(:eval).raises(Redis::CommandError.new("READONLY"))
+      Discourse.redis.without_namespace.stubs(:evalsha).raises(Redis::CommandError.new("READONLY"))
       Discourse.redis.without_namespace.stubs(:set).raises(Redis::CommandError.new("READONLY"))
 
       # flush will be deferred no error raised
       inc(:http_total, autoflush: 3)
       ApplicationRequest.write_cache!
 
-      Discourse.redis.without_namespace.unstub(:incr)
+      Discourse.redis.without_namespace.unstub(:eval)
+      Discourse.redis.without_namespace.unstub(:evalsha)
       Discourse.redis.without_namespace.unstub(:set)
 
       inc(:http_total, autoflush: 3)

spec/models/web_crawler_request_spec.rb

@@ -31,13 +31,15 @@ describe WebCrawlerRequest do
     inc('Googlebot')
     inc('Googlebot')
 
-    Discourse.redis.without_namespace.stubs(:incr).raises(Redis::CommandError.new("READONLY"))
+    Discourse.redis.without_namespace.stubs(:eval).raises(Redis::CommandError.new("READONLY"))
+    Discourse.redis.without_namespace.stubs(:evalsha).raises(Redis::CommandError.new("READONLY"))
     Discourse.redis.without_namespace.stubs(:set).raises(Redis::CommandError.new("READONLY"))
 
     inc('Googlebot', autoflush: 3)
     WebCrawlerRequest.write_cache!
 
-    Discourse.redis.without_namespace.unstub(:incr)
+    Discourse.redis.without_namespace.unstub(:eval)
+    Discourse.redis.without_namespace.unstub(:evalsha)
     Discourse.redis.without_namespace.unstub(:set)
 
     inc('Googlebot', autoflush: 3)