PERF: Use Redis `SET EX GET` instead of LUA script for counting (#15939)

This will prevent Discourse from booting on Redis < 6.2.0

app/models/concerns/cached_counting.rb

@@ -42,19 +42,10 @@ module CachedCounting
       raise NotImplementedError
     end
 
-    GET_AND_RESET = <<~LUA
-      local val = redis.call('get', KEYS[1])
-      redis.call('set', KEYS[1], '0')
-      return val
-    LUA
-
     # this may seem a bit fancy but in so it allows
     # for concurrent calls without double counting
     def get_and_reset(key)
-      namespaced_key = Discourse.redis.namespace_key(key)
-      val = Discourse.redis.without_namespace.eval(GET_AND_RESET, keys: [namespaced_key]).to_i
-      Discourse.redis.expire(key, 259200) # SET removes expiry, so set it again
-      val
+      Discourse.redis.set(key, '0', ex: 259200, get: true).to_i
     end
 
     def request_id(query_params, retries = 0)

config/initializers/001-redis.rb

@@ -8,3 +8,8 @@ end
 # Pending https://github.com/MiniProfiler/rack-mini-profiler/pull/450 and
 # upgrade to Sidekiq 6.1
 Redis.exists_returns_integer = true
+
+if Gem::Version.new(Discourse.redis.info['redis_version']) < Gem::Version.new("6.2.0")
+  STDERR.puts "Discourse requires Redis 6.2.0 or up"
+  exit 1
+end

spec/models/application_request_spec.rb

@@ -30,14 +30,14 @@ describe ApplicationRequest do
       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(: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(:set)
 
       inc(:http_total, autoflush: 3)
       expect(ApplicationRequest.http_total.first.count).to eq(3)

spec/models/web_crawler_request_spec.rb

@@ -32,13 +32,13 @@ describe WebCrawlerRequest do
     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(: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(:set)
 
     inc('Googlebot', autoflush: 3)
     expect(web_crawler_request('Googlebot').count).to eq(3)