PERF: introduce aggressive rate limiting for anonymous (#11129)

Previous to this change our anonymous rate limits acted as a throttle.
New implementation means we now also consider rate limited requests towards
the limit.

This means that if an anonymous user is hammering the server it will not be
able to get any requests through until it subsides with traffic.

lib/middleware/request_tracker.rb

@@ -241,7 +241,8 @@ class Middleware::RequestTracker
         "global_ip_limit_10_#{ip}",
         GlobalSetting.max_reqs_per_ip_per_10_seconds,
         10,
-        global: true
+        global: true,
+        aggressive: true
       )
 
       limiter60 = RateLimiter.new(
@@ -249,7 +250,8 @@ class Middleware::RequestTracker
         "global_ip_limit_60_#{ip}",
         GlobalSetting.max_reqs_per_ip_per_minute,
         60,
-        global: true
+        global: true,
+        aggressive: true
       )
 
       limiter_assets10 = RateLimiter.new(

lib/rate_limiter.rb

@@ -37,13 +37,14 @@ class RateLimiter
     "#{RateLimiter.key_prefix}:#{@user && @user.id}:#{type}"
   end
 
-  def initialize(user, type, max, secs, global: false)
+  def initialize(user, type, max, secs, global: false, aggressive: false)
     @user = user
     @type = type
     @key = build_key(type)
     @max = max
     @secs = secs
     @global = global
+    @aggressive = aggressive
   end
 
   def clear!
@@ -79,13 +80,38 @@ class RateLimiter
     PERFORM_LUA_SHA = Digest::SHA1.hexdigest(PERFORM_LUA)
   end
 
+  unless defined? PERFORM_LUA_AGGRESSIVE
+    PERFORM_LUA_AGGRESSIVE = <<~LUA
+      local now = tonumber(ARGV[1])
+      local secs = tonumber(ARGV[2])
+      local max = tonumber(ARGV[3])
+
+      local key = KEYS[1]
+
+      local return_val = 0
+
+      if ((tonumber(redis.call("LLEN", key)) < max) or
+          (now - tonumber(redis.call("LRANGE", key, -1, -1)[1])) > secs) then
+        return_val = 1
+      else
+        return_val = 0
+      end
+
+      redis.call("LPUSH", key, now)
+      redis.call("LTRIM", key, 0, max - 1)
+      redis.call("EXPIRE", key, secs * 2)
+
+      return return_val
+    LUA
+
+    PERFORM_LUA_AGGRESSIVE_SHA = Digest::SHA1.hexdigest(PERFORM_LUA_AGGRESSIVE)
+  end
+
   def performed!(raise_error: true)
     return true if rate_unlimited?
     now = Time.now.to_i
 
-    if ((max || 0) <= 0) ||
-       (eval_lua(PERFORM_LUA, PERFORM_LUA_SHA, [prefixed_key], [now, @secs, @max]) == 0)
-
+    if ((max || 0) <= 0) || rate_limiter_allowed?(now)
       raise RateLimiter::LimitExceeded.new(seconds_to_wait, @type) if raise_error
       false
     else
@@ -121,6 +147,20 @@ class RateLimiter
 
   private
 
+  def rate_limiter_allowed?(now)
+
+    lua, lua_sha = nil
+    if @aggressive
+      lua = PERFORM_LUA_AGGRESSIVE
+      lua_sha = PERFORM_LUA_AGGRESSIVE_SHA
+    else
+      lua = PERFORM_LUA
+      lua_sha = PERFORM_LUA_SHA
+    end
+
+    eval_lua(lua, lua_sha, [prefixed_key], [now, @secs, @max]) == 0
+  end
+
   def prefixed_key
     if @global
       "GLOBAL::#{key}"

spec/components/rate_limiter_spec.rb

@@ -38,6 +38,48 @@ describe RateLimiter do
       RateLimiter.disable
     end
 
+    context 'aggressive rate limiter' do
+
+      it 'can operate correctly and totally stop limiting' do
+
+        freeze_time
+
+        # 2 requests every 30 seconds
+        limiter = RateLimiter.new(nil, "test", 2, 30, global: true, aggressive: true)
+        limiter.clear!
+
+        limiter.performed!
+        limiter.performed!
+
+        freeze_time 29.seconds.from_now
+
+        expect do
+          limiter.performed!
+        end.to raise_error(RateLimiter::LimitExceeded)
+
+        expect do
+          limiter.performed!
+        end.to raise_error(RateLimiter::LimitExceeded)
+
+        # in aggressive mode both these ^^^ count as an attempt
+        freeze_time 29.seconds.from_now
+
+        expect do
+          limiter.performed!
+        end.to raise_error(RateLimiter::LimitExceeded)
+
+        expect do
+          limiter.performed!
+        end.to raise_error(RateLimiter::LimitExceeded)
+
+        freeze_time 31.seconds.from_now
+
+        limiter.performed!
+        limiter.performed!
+
+      end
+    end
+
     context 'global rate limiter' do
 
       it 'can operate in global mode' do