[Fix] Try harder to clean pending bucket

Issue: #4467

lualib/redis_scripts/ratelimit_cleanup_pending.lua

@@ -0,0 +1,29 @@
+-- This script cleans up the pending requests in Redis.
+
+-- KEYS: Input parameters
+-- KEYS[1] - prefix: The Redis key prefix used to store the bucket information.
+-- KEYS[2] - now: The current time in milliseconds.
+-- KEYS[3] - expire: The expiration time for the Redis key storing the bucket information, in seconds.
+-- KEYS[4] - number_of_recipients: The number of requests to be allowed (or the increase rate).
+
+-- 1. Retrieve the last hit time and initialize variables
+local prefix = KEYS[1]
+local last = redis.call('HGET', prefix, 'l')
+local nrcpt = tonumber(KEYS[4])
+if not last then
+  -- No bucket, no cleanup
+  return 0
+end
+
+
+-- 2. Update the pending values based on the number of recipients (requests)
+local pending = redis.call('HGET', prefix, 'p')
+pending = tonumber(pending or '0')
+if pending < nrcpt then pending = 0 else pending = pending - nrcpt end
+
+-- 3. Set the updated values back to Redis and update the expiration time for the bucket
+redis.call('HMSET', prefix, tostring(pending), 'l', KEYS[2])
+redis.call('EXPIRE', prefix, KEYS[3])
+
+-- 4. Return the updated pending value
+return pending

src/plugins/lua/ratelimit.lua

@@ -62,6 +62,9 @@ local bucket_check_id
 local bucket_update_script = "ratelimit_update.lua"
 local bucket_update_id
 
+local bucket_cleanup_script = "ratelimit_cleanup_pending.lua"
+local bucket_cleanup_id
+
 -- message_func(task, limit_type, prefix, bucket, limit_key)
 local message_func = function(_, limit_type, _, _, _)
   return string.format('Ratelimit "%s" exceeded', limit_type)
@@ -71,6 +74,7 @@ end
 local function load_scripts(_, _)
   bucket_check_id = lua_redis.load_redis_script_from_file(bucket_check_script, redis_params)
   bucket_update_id = lua_redis.load_redis_script_from_file(bucket_update_script, redis_params)
+  bucket_cleanup_id = lua_redis.load_redis_script_from_file(bucket_cleanup_script, redis_params)
 end
 
 local limit_parser
@@ -464,6 +468,7 @@ local function ratelimit_cb(task)
             prefix, bucket.burst, bucket.rate,
             data[2], data[3], data[4], data[5])
 
+        task:cache_set('ratelimit_bucket_touched', true)
         if data[1] == 1 then
           -- set symbol only and do NOT soft reject
           if bucket.symbol then
@@ -530,15 +535,57 @@ local function ratelimit_cb(task)
   end
 end
 
+
+-- This function is used to clean up pending bucket when
+-- the task is somehow being skipped (e.g. greylisting/ratelimit/whatever)
+-- but the ratelimit buckets for this task are touched (e.g. pending has been increased)
+-- See https://github.com/rspamd/rspamd/issues/4467 for more context
+local function maybe_cleanup_pending(task)
+  if task:cache_get('ratelimit_bucket_touched') then
+    local prefixes = task:cache_get('ratelimit_prefixes')
+    if prefixes then
+      for k, v in pairs(prefixes) or E do
+        local bucket = v.bucket
+        local function cleanup_cb(err, data)
+          if err then
+            rspamd_logger.errx('cannot cleanup limit %s: %s %s', k, err, data)
+          else
+            lua_util.debugm(N, task, 'cleaned pending bucked for %s: %s', k, data)
+          end
+        end
+        local _,nrcpt = task:has_recipients('smtp')
+        if not nrcpt or nrcpt <= 0 then
+          nrcpt = 1
+        end
+        local bincr = nrcpt
+        if bucket.skip_recipients then bincr = 1 end
+        local now = task:get_timeval(true)
+        now = lua_util.round(now * 1000.0) -- Get milliseconds
+        lua_redis.exec_redis_script(bucket_cleanup_id,
+            {key = v.hash, task = task, is_write = true},
+            cleanup_cb,
+            {v.hash, tostring(now), tostring(settings.expire), tostring(bincr)})
+      end
+    end
+  end
+end
+
 local function ratelimit_update_cb(task)
-  if task:has_flag('skip') then return end
-  if not settings.allow_local and lua_util.is_rspamc_or_controller(task) then return end
+  if task:has_flag('skip') then
+    maybe_cleanup_pending(task)
+    return
+  end
+  if not settings.allow_local and lua_util.is_rspamc_or_controller(task) then
+    maybe_cleanup_pending(task)
+  end
+
   local prefixes = task:cache_get('ratelimit_prefixes')
 
   if prefixes then
     if task:has_pre_result() then
       -- Already rate limited/greylisted, do nothing
       lua_util.debugm(N, task, 'pre-action has been set, do not update')
+      maybe_cleanup_pending(task)
       return
     end
 
@@ -563,7 +610,7 @@ local function ratelimit_update_cb(task)
               data[1], data[2], data[3])
         end
       end
-      local now = rspamd_util.get_time()
+      local now = task:get_timeval(true)
       now = lua_util.round(now * 1000.0) -- Get milliseconds
       local mult_burst = 1.0
       local mult_rate = 1.0