fix(runloop) reschedule rebuild timers after running them

CHANGELOG.md

@@ -71,6 +71,12 @@
 
 ### Fixes
 
+#### Core
+
+- Only reschedule router and plugin iterator timers after finishing previous
+  execution, avoiding unnecessary concurrent executions.
+  [#8634](https://github.com/Kong/kong/pull/8634)
+
 ## [2.8.0]
 
 ### Deprecations

kong/runloop/handler.lua

@@ -36,7 +36,6 @@ local exit         = ngx.exit
 local exec         = ngx.exec
 local header       = ngx.header
 local timer_at     = ngx.timer.at
-local timer_every  = ngx.timer.every
 local subsystem    = ngx.config.subsystem
 local clear_header = ngx.req.clear_header
 local http_version = ngx.req.http_version
@@ -1113,7 +1112,7 @@ return {
           on_timeout = "return_true",
         }
 
-        timer_every(worker_state_update_frequency, function(premature)
+        local function rebuild_router_timer(premature)
           if premature then
             return
           end
@@ -1126,24 +1125,45 @@ return {
           if not ok then
             log(ERR, "could not rebuild router via timer: ", err)
           end
-        end)
+
+          local _, err = timer_at(worker_state_update_frequency, rebuild_router_timer)
+          if err then
+            log(ERR, "could not schedule timer to rebuild router: ", err)
+          end
+        end
+
+        local _, err = timer_at(worker_state_update_frequency, rebuild_router_timer)
+        if err then
+          log(ERR, "could not schedule timer to rebuild router: ", err)
+        end
 
         local plugins_iterator_async_opts = {
           name = "plugins_iterator",
           timeout = 0,
           on_timeout = "return_true",
         }
 
-        timer_every(worker_state_update_frequency, function(premature)
+        local function rebuild_plugins_iterator_timer(premature)
           if premature then
             return
           end
 
-          local ok, err = rebuild_plugins_iterator(plugins_iterator_async_opts)
-          if not ok then
+          local _, err = rebuild_plugins_iterator(plugins_iterator_async_opts)
+          if err then
             log(ERR, "could not rebuild plugins iterator via timer: ", err)
           end
-        end)
+
+          local _, err = timer_at(worker_state_update_frequency, rebuild_plugins_iterator_timer)
+          if err then
+            log(ERR, "could not schedule timer to rebuild plugins iterator: ", err)
+          end
+        end
+
+        local _, err = timer_at(worker_state_update_frequency, rebuild_plugins_iterator_timer)
+        if err then
+          log(ERR, "could not schedule timer to rebuild plugins iterator: ", err)
+        end
+
       end
     end,
     after = NOOP,

kong/runloop/plugins_iterator.lua

@@ -466,7 +466,10 @@ function PluginsIterator.new(version)
       end
 
       if new_version ~= version then
-        return nil, "plugins iterator was changed while rebuilding it"
+        -- the plugins iterator rebuild is being done by a different process at
+        -- the same time, stop here and let the other one go for it
+        kong.log.info("plugins iterator was changed while rebuilding it")
+        return
       end
     end