chore: export listener stats (#3007)

* chore: export listener stats

Specifically export reached maxclient event.

---------

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/facade/dragonfly_listener.cc

@@ -156,25 +156,30 @@ bool ConfigureKeepAlive(int fd) {
   return true;
 }
 
-thread_local size_t ssl_allocated_bytes = 0;
+struct ListenerStats {
+  size_t tls_allocated_bytes = 0;
+  uint64_t refused_conn_maxclients_reached_cnt = 0;
+};
+
+thread_local ListenerStats listener_tl_stats;
 atomic_int ssl_init_refcount = 0;
 
 void* OverriddenSSLMalloc(size_t size, const char* file, int line) {
   void* res = mi_malloc(size);
-  ssl_allocated_bytes += mi_malloc_usable_size(res);
+  listener_tl_stats.tls_allocated_bytes += mi_malloc_usable_size(res);
   return res;
 }
 
 void* OverriddenSSLRealloc(void* addr, size_t size, const char* file, int line) {
   size_t prev_size = mi_malloc_usable_size(addr);
   void* res = mi_realloc(addr, size);
-  ssl_allocated_bytes += mi_malloc_usable_size(res);
-  ssl_allocated_bytes -= prev_size;
+  listener_tl_stats.tls_allocated_bytes += mi_malloc_usable_size(res);
+  listener_tl_stats.tls_allocated_bytes -= prev_size;
   return res;
 }
 
 void OverriddenSSLFree(void* addr, const char* file, int line) {
-  ssl_allocated_bytes -= mi_malloc_usable_size(addr);
+  listener_tl_stats.tls_allocated_bytes -= mi_malloc_usable_size(addr);
   mi_free(addr);
 }
 
@@ -274,7 +279,11 @@ bool Listener::ReconfigureTLS() {
 }
 
 size_t Listener::TLSUsedMemoryThreadLocal() {
-  return ssl_allocated_bytes;
+  return listener_tl_stats.tls_allocated_bytes;
+}
+
+uint64_t Listener::RefusedConnectionMaxClientsCount() {
+  return listener_tl_stats.refused_conn_maxclients_reached_cnt;
 }
 
 void Listener::PreAcceptLoop(util::ProactorBase* pb) {
@@ -356,6 +365,7 @@ void Listener::OnConnectionClose(util::Connection* conn) {
 }
 
 void Listener::OnMaxConnectionsReached(util::FiberSocketBase* sock) {
+  listener_tl_stats.refused_conn_maxclients_reached_cnt++;
   sock->Write(io::Buffer("-ERR max number of clients reached\r\n"));
 }
 

src/facade/dragonfly_listener.h

@@ -45,6 +45,7 @@ class Listener : public util::ListenerInterface {
 
   // Returns thread-local dynamic memory usage by TLS.
   static size_t TLSUsedMemoryThreadLocal();
+  static uint64_t RefusedConnectionMaxClientsCount();
 
   bool IsPrivilegedInterface() const;
   bool IsMainInterface() const;

src/server/server_family.cc

@@ -1152,6 +1152,11 @@ void PrintPrometheusMetrics(const Metrics& m, StringResponse* resp) {
   AppendMetricWithoutLabels("script_error_total", "", m.facade_stats.reply_stats.script_error_count,
                             MetricType::COUNTER, &resp->body());
 
+  AppendMetricHeader("listener_accept_error_total", "Listener accept errors", MetricType::COUNTER,
+                     &resp->body());
+  AppendMetricValue("listener_accept_error_total", m.refused_conn_max_clients_reached_count,
+                    {"reason"}, {"limit_reached"}, &resp->body());
+
   // DB stats
   AppendMetricWithoutLabels("expired_keys_total", "", m.events.expired_keys, MetricType::COUNTER,
                             &resp->body());
@@ -1860,6 +1865,7 @@ Metrics ServerFamily::GetMetrics() const {
     }
 
     result.tls_bytes += Listener::TLSUsedMemoryThreadLocal();
+    result.refused_conn_max_clients_reached_count += Listener::RefusedConnectionMaxClientsCount();
 
     service_.mutable_registry()->MergeCallStats(index, cmd_stat_cb);
   };

src/server/server_family.h

@@ -96,6 +96,7 @@ struct Metrics {
   uint64_t fiber_switch_cnt = 0;
   uint64_t fiber_switch_delay_usec = 0;
   uint64_t tls_bytes = 0;
+  uint64_t refused_conn_max_clients_reached_count = 0;
 
   // Statistics about fibers running for a long time (more than 1ms).
   uint64_t fiber_longrun_cnt = 0;