conf, router: Make the listen(2) backlog configurable

@oopsoop2 on GitHub reported a performance issue related to the default
listen(2) backlog size of 511 on nginx. They found that increasing it
helped, nginx has a config option to configure this.

They would like to be able to do the same on Unit (which also defaults
to 511 on some systems, incl Linux). This seems reasonable.

This adds a new per-listener 'backlog' config option, e.g

  {
      "listeners": {
          "[::1]:8080": {
              "pass": "routes",
              "backlog": 1024
          },
      }

      ...
  }

This doesn't effect the control socket.

Closes: nginx#1384
Reported-by: <https://github.com/oopsoop2>
Signed-off-by: Andrew Clayton <[email protected]>

src/nxt_conf_validation.c

@@ -176,6 +176,8 @@ static nxt_int_t nxt_conf_vldt_app_name(nxt_conf_validation_t *vldt,
     nxt_conf_value_t *value, void *data);
 static nxt_int_t nxt_conf_vldt_forwarded(nxt_conf_validation_t *vldt,
     nxt_conf_value_t *value, void *data);
+static nxt_int_t nxt_conf_vldt_listen_backlog(nxt_conf_validation_t *vldt,
+    nxt_conf_value_t *value, void *data);
 static nxt_int_t nxt_conf_vldt_app(nxt_conf_validation_t *vldt,
     nxt_str_t *name, nxt_conf_value_t *value);
 static nxt_int_t nxt_conf_vldt_object(nxt_conf_validation_t *vldt,
@@ -424,6 +426,10 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_listener_members[] = {
         .type       = NXT_CONF_VLDT_OBJECT,
         .validator  = nxt_conf_vldt_object,
         .u.members  = nxt_conf_vldt_client_ip_members
+    }, {
+        .name       = nxt_string("backlog"),
+        .type       = NXT_CONF_VLDT_NUMBER,
+        .validator  = nxt_conf_vldt_listen_backlog,
     },
 
 #if (NXT_TLS)
@@ -2677,6 +2683,32 @@ nxt_conf_vldt_forwarded(nxt_conf_validation_t *vldt, nxt_conf_value_t *value,
 }
 
 
+static nxt_int_t
+nxt_conf_vldt_listen_backlog(nxt_conf_validation_t *vldt,
+    nxt_conf_value_t *value, void *data)
+{
+    int64_t  backlog;
+
+    backlog = nxt_conf_get_number(value);
+
+    /*
+     * POSIX allows this to be 0 and some systems use -1 to
+     * indicate to use the OS's default value.
+     */
+    if (backlog < -1) {
+        return nxt_conf_vldt_error(vldt, "The \"backlog\" number must be "
+                                   "equal to or greater than -1.");
+    }
+
+    if (backlog > NXT_INT32_T_MAX) {
+        return nxt_conf_vldt_error(vldt, "The \"backlog\" number must "
+                                   "not exceed %d.", NXT_INT32_T_MAX);
+    }
+
+    return NXT_OK;
+}
+
+
 static nxt_int_t
 nxt_conf_vldt_app(nxt_conf_validation_t *vldt, nxt_str_t *name,
     nxt_conf_value_t *value)

src/nxt_router.c

@@ -166,7 +166,7 @@ static void nxt_router_app_prefork_ready(nxt_task_t *task,
 static void nxt_router_app_prefork_error(nxt_task_t *task,
     nxt_port_recv_msg_t *msg, void *data);
 static nxt_socket_conf_t *nxt_router_socket_conf(nxt_task_t *task,
-    nxt_router_temp_conf_t *tmcf, nxt_str_t *name);
+    nxt_router_temp_conf_t *tmcf, nxt_str_t *name, int backlog);
 static nxt_int_t nxt_router_listen_socket_find(nxt_router_temp_conf_t *tmcf,
     nxt_socket_conf_t *nskcf, nxt_sockaddr_t *sa);
 
@@ -1959,12 +1959,22 @@ nxt_router_conf_create(nxt_task_t *task, nxt_router_temp_conf_t *tmcf,
         next = 0;
 
         for ( ;; ) {
+            int               listen_backlog = -1;
+            nxt_conf_value_t  *backlog;
+
+            static const nxt_str_t  backlog_path = nxt_string("backlog");
+
             listener = nxt_conf_next_object_member(listeners, &name, &next);
             if (listener == NULL) {
                 break;
             }
 
-            skcf = nxt_router_socket_conf(task, tmcf, &name);
+            backlog = nxt_conf_get_object_member(listener, &backlog_path, NULL);
+            if (backlog != NULL) {
+                listen_backlog = nxt_conf_get_number(backlog);
+            }
+
+            skcf = nxt_router_socket_conf(task, tmcf, &name, listen_backlog);
             if (skcf == NULL) {
                 goto fail;
             }
@@ -2684,7 +2694,7 @@ nxt_router_application_init(nxt_router_conf_t *rtcf, nxt_str_t *name,
 
 static nxt_socket_conf_t *
 nxt_router_socket_conf(nxt_task_t *task, nxt_router_temp_conf_t *tmcf,
-    nxt_str_t *name)
+    nxt_str_t *name, int backlog)
 {
     size_t               size;
     nxt_int_t            ret;
@@ -2728,7 +2738,7 @@ nxt_router_socket_conf(nxt_task_t *task, nxt_router_temp_conf_t *tmcf,
         nxt_listen_socket_remote_size(ls);
 
         ls->socket = -1;
-        ls->backlog = NXT_LISTEN_BACKLOG;
+        ls->backlog = backlog > -1 ? backlog : NXT_LISTEN_BACKLOG;
         ls->flags = NXT_NONBLOCK;
         ls->read_after_accept = 1;
     }
@@ -2875,7 +2885,7 @@ nxt_router_listen_socket_ready(nxt_task_t *task, nxt_port_recv_msg_t *msg,
 
     nxt_socket_defer_accept(task, s, rpc->socket_conf->listen->sockaddr);
 
-    ret = nxt_listen_socket(task, s, NXT_LISTEN_BACKLOG);
+    ret = nxt_listen_socket(task, s, rpc->socket_conf->listen->backlog);
     if (nxt_slow_path(ret != NXT_OK)) {
         goto fail;
     }