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). This seems reasonable.

NOTE: On Linux before commit 97c15fa38 ("socket: Use a default listen
backlog of -1 on Linux") we defaulted to 511. Since that commit we
default to the Kernels default, which before 5.4 is 128 and after is
4096.

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: https://github.com/nginx/unit/issues/1384
Reported-by: <https://github.com/oopsoop2>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/nxt_conf_validation.c b/src/nxt_conf_validation.c
index c9c51ac1..8f31bd18 100644
--- a/src/nxt_conf_validation.c
+++ b/src/nxt_conf_validation.c
@@ -178,6 +178,8 @@ 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,
@@ -430,6 +432,10 @@ 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)
@@ -2705,6 +2711,32 @@ 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)
 {