multisocket plugin (#6882)

* multisocket plugin improves performance in multiprocessor systems

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* - refactoring
- update doc

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* remove port from reuseport plugin README

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* rename reuseport plugin to numsockets plugin

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* Add Recommendations to numsockets README

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* added numsockets test; made NUM_SOCKETS mandatory in doc

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* restart and whoami tests for numsockets plugin

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* default value for numsockets

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* caddy up

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* add numsockets to plugin.cfg

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* - rename numsockets plugin to multisocket
- default as GOMAXPROCS
- update README

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

* resolve conflicts

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

---------

Signed-off-by: Viktor Rodionov <33463837+Shmillerov@users.noreply.github.com>

plugin/multisocket/multisocket.go

@@ -0,0 +1,51 @@
+package multisocket
+
+import (
+	"fmt"
+	"runtime"
+	"strconv"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+)
+
+const pluginName = "multisocket"
+
+func init() { plugin.Register(pluginName, setup) }
+
+func setup(c *caddy.Controller) error {
+	err := parseNumSockets(c)
+	if err != nil {
+		return plugin.Error(pluginName, err)
+	}
+	return nil
+}
+
+func parseNumSockets(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+	c.Next() // "multisocket"
+
+	args := c.RemainingArgs()
+
+	if len(args) > 1 || c.Next() {
+		return c.ArgErr()
+	}
+
+	if len(args) == 0 {
+		// Nothing specified; use default that is equal to GOMAXPROCS.
+		config.NumSockets = runtime.GOMAXPROCS(0)
+		return nil
+	}
+
+	numSockets, err := strconv.Atoi(args[0])
+	if err != nil {
+		return fmt.Errorf("invalid num sockets: %w", err)
+	}
+	if numSockets < 1 {
+		return fmt.Errorf("num sockets can not be zero or negative: %d", numSockets)
+	}
+	config.NumSockets = numSockets
+
+	return nil
+}