private/server: FreeBSD TCP fastopen support detection

Detects whether or not TCP fastopen is supported by running sysctl to
grab the current value of net.inet.tcp.fastopen.server_enable.

If not enabled, directs the operator to use sysctl to set it temporarily
and /etc/sysctl.conf to set it on-boot.

Setting the socket option always works whether it is enabled or not.

Verified that fastopen works on FreeBSD 13. Did not attempt on earlier
versions but support has been there since FreeBSD 10.

Change-Id: I2e0c457558a6fa7b7a1b18bc3c6684aff50b81a2

private/server/fastopen_freebsd.go

@@ -0,0 +1,51 @@
+// Copyright (C) 2023 Storj Labs, Inc.
+// See LICENSE for copying information.
+
+package server
+
+import (
+	"os/exec"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+
+	"go.uber.org/zap"
+)
+
+const tcpFastOpen = 1025
+
+func setTCPFastOpen(fd uintptr, _queue int) error {
+	return syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, tcpFastOpen, 1)
+}
+
+var tryInitFastOpenOnce sync.Once
+var initFastOpenPossiblyEnabled bool
+
+// tryInitFastOpen returns true if fastopen support is possibly enabled.
+func tryInitFastOpen(log *zap.Logger) bool {
+	tryInitFastOpenOnce.Do(func() {
+		initFastOpenPossiblyEnabled = true
+		output, err := exec.Command("sysctl", "-n", "net.inet.tcp.fastopen.server_enable").Output()
+		if err != nil {
+			log.Sugar().Infof("kernel support for tcp fast open unknown")
+			initFastOpenPossiblyEnabled = true
+			return
+		}
+		enabled, err := strconv.ParseBool(strings.TrimSpace(string(output)))
+		if err != nil {
+			log.Sugar().Infof("kernel support for tcp fast open unparsable")
+			initFastOpenPossiblyEnabled = true
+			return
+		}
+		if enabled {
+			log.Sugar().Infof("kernel support for server-side tcp fast open enabled.")
+		} else {
+			log.Sugar().Infof("kernel support for server-side tcp fast open not enabled.")
+			log.Sugar().Infof("enable with: sysctl net.inet.tcp.fastopen.server_enable=1")
+			log.Sugar().Infof("enable on-boot by setting net.inet.tcp.fastopen.server_enable=1 in /etc/sysctl.conf")
+		}
+		initFastOpenPossiblyEnabled = enabled
+	})
+	return initFastOpenPossiblyEnabled
+}

private/server/fastopen_other.go

@@ -1,8 +1,8 @@
 // Copyright (C) 2023 Storj Labs, Inc.
 // See LICENSE for copying information.
 
-//go:build !linux && !windows
+//go:build !linux && !windows && !freebsd
-// +build !linux,!windows
+// +build !linux,!windows,!freebsd
 
 package server