📡 linux: use Go's listener backlog as TFO backlog

Previously we hardcoded Linux's default net.core.somaxconn (4096) as the default TFO backlog. Turns out it's quite easy to use linkname to retrieve Go's listener backlog, so we changed to do this in this commit, as well as fixing some related comments.

sockopt.go

@@ -8,7 +8,8 @@ func SetTFOListener(fd uintptr) error {
 }
 
 // SetTFOListenerWithBacklog enables TCP Fast Open on the listener with the given backlog.
-// If the platform does not support custom backlog, the specified backlog is ignored.
+// If the backlog is 0, Go std's listen(2) backlog is used.
+// If the platform does not support custom backlog values, the given backlog is ignored.
 func SetTFOListenerWithBacklog(fd uintptr, backlog int) error {
 	return setTFOListenerWithBacklog(fd, backlog) // sockopt_linux.go, sockopt_listen_generic.go, sockopt_stub.go
 }

sockopt_linux.go

@@ -1,22 +1,34 @@
 package tfo
 
-import "golang.org/x/sys/unix"
+import (
+	_ "unsafe"
+
+	"golang.org/x/sys/unix"
+)
 
 // TCPFastopenQueueLength is the maximum number of total pending TFO connection requests,
 // see https://datatracker.ietf.org/doc/html/rfc7413#section-5.1 for why this limit exists.
-// The current value aligns with Go std's listen(2) backlog (4096, as of the current version).
+// The current value is the default net.core.somaxconn on Linux.
 //
 // Deprecated: This constant is no longer used in this module and will be removed in v3.
 const TCPFastopenQueueLength = 4096
 
 func setTFOListener(fd uintptr) error {
-	return setTFOListenerWithBacklog(fd, defaultBacklog)
+	return setTFOListenerWithBacklog(fd, 0)
 }
 
 func setTFOListenerWithBacklog(fd uintptr, backlog int) error {
+	if backlog == 0 {
+		backlog = listenerBacklog()
+	}
 	return setTFO(int(fd), backlog)
 }
 
+// listenerBacklog is linked from src/net/net.go
+//
+//go:linkname listenerBacklog net.listenerBacklog
+func listenerBacklog() int
+
 func setTFODialer(fd uintptr) error {
 	return unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_FASTOPEN_CONNECT, 1)
 }

tfo.go

@@ -42,7 +42,7 @@ type ListenConfig struct {
 	net.ListenConfig
 
 	// Backlog specifies the maximum number of pending TFO connections on supported platforms.
-	// If the value is 0, Go std's listen(2) backlog (4096, as of the current version) is used.
+	// If the value is 0, Go std's listen(2) backlog is used.
 	// If the value is negative, TFO is disabled.
 	Backlog int
 

tfo_listen_generic.go

@@ -9,16 +9,9 @@ import (
 	"syscall"
 )
 
-// defaultBacklog is Go std's listen(2) backlog.
-// We use this as the default TFO backlog.
-const defaultBacklog = 4096
-
 func (lc *ListenConfig) listenTFO(ctx context.Context, network, address string) (net.Listener, error) {
 	ctrlFn := lc.Control
 	backlog := lc.Backlog
-	if backlog == 0 {
-		backlog = defaultBacklog
-	}
 	llc := *lc
 	llc.Control = func(network, address string, c syscall.RawConn) (err error) {
 		if ctrlFn != nil {