netx: add SetTCPUserTimeout to drop dead TCP connections fast

go.mod

@@ -7,4 +7,4 @@ require (
 	golang.org/x/crypto v0.50.0
 )
 
-require golang.org/x/sys v0.43.0 // indirect
+require golang.org/x/sys v0.43.0

netx/tcpusertimeout_linux.go

@@ -0,0 +1,36 @@
+//go:build linux
+
+// Package netx holds network helpers that the standard net package doesn't have.
+package netx
+
+import (
+	"crypto/tls"
+	"net"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+// SetTCPUserTimeout makes the kernel drop conn when sent data stays unacknowledged for timeout, instead of
+// waiting for its ~15 min retry limit (e.g. when the peer's node is gone). Non-TCP connections are left alone.
+func SetTCPUserTimeout(conn net.Conn, timeout time.Duration) error {
+	if tlsConn, ok := conn.(*tls.Conn); ok {
+		conn = tlsConn.NetConn()
+	}
+	tcpConn, ok := conn.(*net.TCPConn)
+	if !ok {
+		return nil
+	}
+	raw, err := tcpConn.SyscallConn()
+	if err != nil {
+		return err
+	}
+	var sockErr error
+	ctrlfn := func(fd uintptr) {
+		sockErr = unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_USER_TIMEOUT, int(timeout.Milliseconds()))
+	}
+	if err := raw.Control(ctrlfn); err != nil {
+		return err
+	}
+	return sockErr
+}

netx/tcpusertimeout_other.go

@@ -0,0 +1,14 @@
+//go:build !linux
+
+// Package netx holds network helpers that the standard net package doesn't have.
+package netx
+
+import (
+	"net"
+	"time"
+)
+
+// SetTCPUserTimeout does nothing: TCP_USER_TIMEOUT is Linux only.
+func SetTCPUserTimeout(net.Conn, time.Duration) error {
+	return nil
+}