net/ktimeout: add a package to set TCP user timeout

Setting a user timeout will be a more practical tuning knob for a number
of endpoints, this provides a way to set it.

Updates tailscale/corp#17587

Signed-off-by: James Tucker <james@tailscale.com>

net/ktimeout/ktimeout.go

@@ -0,0 +1,36 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package ktimeout configures kernel TCP stack timeouts via the provided
+// control functions. Platform support varies; on unsupported platforms control
+// functions may be entirely no-ops.
+package ktimeout
+
+import (
+	"fmt"
+	"syscall"
+	"time"
+)
+
+// UserTimeout returns a control function that sets the TCP user timeout
+// (TCP_USER_TIMEOUT on linux). A user timeout specifies the maximum age of
+// unacknowledged data on the connection (either in buffer, or sent but not
+// acknowledged) before the connection is terminated. This timer has no effect
+// on limiting the lifetime of idle connections. This may be entirely local to
+// the network stack or may also apply RFC 5482 options to packets.
+func UserTimeout(timeout time.Duration) func(network, address string, c syscall.RawConn) error {
+	return func(network, address string, c syscall.RawConn) error {
+		switch network {
+		case "tcp", "tcp4", "tcp6":
+		default:
+			return fmt.Errorf("ktimeout.UserTimeout: unsupported network: %s", network)
+		}
+		var err error
+		if e := c.Control(func(fd uintptr) {
+			err = SetUserTimeout(fd, timeout)
+		}); e != nil {
+			return e
+		}
+		return err
+	}
+}