net/sockopts,wgengine/magicsock: export socket buffer sizing logic (#16909)

For eventual use by net/udprelay.Server

Updates tailscale/corp#31164

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/sockopts/sockopts_unix_test.go

@@ -0,0 +1,61 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build unix
+
+package sockopts
+
+import (
+	"net"
+	"syscall"
+	"testing"
+
+	"tailscale.com/types/nettype"
+)
+
+func TestSetBufferSize(t *testing.T) {
+	c, err := net.ListenPacket("udp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	rc, err := c.(*net.UDPConn).SyscallConn()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	getBufs := func() (int, int) {
+		var rcv, snd int
+		rc.Control(func(fd uintptr) {
+			rcv, err = syscall.GetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVBUF)
+			if err != nil {
+				t.Errorf("getsockopt(SO_RCVBUF): %v", err)
+			}
+			snd, err = syscall.GetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_SNDBUF)
+			if err != nil {
+				t.Errorf("getsockopt(SO_SNDBUF): %v", err)
+			}
+		})
+		return rcv, snd
+	}
+
+	curRcv, curSnd := getBufs()
+
+	SetBufferSize(c.(nettype.PacketConn), ReadDirection, 7<<20)
+	SetBufferSize(c.(nettype.PacketConn), WriteDirection, 7<<20)
+
+	newRcv, newSnd := getBufs()
+
+	if curRcv > newRcv {
+		t.Errorf("SO_RCVBUF decreased: %v -> %v", curRcv, newRcv)
+	}
+	if curSnd > newSnd {
+		t.Errorf("SO_SNDBUF decreased: %v -> %v", curSnd, newSnd)
+	}
+
+	// On many systems we may not increase the value, particularly running as a
+	// regular user, so log the information for manual verification.
+	t.Logf("SO_RCVBUF: %v -> %v", curRcv, newRcv)
+	t.Logf("SO_SNDBUF: %v -> %v", curRcv, newRcv)
+}