all: move network monitoring from wgengine/monitor to net/netmon

We're using it in more and more places, and it's not really specific to
our use of Wireguard (and does more just link/interface monitoring).

Also removes the separate interface we had for it in sockstats -- it's
a small enough package (we already pull in all of its dependencies
via other paths) that it's not worth the extra complexity.

Updates #7621
Updates #7850

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/netmon/netmon_linux_test.go

@@ -0,0 +1,99 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netmon
+
+import (
+	"net"
+	"net/netip"
+	"testing"
+
+	"github.com/jsimonetti/rtnetlink"
+	"github.com/mdlayher/netlink"
+	"golang.org/x/sys/unix"
+)
+
+func newAddrMsg(iface uint32, addr string, typ netlink.HeaderType) netlink.Message {
+	ip := net.ParseIP(addr)
+	if ip == nil {
+		panic("newAddrMsg: invalid addr: " + addr)
+	}
+
+	addrMsg := rtnetlink.AddressMessage{
+		Index: iface,
+		Attributes: &rtnetlink.AddressAttributes{
+			Address: ip,
+		},
+	}
+
+	b, err := addrMsg.MarshalBinary()
+	if err != nil {
+		panic(err)
+	}
+
+	return netlink.Message{
+		Header: netlink.Header{Type: typ},
+		Data:   b,
+	}
+}
+
+// See issue #4282 and nlConn.addrCache.
+func TestIgnoreDuplicateNEWADDR(t *testing.T) {
+	mustReceive := func(c *nlConn) message {
+		msg, err := c.Receive()
+		if err != nil {
+			t.Fatalf("mustReceive: unwanted error: %s", err)
+		}
+		return msg
+	}
+
+	t.Run("suppress duplicate NEWADDRs", func(t *testing.T) {
+		c := nlConn{
+			buffered: []netlink.Message{
+				newAddrMsg(1, "192.168.0.5", unix.RTM_NEWADDR),
+				newAddrMsg(1, "192.168.0.5", unix.RTM_NEWADDR),
+			},
+			addrCache: make(map[uint32]map[netip.Addr]bool),
+		}
+
+		msg := mustReceive(&c)
+		if _, ok := msg.(*newAddrMessage); !ok {
+			t.Fatalf("want newAddrMessage, got %T %v", msg, msg)
+		}
+
+		msg = mustReceive(&c)
+		if _, ok := msg.(ignoreMessage); !ok {
+			t.Fatalf("want ignoreMessage, got %T %v", msg, msg)
+		}
+	})
+
+	t.Run("do not suppress after DELADDR", func(t *testing.T) {
+		c := nlConn{
+			buffered: []netlink.Message{
+				newAddrMsg(1, "192.168.0.5", unix.RTM_NEWADDR),
+				newAddrMsg(1, "192.168.0.5", unix.RTM_DELADDR),
+				newAddrMsg(1, "192.168.0.5", unix.RTM_NEWADDR),
+			},
+			addrCache: make(map[uint32]map[netip.Addr]bool),
+		}
+
+		msg := mustReceive(&c)
+		if _, ok := msg.(*newAddrMessage); !ok {
+			t.Fatalf("want newAddrMessage, got %T %v", msg, msg)
+		}
+
+		msg = mustReceive(&c)
+		if m, ok := msg.(*newAddrMessage); !ok {
+			t.Fatalf("want newAddrMessage, got %T %v", msg, msg)
+		} else {
+			if !m.Delete {
+				t.Fatalf("want delete, got %#v", m)
+			}
+		}
+
+		msg = mustReceive(&c)
+		if _, ok := msg.(*newAddrMessage); !ok {
+			t.Fatalf("want newAddrMessage, got %T %v", msg, msg)
+		}
+	})
+}