net/{interfaces,netmon}, all: merge net/interfaces package into net/netmon

In prep for most of the package funcs in net/interfaces to become
methods in a long-lived netmon.Monitor that can cache things.  (Many
of the funcs are very heavy to call regularly, whereas the long-lived
netmon.Monitor can subscribe to things from the OS and remember
answers to questions it's asked regularly later)

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: Ie4e8dedb70136af2d611b990b865a822cd1797e5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netmon/interfaces_darwin_test.go

@@ -0,0 +1,114 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netmon
+
+import (
+	"errors"
+	"io"
+	"net/netip"
+	"os/exec"
+	"testing"
+
+	"go4.org/mem"
+	"tailscale.com/util/lineread"
+	"tailscale.com/version"
+)
+
+func TestLikelyHomeRouterIPSyscallExec(t *testing.T) {
+	syscallIP, _, syscallOK := likelyHomeRouterIPBSDFetchRIB()
+	netstatIP, netstatIf, netstatOK := likelyHomeRouterIPDarwinExec()
+
+	if syscallOK != netstatOK || syscallIP != netstatIP {
+		t.Errorf("syscall() = %v, %v, netstat = %v, %v",
+			syscallIP, syscallOK,
+			netstatIP, netstatOK,
+		)
+	}
+
+	if !syscallOK {
+		return
+	}
+
+	def, err := defaultRoute()
+	if err != nil {
+		t.Errorf("defaultRoute() error: %v", err)
+	}
+
+	if def.InterfaceName != netstatIf {
+		t.Errorf("syscall default route interface %s differs from netstat %s", def.InterfaceName, netstatIf)
+	}
+}
+
+/*
+Parse out 10.0.0.1 and en0 from:
+
+$ netstat -r -n -f inet
+Routing tables
+
+Internet:
+Destination        Gateway            Flags        Netif Expire
+default            10.0.0.1           UGSc           en0
+default            link#14            UCSI         utun2
+10/16              link#4             UCS            en0      !
+10.0.0.1/32        link#4             UCS            en0      !
+...
+*/
+func likelyHomeRouterIPDarwinExec() (ret netip.Addr, netif string, ok bool) {
+	if version.IsMobile() {
+		// Don't try to do subprocesses on iOS. Ends up with log spam like:
+		// kernel: "Sandbox: IPNExtension(86580) deny(1) process-fork"
+		// This is why we have likelyHomeRouterIPDarwinSyscall.
+		return ret, "", false
+	}
+	cmd := exec.Command("/usr/sbin/netstat", "-r", "-n", "-f", "inet")
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return
+	}
+	if err := cmd.Start(); err != nil {
+		return
+	}
+	defer cmd.Wait()
+	defer io.Copy(io.Discard, stdout) // clear the pipe to prevent hangs
+
+	var f []mem.RO
+	lineread.Reader(stdout, func(lineb []byte) error {
+		line := mem.B(lineb)
+		if !mem.Contains(line, mem.S("default")) {
+			return nil
+		}
+		f = mem.AppendFields(f[:0], line)
+		if len(f) < 4 || !f[0].EqualString("default") {
+			return nil
+		}
+		ipm, flagsm, netifm := f[1], f[2], f[3]
+		if !mem.Contains(flagsm, mem.S("G")) {
+			return nil
+		}
+		if mem.Contains(flagsm, mem.S("I")) {
+			return nil
+		}
+		ip, err := netip.ParseAddr(string(mem.Append(nil, ipm)))
+		if err == nil && ip.IsPrivate() {
+			ret = ip
+			netif = netifm.StringCopy()
+			// We've found what we're looking for.
+			return errStopReadingNetstatTable
+		}
+		return nil
+	})
+	return ret, netif, ret.IsValid()
+}
+
+func TestFetchRoutingTable(t *testing.T) {
+	// Issue 1345: this used to be flaky on darwin.
+	for range 20 {
+		_, err := fetchRoutingTable()
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+var errStopReadingNetstatTable = errors.New("found private gateway")