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_linux_test.go

@@ -0,0 +1,116 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netmon
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"tailscale.com/tstest"
+)
+
+// test the specific /proc/net/route path as found on Google Cloud Run instances
+func TestGoogleCloudRunDefaultRouteInterface(t *testing.T) {
+	dir := t.TempDir()
+	tstest.Replace(t, &procNetRoutePath, filepath.Join(dir, "CloudRun"))
+	buf := []byte("Iface\tDestination\tGateway\tFlags\tRefCnt\tUse\tMetric\tMask\tMTU\tWindow\tIRTT\n" +
+		"eth0\t8008FEA9\t00000000\t0001\t0\t0\t0\t01FFFFFF\t0\t0\t0\n" +
+		"eth1\t00000000\t00000000\t0001\t0\t0\t0\t00000000\t0\t0\t0\n")
+	err := os.WriteFile(procNetRoutePath, buf, 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got, err := DefaultRouteInterface()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got != "eth1" {
+		t.Fatalf("got %s, want eth1", got)
+	}
+}
+
+// we read chunks of /proc/net/route at a time, test that files longer than the chunk
+// size can be handled.
+func TestExtremelyLongProcNetRoute(t *testing.T) {
+	dir := t.TempDir()
+	tstest.Replace(t, &procNetRoutePath, filepath.Join(dir, "VeryLong"))
+	f, err := os.Create(procNetRoutePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = f.Write([]byte("Iface\tDestination\tGateway\tFlags\tRefCnt\tUse\tMetric\tMask\tMTU\tWindow\tIRTT\n"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for n := 0; n <= 900; n++ {
+		line := fmt.Sprintf("eth%d\t8008FEA9\t00000000\t0001\t0\t0\t0\t01FFFFFF\t0\t0\t0\n", n)
+		_, err := f.Write([]byte(line))
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	_, err = f.Write([]byte("tokenring1\t00000000\t00000000\t0001\t0\t0\t0\t00000000\t0\t0\t0\n"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := DefaultRouteInterface()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got != "tokenring1" {
+		t.Fatalf("got %q, want tokenring1", got)
+	}
+}
+
+// test the specific /proc/net/route path as found on AWS App Runner instances
+func TestAwsAppRunnerDefaultRouteInterface(t *testing.T) {
+	dir := t.TempDir()
+	tstest.Replace(t, &procNetRoutePath, filepath.Join(dir, "CloudRun"))
+	buf := []byte("Iface\tDestination\tGateway\tFlags\tRefCnt\tUse\tMetric\tMask\tMTU\tWindow\tIRTT\n" +
+		"eth0\t00000000\tF9AFFEA9\t0003\t0\t0\t0\t00000000\t0\t0\t0\n" +
+		"*\tFEA9FEA9\t00000000\t0005\t0\t0\t0\tFFFFFFFF\t0\t0\t0\n" +
+		"ecs-eth0\t02AAFEA9\t01ACFEA9\t0007\t0\t0\t0\tFFFFFFFF\t0\t0\t0\n" +
+		"ecs-eth0\t00ACFEA9\t00000000\t0001\t0\t0\t0\t00FFFFFF\t0\t0\t0\n" +
+		"eth0\t00AFFEA9\t00000000\t0001\t0\t0\t0\t00FFFFFF\t0\t0\t0\n")
+	err := os.WriteFile(procNetRoutePath, buf, 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got, err := DefaultRouteInterface()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got != "eth0" {
+		t.Fatalf("got %s, want eth0", got)
+	}
+}
+
+func BenchmarkDefaultRouteInterface(b *testing.B) {
+	b.ReportAllocs()
+	for range b.N {
+		if _, err := DefaultRouteInterface(); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func TestRouteLinuxNetlink(t *testing.T) {
+	d, err := defaultRouteFromNetlink()
+	if errors.Is(err, fs.ErrPermission) {
+		t.Skip(err)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %+v", d)
+}