hostinfo, control/controlclient: tell control when Ubuntu has disabled Tailscale's sources

Fixes #3177
Updates #2500

Change-Id: Iff2a8e27ec7d36a1c210263d6218f20ebed37924
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -622,6 +622,9 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 	if health.NetworkCategoryHealth() != nil {
 		extraDebugFlags = append(extraDebugFlags, "warn-network-category-unhealthy")
 	}
+	if hostinfo.DisabledEtcAptSource() {
+		extraDebugFlags = append(extraDebugFlags, "warn-etc-apt-source-disabled")
+	}
 	if len(extraDebugFlags) > 0 {
 		old := request.DebugFlags
 		request.DebugFlags = append(old[:len(old):len(old)], extraDebugFlags...)

hostinfo/hostinfo.go

@@ -7,11 +7,14 @@
 package hostinfo
 
 import (
+	"bufio"
 	"io"
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"sync/atomic"
+	"time"
 
 	"go4.org/mem"
 	"tailscale.com/tailcfg"
@@ -224,3 +227,55 @@ func inKubernetes() bool {
 	}
 	return false
 }
+
+type etcAptSrcResult struct {
+	mod      time.Time
+	disabled bool
+}
+
+var etcAptSrcCache atomic.Value // of etcAptSrcResult
+
+// DisabledEtcAptSource reports whether Ubuntu (or similar) has disabled
+// the /etc/apt/sources.list.d/tailscale.list file contents upon upgrade
+// to a new release of the distro.
+//
+// See https://github.com/tailscale/tailscale/issues/3177
+func DisabledEtcAptSource() bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+	const path = "/etc/apt/sources.list.d/tailscale.list"
+	fi, err := os.Stat(path)
+	if err != nil || !fi.Mode().IsRegular() {
+		return false
+	}
+	mod := fi.ModTime()
+	if c, ok := etcAptSrcCache.Load().(etcAptSrcResult); ok && c.mod == mod {
+		return c.disabled
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	v := etcAptSourceFileIsDisabled(f)
+	etcAptSrcCache.Store(etcAptSrcResult{mod: mod, disabled: v})
+	return v
+}
+
+func etcAptSourceFileIsDisabled(r io.Reader) bool {
+	bs := bufio.NewScanner(r)
+	disabled := false // did we find the "disabled on upgrade" comment?
+	for bs.Scan() {
+		line := strings.TrimSpace(bs.Text())
+		if strings.Contains(line, "# disabled on upgrade") {
+			disabled = true
+		}
+		if line == "" || line[0] == '#' {
+			continue
+		}
+		// Well, it has some contents in it at least.
+		return false
+	}
+	return disabled
+}

hostinfo/hostinfo_test.go

@@ -6,6 +6,7 @@ package hostinfo
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 )
 
@@ -27,3 +28,25 @@ func TestOSVersion(t *testing.T) {
 	}
 	t.Logf("Got: %#q", osVersion())
 }
+
+func TestEtcAptSourceFileIsDisabled(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{"empty", "", false},
+		{"normal", "deb foo\n", false},
+		{"normal-commented", "# deb foo\n", false},
+		{"normal-disabled-by-ubuntu", "# deb foo # disabled on upgrade to dingus\n", true},
+		{"normal-disabled-then-uncommented", "deb foo # disabled on upgrade to dingus\n", false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := etcAptSourceFileIsDisabled(strings.NewReader(tt.in))
+			if got != tt.want {
+				t.Errorf("got %v; want %v", got, tt.want)
+			}
+		})
+	}
+}