ipn: make LoadPrefs return os.ErrNotExist when reading corrupted files

It appears some users have corrupted pref.conf files. Have LoadPrefs
treat these files as non-existent. This way tailscale will make user
login, and not crash.

Fixes #954

Signed-off-by: Alex Brainman <alex.brainman@gmail.com>

ipn/prefs.go

@@ -5,6 +5,7 @@
 package ipn
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -276,6 +277,9 @@ func LoadPrefs(filename string) (*Prefs, error) {
 	if err != nil {
 		return nil, fmt.Errorf("LoadPrefs open: %w", err) // err includes path
 	}
+	if bytes.Contains(data, jsonEscapedZero) {
+		return nil, os.ErrNotExist
+	}
 	p, err := PrefsFromBytes(data, false)
 	if err != nil {
 		return nil, fmt.Errorf("LoadPrefs(%q) decode: %w", filename, err)

ipn/prefs_test.go

@@ -7,6 +7,7 @@ package ipn
 import (
 	"errors"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"reflect"
 	"testing"
@@ -371,3 +372,25 @@ func TestLoadPrefsNotExist(t *testing.T) {
 	}
 	t.Fatalf("unexpected prefs=%#v, err=%v", p, err)
 }
+
+// TestLoadPrefsFileWithZeroInIt verifies that LoadPrefs hanldes corrupted input files.
+// See issue #954 for details.
+func TestLoadPrefsFileWithZeroInIt(t *testing.T) {
+	f, err := ioutil.TempFile("", "TestLoadPrefsFileWithZeroInIt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := f.Name()
+	if _, err := f.Write(jsonEscapedZero); err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+	defer os.Remove(path)
+
+	p, err := LoadPrefs(path)
+	if errors.Is(err, os.ErrNotExist) {
+		// expected.
+		return
+	}
+	t.Fatalf("unexpected prefs=%#v, err=%v", p, err)
+}