Introduce a state store to LocalBackend.

The store is passed-in by callers of NewLocalBackend and
ipnserver.Run, but currently all callers are hardcoded to
an in-memory store. The store is unused.

Signed-Off-By: David Anderson <dave@natulte.net>

ipn/backend.go

@@ -50,11 +50,44 @@ type Notify struct {
 	BackendLogID  *string       // public logtail id used by backend
 }
 
+// StateKey is an opaque identifier for a set of LocalBackend state
+// (preferences, private keys, etc.).
+//
+// The reason we need this is that the Tailscale agent may be running
+// on a multi-user machine, in a context where a single daemon is
+// shared by several consecutive users. Ideally we would just use the
+// username of the connected frontend as the StateKey.
+//
+// However, on Windows, there seems to be no safe way to figure out
+// the owning user of a process connected over IPC mechanisms
+// (sockets, named pipes). So instead, on Windows, we use a
+// capability-oriented system where the frontend generates a random
+// identifier for itself, and uses that as the StateKey when talking
+// to the backend. That way, while we can't identify an OS user by
+// name, we can tell two different users apart, because they'll have
+// different opaque state keys (and no access to each others's keys).
+//
+// It would be much nicer if we could just figure out the OS user that
+// owns the connected frontend, but here we are.
+type StateKey string
+
 type Options struct {
-	FrontendLogID string // public logtail id used by frontend
-	ServerURL     string
-	Prefs         *Prefs
-	Notify        func(n Notify) `json:"-"`
+	// Public logtail id used by frontend.
+	FrontendLogID string
+	// Base URL for the tailcontrol server to talk to.
+	ServerURL string
+	// StateKey and Prefs together define the state the backend should
+	// use:
+	//  - StateKey=="" && Prefs!=nil: use Prefs for internal state,
+	//    don't persist changes in the backend.
+	//  - StateKey!="" && Prefs==nil: load the given backend-side
+	//    state and use/update that.
+	//  - StateKey!="" && Prefs!=nil: like the previous case, but do
+	//    an initial overwrite of backend state with Prefs.
+	StateKey StateKey
+	Prefs    *Prefs
+	// Callback for backend events.
+	Notify func(Notify) `json:"-"`
 }
 
 type Backend interface {

ipn/e2e_test.go

@@ -158,7 +158,7 @@ func newNode(t *testing.T, prefix string, https *httptest.Server) testNode {
 	if err != nil {
 		t.Fatalf("NewFakeEngine: %v\n", err)
 	}
-	n, err := NewLocalBackend(logf, prefix, e1)
+	n, err := NewLocalBackend(logf, prefix, &MemoryStore{}, e1)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v\n", err)
 	}

ipn/ipnserver/server.go

@@ -29,6 +29,7 @@ import (
 )
 
 type Options struct {
+	StatePath          string
 	SurviveDisconnects bool
 	AllowQuit          bool
 }
@@ -58,7 +59,17 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 		return fmt.Errorf("safesocket.Listen: %v", err)
 	}
 
-	b, err := ipn.NewLocalBackend(logf, logid, e)
+	var store ipn.StateStore
+	if opts.StatePath != "" {
+		store, err = ipn.NewFileStore(opts.StatePath)
+		if err != nil {
+			return fmt.Errorf("ipn.NewFileStore(%q): %v", opts.StatePath, err)
+		}
+	} else {
+		store = &ipn.MemoryStore{}
+	}
+
+	b, err := ipn.NewLocalBackend(logf, logid, store, e)
 	if err != nil {
 		return fmt.Errorf("NewLocalBackend: %v", err)
 	}

ipn/local.go

@@ -5,6 +5,7 @@
 package ipn
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"strings"
@@ -28,6 +29,7 @@ type LocalBackend struct {
 	notify          func(n Notify)
 	c               *controlclient.Client
 	e               wgengine.Engine
+	store           StateStore
 	serverURL       string
 	backendLogID    string
 	portpoll        *portlist.Poller // may be nil
@@ -36,6 +38,7 @@ type LocalBackend struct {
 
 	// The mutex protects the following elements.
 	mu           sync.Mutex
+	stateKey     StateKey
 	prefs        Prefs
 	state        State
 	hiCache      tailcfg.Hostinfo
@@ -52,7 +55,9 @@ type LocalBackend struct {
 	statusChanged *sync.Cond
 }
 
-func NewLocalBackend(logf logger.Logf, logid string, e wgengine.Engine) (*LocalBackend, error) {
+// NewLocalBackend returns a new LocalBackend that is ready to run,
+// but is not actually running.
+func NewLocalBackend(logf logger.Logf, logid string, store StateStore, e wgengine.Engine) (*LocalBackend, error) {
 	if e == nil {
 		panic("ipn.NewLocalBackend: wgengine must not be nil")
 	}
@@ -68,6 +73,7 @@ func NewLocalBackend(logf logger.Logf, logid string, e wgengine.Engine) (*LocalB
 	b := LocalBackend{
 		logf:         logf,
 		e:            e,
+		store:        store,
 		backendLogID: logid,
 		state:        NoState,
 		portpoll:     portpoll,
@@ -113,8 +119,8 @@ func (b *LocalBackend) SetCmpDiff(cmpDiff func(x, y interface{}) string) {
 }
 
 func (b *LocalBackend) Start(opts Options) error {
-	if opts.Prefs == nil {
-		panic("Prefs can't be nil yet")
+	if opts.Prefs == nil && opts.StateKey == "" {
+		return errors.New("no state key or prefs provided")
 	}
 
 	if b.c != nil {
@@ -128,7 +134,11 @@ func (b *LocalBackend) Start(opts Options) error {
 		b.c.Shutdown()
 	}
 
-	b.logf("Start: %v\n", opts.Prefs.Pretty())
+	if opts.Prefs != nil {
+		b.logf("Start: %v\n", opts.Prefs.Pretty())
+	} else {
+		b.logf("Start\n")
+	}
 
 	hi := controlclient.NewHostinfo()
 	hi.BackendLogID = b.backendLogID
@@ -139,9 +149,12 @@ func (b *LocalBackend) Start(opts Options) error {
 	b.hiCache = hi
 	b.state = NoState
 	b.serverURL = opts.ServerURL
-	if opts.Prefs != nil {
-		b.prefs = *opts.Prefs
+
+	if err := b.loadStateWithLock(opts.StateKey, opts.Prefs); err != nil {
+		b.mu.Unlock()
+		return fmt.Errorf("loading requested state: %v", err)
 	}
+
 	b.notify = opts.Notify
 	b.netMapCache = nil
 	b.mu.Unlock()
@@ -187,6 +200,11 @@ func (b *LocalBackend) Start(opts Options) error {
 		if new.Persist != nil {
 			persist := *new.Persist // copy
 			b.prefs.Persist = &persist
+			if b.stateKey != "" {
+				if err := b.store.WriteState(b.stateKey, b.prefs.ToBytes()); err != nil {
+					b.logf("Failed to save new controlclient state: %v", err)
+				}
+			}
 			np := b.prefs
 			b.send(Notify{Prefs: &np})
 		}
@@ -257,6 +275,8 @@ func (b *LocalBackend) Start(opts Options) error {
 	blid := b.backendLogID
 	b.logf("Backend: logs: be:%v fe:%v\n", blid, opts.FrontendLogID)
 	b.send(Notify{BackendLogID: &blid})
+	nprefs := b.prefs // make a copy
+	b.send(Notify{Prefs: &nprefs})
 
 	cli.Login(nil, controlclient.LoginDefault)
 	return nil
@@ -338,6 +358,42 @@ func (b *LocalBackend) popBrowserAuthNow() {
 	}
 }
 
+func (b *LocalBackend) loadStateWithLock(key StateKey, prefs *Prefs) error {
+	switch {
+	case key != "" && prefs != nil:
+		b.logf("Importing frontend prefs into backend store")
+		if err := b.store.WriteState(key, prefs.ToBytes()); err != nil {
+			return fmt.Errorf("store.WriteState: %v", err)
+		}
+		fallthrough
+	case key != "":
+		b.logf("Using backend prefs")
+		bs, err := b.store.ReadState(key)
+		if err != nil {
+			if err == ErrStateNotExist {
+				b.prefs = NewPrefs()
+				b.stateKey = key
+				b.logf("Created empty state for %q", key)
+				return nil
+			}
+			return fmt.Errorf("store.ReadState(%q): %v", key, err)
+		}
+		b.prefs, err = PrefsFromBytes(bs, false)
+		if err != nil {
+			return fmt.Errorf("PrefsFromBytes: %v", err)
+		}
+		b.stateKey = key
+	case prefs != nil:
+		b.logf("Using frontend prefs")
+		b.prefs = *prefs
+		b.stateKey = ""
+	default:
+		panic("state key and prefs are unset")
+	}
+
+	return nil
+}
+
 func (b *LocalBackend) State() State {
 	b.mu.Lock()
 	defer b.mu.Unlock()
@@ -436,6 +492,11 @@ func (b *LocalBackend) SetPrefs(new Prefs) {
 	old := b.prefs
 	new.Persist = old.Persist // caller isn't allowed to override this
 	b.prefs = new
+	if b.stateKey != "" {
+		if err := b.store.WriteState(b.stateKey, b.prefs.ToBytes()); err != nil {
+			b.logf("Failed to save new controlclient state: %v", err)
+		}
+	}
 	b.mu.Unlock()
 
 	if old.WantRunning != new.WantRunning {

ipn/prefs.go

@@ -33,6 +33,9 @@ type Prefs struct {
 	Persist *controlclient.Persist `json:"Config"`
 }
 
+// IsEmpty reports whether p is nil or pointing to a Prefs zero value.
+func (uc *Prefs) IsEmpty() bool { return uc == nil || *uc == Prefs{} }
+
 func (uc *Prefs) Pretty() string {
 	var ucp string
 	if uc.Persist != nil {

ipn/store.go

@@ -0,0 +1,117 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipn
+
+import (
+	"encoding/json"
+	"errors"
+	"io/ioutil"
+	"os"
+	"sync"
+
+	"tailscale.com/atomicfile"
+)
+
+// ErrStateNotExist is returned by StateStore.ReadState when the
+// requested state id doesn't exist.
+var ErrStateNotExist = errors.New("no state with given id")
+
+// StateStore persists state, and produces it back on request.
+type StateStore interface {
+	// ReadState returns the bytes associated with id. Returns (nil,
+	// ErrStateNotExist) if the id doesn't have associated state.
+	ReadState(id StateKey) ([]byte, error)
+	// WriteState saves bs as the state associated with id.
+	WriteState(id StateKey, bs []byte) error
+}
+
+// MemoryStore is a store that keeps state in memory only.
+type MemoryStore struct {
+	mu    sync.Mutex
+	cache map[StateKey][]byte
+}
+
+func (s *MemoryStore) ReadState(id StateKey) ([]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.cache == nil {
+		s.cache = map[StateKey][]byte{}
+	}
+	bs, ok := s.cache[id]
+	if !ok {
+		return nil, ErrStateNotExist
+	}
+	return bs, nil
+}
+
+func (s *MemoryStore) WriteState(id StateKey, bs []byte) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.cache == nil {
+		s.cache = map[StateKey][]byte{}
+	}
+	s.cache[id] = append([]byte(nil), bs...)
+	return nil
+}
+
+// FileStore is a StateStore that uses a JSON file for persistence.
+type FileStore struct {
+	path string
+
+	mu    sync.RWMutex
+	cache map[StateKey][]byte
+}
+
+// NewFileStore returns a new file store that persists to path.
+func NewFileStore(path string) (*FileStore, error) {
+	bs, err := ioutil.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			// Write out an initial file, to verify that we can write
+			// to the path.
+			if err = atomicfile.WriteFile(path, []byte("{}"), 0600); err != nil {
+				return nil, err
+			}
+			return &FileStore{
+				path:  path,
+				cache: map[StateKey][]byte{},
+			}, nil
+		}
+		return nil, err
+	}
+
+	ret := &FileStore{
+		path:  path,
+		cache: map[StateKey][]byte{},
+	}
+	if err := json.Unmarshal(bs, &ret.cache); err != nil {
+		return nil, err
+	}
+
+	return ret, nil
+}
+
+// ReadState returns the bytes persisted for id, if any.
+func (s *FileStore) ReadState(id StateKey) ([]byte, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	bs, ok := s.cache[id]
+	if !ok {
+		return nil, ErrStateNotExist
+	}
+	return bs, nil
+}
+
+// WriteState persists bs under the key id.
+func (s *FileStore) WriteState(id StateKey, bs []byte) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.cache[id] = append([]byte(nil), bs...)
+	bs, err := json.MarshalIndent(s.cache, "", "  ")
+	if err != nil {
+		return err
+	}
+	return atomicfile.WriteFile(s.path, bs, 0600)
+}

ipn/store_test.go

@@ -0,0 +1,121 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipn
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func testStoreSemantics(t *testing.T, store StateStore) {
+	t.Helper()
+
+	tests := []struct {
+		// if true, data is data to write. If false, data is expected
+		// output of read.
+		write bool
+		id    StateKey
+		data  string
+		// If write=false, true if we expect a not-exist error.
+		notExists bool
+	}{
+		{
+			id:        "foo",
+			notExists: true,
+		},
+		{
+			write: true,
+			id:    "foo",
+			data:  "bar",
+		},
+		{
+			id:   "foo",
+			data: "bar",
+		},
+		{
+			id:        "baz",
+			notExists: true,
+		},
+		{
+			write: true,
+			id:    "baz",
+			data:  "quux",
+		},
+		{
+			id:   "foo",
+			data: "bar",
+		},
+		{
+			id:   "baz",
+			data: "quux",
+		},
+	}
+
+	for _, test := range tests {
+		if test.write {
+			if err := store.WriteState(test.id, []byte(test.data)); err != nil {
+				t.Errorf("writing %q to %q: %v", test.data, test.id, err)
+			}
+		} else {
+			bs, err := store.ReadState(test.id)
+			if err != nil {
+				if test.notExists && err == ErrStateNotExist {
+					continue
+				}
+				t.Errorf("reading %q: %v", test.id, err)
+				continue
+			}
+			if string(bs) != test.data {
+				t.Errorf("reading %q: got %q, want %q", test.id, string(bs), test.data)
+			}
+		}
+	}
+}
+
+func TestMemoryStore(t *testing.T) {
+	store := &MemoryStore{}
+	testStoreSemantics(t, store)
+}
+
+func TestFileStore(t *testing.T) {
+	f, err := ioutil.TempFile("", "test_ipn_store")
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := f.Name()
+	f.Close()
+	if err := os.Remove(path); err != nil {
+		t.Fatal(err)
+	}
+
+	store, err := NewFileStore(path)
+	if err != nil {
+		t.Fatalf("creating file store failed: %v", err)
+	}
+
+	testStoreSemantics(t, store)
+
+	// Build a brand new file store and check that both IDs written
+	// above are still there.
+	store, err = NewFileStore(path)
+	if err != nil {
+		t.Fatalf("creating second file store failed: %v", err)
+	}
+
+	expected := map[StateKey]string{
+		"foo": "bar",
+		"baz": "quux",
+	}
+	for id, want := range expected {
+		bs, err := store.ReadState(id)
+		if err != nil {
+			t.Errorf("reading %q (2nd store): %v", id, err)
+		}
+		if string(bs) != want {
+			t.Errorf("reading %q (2nd store): got %q, want %q", id, string(bs), want)
+		}
+	}
+}