lib/api: Save session & CSRF tokens to database, add option to stay logged in (fixes #9151) (#9284)

This adds a "token manager" which handles storing and checking expired
tokens, used for both sessions and CSRF tokens. It removes the old,
corresponding functionality for CSRFs which saved things in a file. The
result is less crap in the state directory, and active login sessions
now survive a Syncthing restart (this really annoyed me).

It also adds a boolean on login to create a longer-lived session cookie,
which is now possible and useful. Thus we can remain logged in over
browser restarts, which was also annoying... :)

<img width="1001" alt="Screenshot 2023-12-12 at 09 56 34"
src="https://github.com/syncthing/syncthing/assets/125426/55cb20c8-78fc-453e-825d-655b94c8623b">

Best viewed with whitespace-insensitive diff, as a bunch of the auth
functions became methods instead of closures which changed indentation.

lib/api/tokenmanager.go

@@ -0,0 +1,137 @@
+// Copyright (C) 2024 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package api
+
+import (
+	"time"
+
+	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/rand"
+	"github.com/syncthing/syncthing/lib/sync"
+	"golang.org/x/exp/slices"
+)
+
+type tokenManager struct {
+	key      string
+	miscDB   *db.NamespacedKV
+	lifetime time.Duration
+	maxItems int
+
+	timeNow func() time.Time // can be overridden for testing
+
+	mut       sync.Mutex
+	tokens    *TokenSet
+	saveTimer *time.Timer
+}
+
+func newTokenManager(key string, miscDB *db.NamespacedKV, lifetime time.Duration, maxItems int) *tokenManager {
+	tokens := &TokenSet{
+		Tokens: make(map[string]int64),
+	}
+	if bs, ok, _ := miscDB.Bytes(key); ok {
+		_ = tokens.Unmarshal(bs) // best effort
+	}
+	return &tokenManager{
+		key:      key,
+		miscDB:   miscDB,
+		lifetime: lifetime,
+		maxItems: maxItems,
+		timeNow:  time.Now,
+		mut:      sync.NewMutex(),
+		tokens:   tokens,
+	}
+}
+
+// Check returns true if the token is valid, and updates the token's expiry
+// time. The token is removed if it is expired.
+func (m *tokenManager) Check(token string) bool {
+	m.mut.Lock()
+	defer m.mut.Unlock()
+
+	expires, ok := m.tokens.Tokens[token]
+	if ok {
+		if expires < m.timeNow().UnixNano() {
+			// The token is expired.
+			m.saveLocked() // removes expired tokens
+			return false
+		}
+
+		// Give the token further life.
+		m.tokens.Tokens[token] = m.timeNow().Add(m.lifetime).UnixNano()
+		m.saveLocked()
+	}
+	return ok
+}
+
+// New creates a new token and returns it.
+func (m *tokenManager) New() string {
+	token := rand.String(randomTokenLength)
+
+	m.mut.Lock()
+	defer m.mut.Unlock()
+
+	m.tokens.Tokens[token] = m.timeNow().Add(m.lifetime).UnixNano()
+	m.saveLocked()
+
+	return token
+}
+
+// Delete removes a token.
+func (m *tokenManager) Delete(token string) {
+	m.mut.Lock()
+	defer m.mut.Unlock()
+
+	delete(m.tokens.Tokens, token)
+	m.saveLocked()
+}
+
+func (m *tokenManager) saveLocked() {
+	// Remove expired tokens.
+	now := m.timeNow().UnixNano()
+	for token, expiry := range m.tokens.Tokens {
+		if expiry < now {
+			delete(m.tokens.Tokens, token)
+		}
+	}
+
+	// If we have a limit on the number of tokens, remove the oldest ones.
+	if m.maxItems > 0 && len(m.tokens.Tokens) > m.maxItems {
+		// Sort the tokens by expiry time, oldest first.
+		type tokenExpiry struct {
+			token  string
+			expiry int64
+		}
+		var tokens []tokenExpiry
+		for token, expiry := range m.tokens.Tokens {
+			tokens = append(tokens, tokenExpiry{token, expiry})
+		}
+		slices.SortFunc(tokens, func(i, j tokenExpiry) int {
+			return int(i.expiry - j.expiry)
+		})
+		// Remove the oldest tokens.
+		for _, token := range tokens[:len(tokens)-m.maxItems] {
+			delete(m.tokens.Tokens, token.token)
+		}
+	}
+
+	// Postpone saving until one second of inactivity.
+	if m.saveTimer == nil {
+		m.saveTimer = time.AfterFunc(time.Second, m.scheduledSave)
+	} else {
+		m.saveTimer.Reset(time.Second)
+	}
+}
+
+func (m *tokenManager) scheduledSave() {
+	m.mut.Lock()
+	defer m.mut.Unlock()
+
+	m.saveTimer = nil
+
+	bs, _ := m.tokens.Marshal()      // can't fail
+	_ = m.miscDB.PutBytes(m.key, bs) // can fail, but what are we going to do?
+}