appview/oauth: invalidate sessions if inactive for too long

if sessions are inactive for too long, tokens will not be refreshed, and
calling authorized xrpc methods will error out with invalid_grant. this
changeset does two things:

- tracks the last time a session was active using a new redis pair:
`oauth:session_meta:<did>:<session>`, this is updated every time
`SaveSession` is called
- checks for session inactivity every time `GetSession` is called, and
deletes the session if so

this way, `GetSession` will never return a session with expired tokens.

Signed-off-by: oppiliappan <[e-mail]>

appview/oauth/oauth.go

···
 
 	jwksUri := clientUri + "/oauth/jwks.json"
 
-	authStore, err := NewRedisStore(config.Redis.ToURL())
+	authStore, err := NewRedisStore(&RedisStoreConfig{
+		RedisURL:                  config.Redis.ToURL(),
+		SessionExpiryDuration:     time.Hour * 24 * 90,
+		SessionInactivityDuration: time.Hour * 24 * 14,
+		AuthRequestExpiryDuration: time.Minute * 30,
+	})
 	if err != nil {
 		return nil, err
 	}

appview/oauth/store.go

···
 	"github.com/redis/go-redis/v9"
 )
 
+type RedisStoreConfig struct {
+	RedisURL string
+
+	// The purpose of these limits is to avoid dead sessions hanging around in the db indefinitely.
+	// The durations here should be *at least as long as* the expected duration of the oauth session itself.
+	SessionExpiryDuration     time.Duration // duration since session creation (max TTL)
+	SessionInactivityDuration time.Duration // duration since last session update
+	AuthRequestExpiryDuration time.Duration // duration since auth request creation
+}
+
 // redis-backed implementation of ClientAuthStore.
 type RedisStore struct {
-	client         *redis.Client
-	SessionTTL     time.Duration
-	AuthRequestTTL time.Duration
+	client *redis.Client
+	cfg    *RedisStoreConfig
 }
 
 var _ oauth.ClientAuthStore = &RedisStore{}
 
-func NewRedisStore(redisURL string) (*RedisStore, error) {
-	opts, err := redis.ParseURL(redisURL)
+type sessionMetadata struct {
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+func NewRedisStore(cfg *RedisStoreConfig) (*RedisStore, error) {
+	if cfg == nil {
+		return nil, fmt.Errorf("missing cfg")
+	}
+	if cfg.RedisURL == "" {
+		return nil, fmt.Errorf("missing RedisURL")
+	}
+	if cfg.SessionExpiryDuration == 0 {
+		return nil, fmt.Errorf("missing SessionExpiryDuration")
+	}
+	if cfg.SessionInactivityDuration == 0 {
+		return nil, fmt.Errorf("missing SessionInactivityDuration")
+	}
+	if cfg.AuthRequestExpiryDuration == 0 {
+		return nil, fmt.Errorf("missing AuthRequestExpiryDuration")
+	}
+
+	opts, err := redis.ParseURL(cfg.RedisURL)
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse redis URL: %w", err)
 	}
···
 	}
 
 	return &RedisStore{
-		client:         client,
-		SessionTTL:     30 * 24 * time.Hour, // 30 days
-		AuthRequestTTL: 10 * time.Minute,    // 10 minutes
+		client: client,
+		cfg:    cfg,
 	}, nil
 }
 
···
 	return fmt.Sprintf("oauth:session:%s:%s", did, sessionID)
 }
 
+func sessionMetadataKey(did syntax.DID, sessionID string) string {
+	return fmt.Sprintf("oauth:session_meta:%s:%s", did, sessionID)
+}
+
 func authRequestKey(state string) string {
 	return fmt.Sprintf("oauth:auth_request:%s", state)
 }
 
 func (r *RedisStore) GetSession(ctx context.Context, did syntax.DID, sessionID string) (*oauth.ClientSessionData, error) {
 	key := sessionKey(did, sessionID)
+	metaKey := sessionMetadataKey(did, sessionID)
+
+	// Check metadata for inactivity expiry
+	metaData, err := r.client.Get(ctx, metaKey).Bytes()
+	if err == redis.Nil {
+		return nil, fmt.Errorf("session not found: %s", did)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("failed to get session metadata: %w", err)
+	}
+
+	var meta sessionMetadata
+	if err := json.Unmarshal(metaData, &meta); err != nil {
+		return nil, fmt.Errorf("failed to unmarshal session metadata: %w", err)
+	}
+
+	// Check if session has been inactive for too long
+	inactiveThreshold := time.Now().Add(-r.cfg.SessionInactivityDuration)
+	if meta.UpdatedAt.Before(inactiveThreshold) {
+		// Session is inactive, delete it
+		r.client.Del(ctx, key, metaKey)
+		return nil, fmt.Errorf("session expired due to inactivity: %s", did)
+	}
+
+	// Get the actual session data
 	data, err := r.client.Get(ctx, key).Bytes()
 	if err == redis.Nil {
 		return nil, fmt.Errorf("session not found: %s", did)
···
 
 func (r *RedisStore) SaveSession(ctx context.Context, sess oauth.ClientSessionData) error {
 	key := sessionKey(sess.AccountDID, sess.SessionID)
+	metaKey := sessionMetadataKey(sess.AccountDID, sess.SessionID)
 
 	data, err := json.Marshal(sess)
 	if err != nil {
 		return fmt.Errorf("failed to marshal session: %w", err)
 	}
 
-	if err := r.client.Set(ctx, key, data, r.SessionTTL).Err(); err != nil {
+	// Check if session already exists to preserve CreatedAt
+	var meta sessionMetadata
+	existingMetaData, err := r.client.Get(ctx, metaKey).Bytes()
+	if err == redis.Nil {
+		// New session
+		meta = sessionMetadata{
+			CreatedAt: time.Now(),
+			UpdatedAt: time.Now(),
+		}
+	} else if err != nil {
+		return fmt.Errorf("failed to check existing session metadata: %w", err)
+	} else {
+		// Existing session - preserve CreatedAt, update UpdatedAt
+		if err := json.Unmarshal(existingMetaData, &meta); err != nil {
+			return fmt.Errorf("failed to unmarshal existing session metadata: %w", err)
+		}
+		meta.UpdatedAt = time.Now()
+	}
+
+	// Calculate remaining TTL based on creation time
+	remainingTTL := r.cfg.SessionExpiryDuration - time.Since(meta.CreatedAt)
+	if remainingTTL <= 0 {
+		return fmt.Errorf("session has expired")
+	}
+
+	// Use the shorter of: remaining TTL or inactivity duration
+	ttl := min(r.cfg.SessionInactivityDuration, remainingTTL)
+
+	// Save session data
+	if err := r.client.Set(ctx, key, data, ttl).Err(); err != nil {
 		return fmt.Errorf("failed to save session: %w", err)
 	}
 
+	// Save metadata
+	metaData, err := json.Marshal(meta)
+	if err != nil {
+		return fmt.Errorf("failed to marshal session metadata: %w", err)
+	}
+	if err := r.client.Set(ctx, metaKey, metaData, ttl).Err(); err != nil {
+		return fmt.Errorf("failed to save session metadata: %w", err)
+	}
+
 	return nil
 }
 
 func (r *RedisStore) DeleteSession(ctx context.Context, did syntax.DID, sessionID string) error {
 	key := sessionKey(did, sessionID)
-	if err := r.client.Del(ctx, key).Err(); err != nil {
+	metaKey := sessionMetadataKey(did, sessionID)
+
+	if err := r.client.Del(ctx, key, metaKey).Err(); err != nil {
 		return fmt.Errorf("failed to delete session: %w", err)
 	}
 	return nil
···
 		return fmt.Errorf("failed to marshal auth request: %w", err)
 	}
 
-	if err := r.client.Set(ctx, key, data, r.AuthRequestTTL).Err(); err != nil {
+	if err := r.client.Set(ctx, key, data, r.cfg.AuthRequestExpiryDuration).Err(); err != nil {
 		return fmt.Errorf("failed to save auth request: %w", err)
 	}