plezy/lib/utils/failover_http_client.dart

import 'app_logger.dart';
import 'media_server_http_client.dart';
import '../exceptions/media_server_exceptions.dart';

/// [MediaServerHttpClient] with endpoint failover, shared by both backends
/// (the single implementation of what used to be `PlexClient._getWithFailover`
/// and `_JellyfinFailoverHttpClient`).
///
/// Semantics — decided once, here ([retryTransientMediaServerCall]'s doc
/// cross-references this):
///
/// - **Failover is for GETs and explicitly replay-safe POSTs.** Mutations
///   (POST/PUT/DELETE) fail fast by default on both backends: replaying a
///   mutation against a second endpoint when the first was flaky-but-alive
///   risks double-application. A POST whose replay is harmless may opt in
///   with `allowEndpointFailover: true` (Plex play-queue creation does — an
///   orphaned duplicate queue on the server is inert).
/// - **Trigger:** a transient transport failure
///   ([MediaServerHttpException.isTransient]) or a 5xx — whether thrown or
///   returned as a response. 4xx answers never trigger failover.
/// - **One authenticated retry per cascade.** Candidate validation may skip
///   rejected endpoints before that retry. A failed retry (transport error or
///   error status) resets the list to the preferred endpoint and fires
///   [onAllEndpointsExhausted]; the next cascade starts from the best candidate
///   again. Concurrent requests are generation-stamped so a request raced by a
///   switch doesn't cascade a second time.
/// - **Persistence is two-phase:** the switch is applied with
///   `persist: false` for the retry, and only a successful retry persists the
///   winner (`persist: true`).
/// - **Retry interplay:** [retryTransientMediaServerCall] is for
///   *slow-but-working* endpoints (per-surface timeout budgets); surfaces
///   that wrap it pass `allowEndpointFailover: false` so a slow row doesn't
///   move the whole client off an otherwise working endpoint. Failover is for
///   *dead* endpoints.
///
/// Endpoint orchestration diagnostics never contain raw endpoint literals.
/// Backends still register configured endpoints before construction to protect
/// unavoidable lower-level HTTP diagnostics.
class FailoverHttpClient extends MediaServerHttpClient {
  /// [prioritizedEndpoints] may be empty (failover disabled — plain client
  /// behavior). A single-entry list still arms [onAllEndpointsExhausted]:
  /// the lone endpoint failing *is* exhaustion, and the owning manager uses
  /// that to flip server status and reconnect.
  FailoverHttpClient({
    super.client,
    required super.baseUrl,
    required super.defaultHeaders,
    super.connectTimeout,
    super.receiveTimeout,
    required this.logLabel,
    required List<String> prioritizedEndpoints,
    required this.onEndpointSwitch,
    this.onAllEndpointsExhausted,
    this.validateCandidate,
  }) : _endpointManager = prioritizedEndpoints.isNotEmpty ? EndpointFailoverManager(prioritizedEndpoints) : null;

  /// Backend name for log lines ('Plex' / 'Jellyfin') — keeps failover logs
  /// greppable per backend now that the implementation is shared.
  final String logLabel;
  final EndpointFailoverManager? _endpointManager;

  /// Applies a base-URL change on the owning client. The callback must update
  /// this client's [baseUrl] alongside its own config/connection snapshot
  /// (the two-phase protocol calls it with `persist: false` before the retry
  /// and `persist: true` only after a success — persistence must not be gated
  /// on the URL having changed, since the second call sees it already applied).
  final Future<void> Function(String newBaseUrl, {required bool persist}) onEndpointSwitch;

  /// Optional trust gate run after a fallback is selected but before any
  /// switch callback, base-URL mutation, or authenticated retry. The active
  /// request's abort controller must cancel validation as well as the retry.
  final Future<bool> Function(String candidateBaseUrl, AbortController? abort)? validateCandidate;

  /// Fired when a cascade ends without a working endpoint (or the retry
  /// itself fails). The owning manager debounces this into a server-offline
  /// flip + reconnection.
  final void Function()? onAllEndpointsExhausted;

  bool _failoverSwitching = false;

  /// Endpoints currently configured, preferred-first (test/diagnostic view).
  List<String> get endpoints => _endpointManager?.endpoints ?? const [];

  /// Replace the endpoint list after a connection refresh, keeping
  /// [currentBaseUrl] active when provided (it must be present in the list).
  void resetEndpoints(List<String> prioritizedEndpoints, {String? currentBaseUrl}) {
    _endpointManager?.reset(prioritizedEndpoints, currentBaseUrl: currentBaseUrl);
  }

  @override
  Future<MediaServerResponse> get(
    String path, {
    Map<String, dynamic>? queryParameters,
    Map<String, String>? headers,
    Duration? timeout,
    AbortController? abort,
    bool allowEndpointFailover = true,
  }) => _sendWithFailover(
    verb: 'GET',
    allowEndpointFailover: allowEndpointFailover,
    abort: abort,
    send: () => super.get(path, queryParameters: queryParameters, headers: headers, timeout: timeout, abort: abort),
  );

  /// POSTs fail fast by default (see the class doc); only replay-safe POSTs
  /// pass `allowEndpointFailover: true`.
  @override
  Future<MediaServerResponse> post(
    String path, {
    Map<String, dynamic>? queryParameters,
    Map<String, String>? headers,
    Object? body,
    Duration? timeout,
    AbortController? abort,
    bool allowEndpointFailover = false,
  }) => _sendWithFailover(
    verb: 'POST',
    allowEndpointFailover: allowEndpointFailover,
    abort: abort,
    send: () => super.post(
      path,
      queryParameters: queryParameters,
      headers: headers,
      body: body,
      timeout: timeout,
      abort: abort,
    ),
  );

  Future<MediaServerResponse> _sendWithFailover({
    required String verb,
    required bool allowEndpointFailover,
    required Future<MediaServerResponse> Function() send,
    AbortController? abort,
  }) async {
    final generation = _endpointManager?.generation;
    final MediaServerResponse response;
    try {
      response = await send();
    } on MediaServerHttpException catch (e) {
      if (!allowEndpointFailover || !_shouldAttemptFailover(exception: e) || !_canFailover(generation)) {
        rethrow;
      }
      final retried = await _failoverOnce(verb: verb, send: send, abort: abort);
      if (retried == null) rethrow;
      return retried;
    }
    if (!allowEndpointFailover ||
        !_shouldAttemptFailover(statusCode: response.statusCode) ||
        !_canFailover(generation)) {
      return response;
    }
    return await _failoverOnce(verb: verb, send: send, abort: abort) ?? response;
  }

  bool _canFailover(int? requestGeneration) {
    final manager = _endpointManager;
    return manager != null && !_failoverSwitching && requestGeneration == manager.generation;
  }

  bool _shouldAttemptFailover({MediaServerHttpException? exception, int? statusCode}) {
    if (exception != null) {
      if (exception.isTransient) return true;
      statusCode = exception.statusCode;
    }
    return statusCode != null && statusCode >= 500 && statusCode <= 599;
  }

  /// One step of the cascade: validate candidates in priority order, move to
  /// the first accepted endpoint, and retry the authenticated request once.
  ///
  /// Returns the retry's response on success. Returns `null` when no accepted
  /// fallback exists (after firing [onAllEndpointsExhausted]) — the caller
  /// surfaces its original failure. A retry that answers with an error status
  /// is returned as-is (the caller's status handling applies), and a retry that
  /// throws rethrows; both count as exhaustion: the list resets to the
  /// preferred endpoint so the next cascade starts from the best candidate.
  Future<MediaServerResponse?> _failoverOnce({
    required String verb,
    required Future<MediaServerResponse> Function() send,
    AbortController? abort,
  }) async {
    final manager = _endpointManager!;
    if (!manager.hasFallback) {
      await _resetToPreferred(manager);
      onAllEndpointsExhausted?.call();
      return null;
    }

    final endpoints = manager.endpoints;
    final currentIndex = endpoints.indexOf(manager.current);
    if (currentIndex < 0 || currentIndex >= endpoints.length - 1) return null;
    final candidateGeneration = manager.generation;

    _failoverSwitching = true;
    try {
      final validator = validateCandidate;
      String? selectedBaseUrl;
      for (var candidateIndex = currentIndex + 1; candidateIndex < endpoints.length; candidateIndex++) {
        final candidateBaseUrl = endpoints[candidateIndex];
        var accepted = validator == null;
        if (validator != null) {
          try {
            accepted = await validator(candidateBaseUrl, abort);
          } catch (error) {
            if (error is MediaServerHttpException && error.isCancellation) rethrow;
            accepted = false;
          }
        }
        if (accepted) {
          selectedBaseUrl = candidateBaseUrl;
          break;
        }
      }
      if (selectedBaseUrl == null) {
        // Validation happens before moving the cursor, so the last accepted
        // endpoint remains authoritative for both the manager and live client.
        onAllEndpointsExhausted?.call();
        return null;
      }
      abort?.throwIfAborted();

      if (manager.generation != candidateGeneration || manager.current != endpoints[currentIndex]) {
        return null;
      }
      String? movedBaseUrl;
      do {
        movedBaseUrl = manager.moveToNext();
      } while (movedBaseUrl != null && movedBaseUrl != selectedBaseUrl);
      if (movedBaseUrl != selectedBaseUrl) return null;
      appLogger.i('Switching $logLabel endpoint after $verb failure');
      await onEndpointSwitch(selectedBaseUrl, persist: false);
      final response = await send();
      if (response.statusCode < 400) {
        appLogger.i('$logLabel endpoint failover retry succeeded');
        await onEndpointSwitch(selectedBaseUrl, persist: true);
        return response;
      }
      await _resetToPreferred(manager);
      onAllEndpointsExhausted?.call();
      return response;
    } catch (error) {
      await _resetToPreferred(manager);
      if (error is! MediaServerHttpException || !error.isCancellation) {
        onAllEndpointsExhausted?.call();
      }
      rethrow;
    } finally {
      _failoverSwitching = false;
    }
  }

  Future<void> _resetToPreferred(EndpointFailoverManager manager) async {
    final resetBaseUrl = manager.resetToFirst();
    if (resetBaseUrl != null) {
      await onEndpointSwitch(resetBaseUrl, persist: false);
    }
  }
}

/// Maintains the list of endpoints we can cycle through when one fails.
class EndpointFailoverManager {
  EndpointFailoverManager(List<String> urls) {
    _setEndpoints(urls);
  }

  late List<String> _endpoints;
  int _currentIndex = 0;

  /// Incremented every time the active endpoint changes. Requests stamped with
  /// an older generation should not trigger additional failover cascades.
  int _generation = 0;
  int get generation => _generation;

  List<String> get endpoints => List.unmodifiable(_endpoints);

  String get current => _endpoints[_currentIndex];

  bool get hasFallback => _currentIndex < _endpoints.length - 1;

  /// Move to the next endpoint, returning its URL or null if exhausted.
  String? moveToNext() {
    if (!hasFallback) return null;
    _currentIndex++;
    _generation++;
    return _endpoints[_currentIndex];
  }

  /// Reset back to the first (preferred) endpoint. Called when all endpoints
  /// are exhausted so the next failure cycle starts from the best candidate.
  String? resetToFirst() {
    if (_currentIndex != 0) {
      _currentIndex = 0;
      _generation++;
      appLogger.d('Failover endpoint list reset to first candidate');
      return _endpoints[_currentIndex];
    }
    return null;
  }

  /// Replace the endpoint list and optionally set the active endpoint.
  void reset(List<String> urls, {String? currentBaseUrl}) {
    _setEndpoints(urls);
    if (currentBaseUrl != null) {
      final index = _endpoints.indexOf(currentBaseUrl);
      _currentIndex = index >= 0 ? index : 0;
    } else {
      _currentIndex = 0;
    }
    _generation++;
  }

  void _setEndpoints(List<String> urls) {
    final sanitized = <String>[];
    final seen = <String>{};
    for (final url in urls) {
      if (url.isEmpty || seen.contains(url)) continue;
      seen.add(url);
      sanitized.add(url);
    }
    if (sanitized.isEmpty) {
      throw ArgumentError('At least one endpoint is required');
    }
    _endpoints = sanitized;
    _currentIndex = _currentIndex.clamp(0, _endpoints.length - 1);
  }
}