Signal-iOS/Signal/src/MessageFetchBGRefreshTask.swift

//
// Copyright 2023 Signal Messenger, LLC
// SPDX-License-Identifier: AGPL-3.0-only
//

import BackgroundTasks
import SignalServiceKit

/**
 * Utility class for managing the BGAppRefreshTask we use as a "keepalive" for
 * registration lock.
 *
 * Ensures that while reglock is active, we try to fetch messages every once in a while
 * even if the app or NSE don't launch, so that the server keeps the account active
 * and reglock alive.
 */
public class MessageFetchBGRefreshTask {

    private static var _shared: MessageFetchBGRefreshTask = {
        return MessageFetchBGRefreshTask(
            dateProvider: { Date() },
            messageFetcherJob: NSObject.messageFetcherJob,
            ows2FAManager: NSObject.ows2FAManager,
            tsAccountManager: DependenciesBridge.shared.tsAccountManager
        )
    }()

    public static var shared: MessageFetchBGRefreshTask? {
        guard AppReadiness.isAppReady else {
            return nil
        }
        return _shared
    }

    // Must be kept in sync with the value in info.plist.
    private static let taskIdentifier = "MessageFetchBGRefreshTask"

    private let dateProvider: DateProvider
    private let messageFetcherJob: MessageFetcherJob
    private let ows2FAManager: OWS2FAManager
    private let tsAccountManager: TSAccountManager

    private init(
        dateProvider: @escaping DateProvider,
        messageFetcherJob: MessageFetcherJob,
        ows2FAManager: OWS2FAManager,
        tsAccountManager: TSAccountManager
    ) {
        self.dateProvider = dateProvider
        self.messageFetcherJob = messageFetcherJob
        self.ows2FAManager = ows2FAManager
        self.tsAccountManager = tsAccountManager
    }

    public static func register() {
        BGTaskScheduler.shared.register(
            forTaskWithIdentifier: Self.taskIdentifier,
            using: nil,
            launchHandler: { task in
                AppReadiness.runNowOrWhenAppDidBecomeReadyAsync {
                    Self.shared?.performTask(task)
                }
            }
        )
    }

    public func scheduleTask() {
        // Note: this file only exists in the main app (Signal/src) so we
        // don't check for that. But if this ever moves, it should check
        // appContext.isMainApp.

        guard ows2FAManager.isRegistrationLockEnabled else {
            // No need to do the keepalive for reglock.
            return
        }

        guard tsAccountManager.registrationStateWithMaybeSneakyTransaction.isRegistered else {
            return
        }

        // Ping server once a day to keep-alive reglock clients.
        // Ideally, we would schedule this for 24 hours _since we last talked to the chat server_.
        // Without knowing that, we risk scheduling this 24 hours out over and over every time you
        // launch the app without internet. That scenario is unlikely, so is left unhandled.
        let interval: TimeInterval = 24 * 60 * 60
        let request = BGAppRefreshTaskRequest(identifier: Self.taskIdentifier)
        request.earliestBeginDate = dateProvider().addingTimeInterval(interval)

        do {
            try BGTaskScheduler.shared.submit(request)
        } catch let error {
            let errorCode = (error as NSError).code
            switch errorCode {
            case BGTaskScheduler.Error.Code.notPermitted.rawValue:
                Logger.warn("Skipping bg task; user permission required.")
            case BGTaskScheduler.Error.Code.tooManyPendingTaskRequests.rawValue:
                // If we reschedule the same identifier, we don't get this error.
                // This means a task with a different identifier was scheduled (not allowed).
                Logger.error("Too many pending bg tasks; only one app refresh task identifier is allowed at any time.")
            case BGTaskScheduler.Error.Code.unavailable.rawValue:
                Logger.error("Trying to schedule bg task from an extension?")
            default:
                Logger.error("Unknown error code scheduling bg task: \(errorCode)")
            }
        }
    }

    private func performTask(_ task: BGTask) {
        Logger.info("performing background fetch")
        AppReadiness.runNowOrWhenUIDidBecomeReadySync {
            self.messageFetcherJob.run().promise
                .timeout(seconds: 10)
                .then {
                    // HACK: Call completion handler after 5 seconds.
                    //
                    // We don't currently have a convenient API to know when message fetching is *done* when
                    // working with the websocket.
                    //
                    // We *could* substantially rewrite the SocketManager to take advantage of the `empty` message
                    // But once our REST endpoint is fixed to properly de-enqueue fallback notifications, we can easily
                    // use the rest endpoint here rather than the websocket and circumvent making changes to critical code.
                    return Guarantee.after(seconds: 5)
                }
                .observe { result in
                    switch result {
                    case .success:
                        Logger.info("success")
                        task.setTaskCompleted(success: true)
                    case .failure:
                        Logger.error("Failing task; failed to fetch messages")
                        task.setTaskCompleted(success: false)
                    }
                    // Schedule the next run now.
                    self.scheduleTask()
                }
        }
    }
}