Signal-iOS/SignalServiceKit/Devices/InactiveLinkedDeviceFinder.swift

//
// Copyright 2024 Signal Messenger, LLC
// SPDX-License-Identifier: AGPL-3.0-only
//

/// Represents an "inactive" linked device.
/// - SeeAlso ``InactiveLinkedDeviceFinder``
public struct InactiveLinkedDevice: Equatable {
    public let displayName: String
    public let expirationDate: Date
}

/// Responsible for finding "inactive" linked devices, or those who have not
/// come online in a long time and are at risk of expiring (and being unlinked)
/// soon.
public protocol InactiveLinkedDeviceFinder {
    /// At most once per day, re-fetch our linked device state.
    ///
    /// - Note
    /// This method does nothing if invoked from a linked device.
    func refreshLinkedDeviceStateIfNecessary() async throws

    /// Find the user's "least active" linked device, i.e. their linked device
    /// that was last seen longest ago.
    ///
    /// - Note
    /// A linked device's expiration time (when it is unlinked) is a function of
    /// its "last seen" time. Consequently, the least-active linked device
    /// returned by this method will also be the next-expiring device.
    ///
    /// - Note
    /// This method returns `nil` if the current device is a linked device.
    func findLeastActiveLinkedDevice(tx: DBReadTransaction) -> InactiveLinkedDevice?

    /// Permanently disables this and any future inactive linked device finders.
    ///
    /// - Important
    /// This is irreversible for the life of this app install. Use with care.
    func permanentlyDisableFinders(tx: DBWriteTransaction)

#if TESTABLE_BUILD
    func reenablePermanentlyDisabledFinders(tx: DBWriteTransaction)
#endif
}

class InactiveLinkedDeviceFinderImpl: InactiveLinkedDeviceFinder {
    private enum Constants {
        /// How long we should wait between device state refreshes.
        static let intervalForDeviceRefresh: TimeInterval = .day

        /// How long before a device expires it is considered "inactive".
        static let intervalBeforeExpirationConsideredInactive: TimeInterval = .week
    }

    private enum StoreKeys {
        static let isPermanentlyDisabled: String = "isPermanentlyDisabled"
    }

    private let dateProvider: DateProvider
    private let db: any DB
    private let deviceService: OWSDeviceService
    private let deviceStore: OWSDeviceStore
    private let kvStore: KeyValueStore
    private let remoteConfigProvider: any RemoteConfigProvider
    private let tsAccountManager: TSAccountManager

    private var intervalForDeviceExpiration: TimeInterval {
        return remoteConfigProvider.currentConfig().messageQueueTime
    }

    private var intervalForDeviceInactivity: TimeInterval {
        return max(0, remoteConfigProvider.currentConfig().messageQueueTime - Constants.intervalBeforeExpirationConsideredInactive)
    }

    private let logger = PrefixedLogger(prefix: "InactiveLinkedDeviceFinder")

    init(
        dateProvider: @escaping DateProvider,
        db: any DB,
        deviceService: OWSDeviceService,
        deviceStore: OWSDeviceStore,
        remoteConfigProvider: any RemoteConfigProvider,
        tsAccountManager: TSAccountManager,
    ) {
        self.dateProvider = dateProvider
        self.db = db
        self.deviceService = deviceService
        self.deviceStore = deviceStore
        self.kvStore = KeyValueStore(collection: "InactiveLinkedDeviceFinderImpl")
        self.remoteConfigProvider = remoteConfigProvider
        self.tsAccountManager = tsAccountManager
    }

    func refreshLinkedDeviceStateIfNecessary() async throws {
        let shouldSkip = db.read { tx -> Bool in
            if kvStore.hasValue(StoreKeys.isPermanentlyDisabled, transaction: tx) {
                // Finder is permanently disabled, no need to refresh.
                return true
            }

            if !tsAccountManager.registrationState(tx: tx).isRegisteredPrimaryDevice {
                // Only refresh state on primaries.
                return true
            }

            return false
        }

        if shouldSkip {
            return
        }

        do {
            _ = try await deviceService.refreshDevices()
        } catch {
            logger.warn("Failed to refresh devices!")
            throw error
        }
    }

    func findLeastActiveLinkedDevice(tx: DBReadTransaction) -> InactiveLinkedDevice? {
        if kvStore.hasValue(StoreKeys.isPermanentlyDisabled, transaction: tx) {
            // Short-circuit if we've been disabled.
            return nil
        }

        if !tsAccountManager.registrationState(tx: tx).isRegisteredPrimaryDevice {
            // Only report linked devices if we are a primary.
            return nil
        }

        let allInactiveLinkedDevices = deviceStore.fetchAll(tx: tx)
            .filter { !$0.deviceId.isPrimary }
            .filter { device in
                // Only keep devices whose inactivity date has passed.
                let inactivityDate = device.lastSeenAt.addingTimeInterval(intervalForDeviceInactivity)
                return inactivityDate < dateProvider()
            }

        return allInactiveLinkedDevices
            .min { lhs, rhs in
                return lhs.lastSeenAt < rhs.lastSeenAt
            }
            .map { device -> InactiveLinkedDevice in
                return InactiveLinkedDevice(
                    displayName: device.displayName,
                    expirationDate: device.lastSeenAt.addingTimeInterval(
                        intervalForDeviceExpiration,
                    ),
                )
            }
    }

    func permanentlyDisableFinders(tx: DBWriteTransaction) {
        kvStore.setBool(true, key: StoreKeys.isPermanentlyDisabled, transaction: tx)
    }

#if TESTABLE_BUILD
    func reenablePermanentlyDisabledFinders(tx: DBWriteTransaction) {
        kvStore.removeValue(forKey: StoreKeys.isPermanentlyDisabled, transaction: tx)
    }
#endif
}