// Copyright © 2022 Rangeproof Pty Ltd. All rights reserved.
//
// stringlint:disable

import UIKit
import CryptoKit
import Combine
import GRDB

#if DEBUG
import Darwin
#endif

// MARK: - Singleton

public extension Singleton {
    static let storage: SingletonConfig<Storage> = Dependencies.create(
        identifier: "storage",
        createInstance: { dependencies in Storage(using: dependencies) }
    )
    static let scheduler: SingletonConfig<ValueObservationScheduler> = Dependencies.create(
        identifier: "scheduler",
        createInstance: { _ in AsyncValueObservationScheduler.async(onQueue: .main) }
    )
}

// MARK: - Log.Category

public extension Log.Category {
    static let storage: Log.Category = .create("Storage", defaultLevel: .info)
}

// MARK: - KeychainStorage

public extension KeychainStorage.DataKey { static let dbCipherKeySpec: Self = "GRDBDatabaseCipherKeySpec" }

// MARK: - Storage

open class Storage {
    public struct CurrentlyRunningMigration: ThreadSafeType {
        public let identifier: TargetMigrations.Identifier
        public let migration: Migration.Type
    }
    
    public static let queuePrefix: String = "SessionDatabase"
    public static let dbFileName: String = "Session.sqlite"
    private static let SQLCipherKeySpecLength: Int = 48
    
    /// If a transaction takes longer than this duration a warning will be logged but the transaction will continue to run
    private static let slowTransactionThreshold: TimeInterval = 3
    
    /// When attempting to do a write the transaction will wait this long to acquite a lock before failing
    private static let writeTransactionStartTimeout: TimeInterval = 5
    
    /// If a transaction takes longer than this duration then we should fail the transaction rather than keep hanging
    ///
    /// **Note:** This timeout only applies to synchronous operations (the assumption being that if we know an operation is going to
    /// take a long time then we should probably be handling it asynchronously rather than a synchronous way)
    private static let transactionDeadlockTimeoutSeconds: Int = 5
    
    private static var sharedDatabaseDirectoryPath: String { "\(SessionFileManager.nonInjectedAppSharedDataDirectoryPath)/database" }
    private static var databasePath: String { "\(Storage.sharedDatabaseDirectoryPath)/\(Storage.dbFileName)" }
    private static var databasePathShm: String { "\(Storage.sharedDatabaseDirectoryPath)/\(Storage.dbFileName)-shm" }
    private static var databasePathWal: String { "\(Storage.sharedDatabaseDirectoryPath)/\(Storage.dbFileName)-wal" }
    
    private let dependencies: Dependencies
    fileprivate var dbWriter: DatabaseWriter?
    internal var testDbWriter: DatabaseWriter? { dbWriter }
    
    // MARK: - Migration Variables
    
    @ThreadSafeObject private var migrationProgressUpdater: ((String, CGFloat) -> ())?
    @ThreadSafe private var internalCurrentlyRunningMigration: CurrentlyRunningMigration? = nil
    @ThreadSafe private var migrationsCompleted: Bool = false
    
    public var hasCompletedMigrations: Bool { migrationsCompleted }
    public var currentlyRunningMigration: CurrentlyRunningMigration? {
        internalCurrentlyRunningMigration
    }
    
    // MARK: - Database State Variables
    
    private var startupError: Error?
    public private(set) var isValid: Bool = false
    public private(set) var isSuspended: Bool = false
    public var isDatabasePasswordAccessible: Bool { ((try? getDatabaseCipherKeySpec()) != nil) }
    
    /// This property gets set the first time we successfully read from the database
    public private(set) var hasSuccessfullyRead: Bool = false
    
    /// This property gets set the first time we successfully write to the database
    public private(set) var hasSuccessfullyWritten: Bool = false
    
    /// This property keeps track of all current database tasks and can be used when suspending the database to explicitly
    /// cancel any currently running tasks
    @ThreadSafeObject private var currentTasks: Set<Task<(), Never>> = []
    
    // MARK: - Initialization
    
    public init(customWriter: DatabaseWriter? = nil, using dependencies: Dependencies) {
        self.dependencies = dependencies
        
        configureDatabase(customWriter: customWriter)
    }
    
    public init(
        testAccessTo databasePath: String,
        encryptedKeyPath: String,
        encryptedKeyPassword: String,
        using dependencies: Dependencies
    ) throws {
        self.dependencies = dependencies
        
        try testAccess(
            databasePath: databasePath,
            encryptedKeyPath: encryptedKeyPath,
            encryptedKeyPassword: encryptedKeyPassword
        )
    }
    
    private func configureDatabase(customWriter: DatabaseWriter? = nil) {
        // Create the database directory if needed and ensure it's protection level is set before attempting to
        // create the database KeySpec or the database itself
        try? dependencies[singleton: .fileManager].ensureDirectoryExists(at: Storage.sharedDatabaseDirectoryPath)
        try? dependencies[singleton: .fileManager].protectFileOrFolder(at: Storage.sharedDatabaseDirectoryPath)
        
        // If a custom writer was provided then use that (for unit testing)
        guard customWriter == nil else {
            dbWriter = customWriter
            isValid = true
            return
        }
        
        /// Generate the database KeySpec if needed (this MUST be done before we try to access the database as a different thread
        /// might attempt to access the database before the key is successfully created)
        ///
        /// We reset the bytes immediately after generation to ensure the database key doesn't hang around in memory unintentionally
        ///
        /// **Note:** If we fail to get/generate the keySpec then don't bother continuing to setup the Database as it'll just be invalid,
        /// in this case the App/Extensions will have logic that checks the `isValid` flag of the database
        do {
            var tmpKeySpec: Data = try getOrGenerateDatabaseKeySpec()
            tmpKeySpec.resetBytes(in: 0..<tmpKeySpec.count)
        }
        catch { return }
        
        // Configure the database and create the DatabasePool for interacting with the database
        var config = Configuration()
        config.label = Storage.queuePrefix
        config.maximumReaderCount = 10                   /// Increase the max read connection limit - Default is 5
        config.busyMode = .timeout(Storage.writeTransactionStartTimeout)

        /// Load in the SQLCipher keys
        config.prepareDatabase { [weak self] db in
            var keySpec: Data = try self?.getOrGenerateDatabaseKeySpec() ?? { throw StorageError.invalidKeySpec }()
            defer { keySpec.resetBytes(in: 0..<keySpec.count) } // Reset content immediately after use
            
            // Use a raw key spec, where the 96 hexadecimal digits are provided
            // (i.e. 64 hex for the 256 bit key, followed by 32 hex for the 128 bit salt)
            // using explicit BLOB syntax, e.g.:
            //
            // x'98483C6EB40B6C31A448C22A66DED3B5E5E8D5119CAC8327B655C8B5C483648101010101010101010101010101010101'
            keySpec = try (keySpec.toHexString().data(using: .utf8) ?? { throw StorageError.invalidKeySpec }())
            keySpec.insert(contentsOf: [120, 39], at: 0)    // "x'" prefix
            keySpec.append(39)                              // "'" suffix
            
            try db.usePassphrase(keySpec)
            
            // According to the SQLCipher docs iOS needs the 'cipher_plaintext_header_size' value set to at least
            // 32 as iOS extends special privileges to the database and needs this header to be in plaintext
            // to determine the file type
            //
            // For more info see: https://www.zetetic.net/sqlcipher/sqlcipher-api/#cipher_plaintext_header_size
            try db.execute(sql: "PRAGMA cipher_plaintext_header_size = 32")
        }
        
        // Create the DatabasePool to allow us to connect to the database and mark the storage as valid
        do {
            do {
                dbWriter = try DatabasePool(
                    path: "\(Storage.sharedDatabaseDirectoryPath)/\(Storage.dbFileName)",
                    configuration: config
                )
            }
            catch {
                switch error {
                    case DatabaseError.SQLITE_BUSY:
                        /// According to the docs in GRDB there are a few edge-cases where opening the database
                        /// can fail due to it reporting a "busy" state, by changing the behaviour from `immediateError`
                        /// to `timeout(1)` we give the database a 1 second grace period to deal with it's issues
                        /// and get back into a valid state - adding this helps the database resolve situations where it
                        /// can get confused due to crashing mid-transaction
                        config.busyMode = .timeout(1)
                        Log.warn(.storage, "Database reported busy state during statup, adding grace period to allow startup to continue")
                        
                        // Try to initialise the dbWriter again (hoping the above resolves the lock)
                        dbWriter = try DatabasePool(
                            path: "\(Storage.sharedDatabaseDirectoryPath)/\(Storage.dbFileName)",
                            configuration: config
                        )
                        
                    default: throw error
                }
            }
            isValid = true
        }
        catch { startupError = error }
    }
    
    // MARK: - Migrations
    
    public typealias KeyedMigration = (key: String, identifier: TargetMigrations.Identifier, migration: Migration.Type)
    
    public static func appliedMigrationIdentifiers(_ db: Database) -> Set<String> {
        let migrator: DatabaseMigrator = DatabaseMigrator()
        
        return (try? migrator.appliedIdentifiers(db))
            .defaulting(to: [])
    }
    
    public static func sortedMigrationInfo(migrationTargets: [MigratableTarget.Type]) -> [KeyedMigration] {
        typealias MigrationInfo = (identifier: TargetMigrations.Identifier, migrations: TargetMigrations.MigrationSet)
        
        return migrationTargets
            .map { target -> TargetMigrations in target.migrations() }
            .sorted()
            .reduce(into: [[MigrationInfo]]()) { result, next in
                next.migrations.enumerated().forEach { index, migrationSet in
                    if result.count <= index {
                        result.append([])
                    }

                    result[index] = (result[index] + [(next.identifier, migrationSet)])
                }
            }
            .reduce(into: []) { result, next in
                next.forEach { identifier, migrations in
                    result.append(contentsOf: migrations.map { (identifier.key(with: $0), identifier, $0) })
                }
            }
    }
    
    public func perform(
        migrationTargets: [MigratableTarget.Type],
        async: Bool = true,
        onProgressUpdate: ((CGFloat, TimeInterval) -> ())?,
        onComplete: @escaping (Result<Void, Error>) -> ()
    ) {
        perform(
            sortedMigrations: Storage.sortedMigrationInfo(migrationTargets: migrationTargets),
            async: async,
            onProgressUpdate: onProgressUpdate,
            onComplete: onComplete
        )
    }
    
    internal func perform(
        sortedMigrations: [KeyedMigration],
        async: Bool,
        onProgressUpdate: ((CGFloat, TimeInterval) -> ())?,
        onComplete: @escaping (Result<Void, Error>) -> ()
    ) {
        guard isValid, let dbWriter: DatabaseWriter = dbWriter else {
            let error: Error = (startupError ?? StorageError.startupFailed)
            Log.error(.storage, "Statup failed with error: \(error)")
            onComplete(.failure(error))
            return
        }
        
        // Setup and run any required migrations
        var migrator: DatabaseMigrator = DatabaseMigrator()
        sortedMigrations.forEach { _, identifier, migration in
            migrator.registerMigration(
                self,
                targetIdentifier: identifier,
                migration: migration,
                using: dependencies
            )
        }
        
        // Determine which migrations need to be performed and gather the relevant settings needed to
        // inform the app of progress/states
        let completedMigrations: [String] = (try? dbWriter.read { db in try migrator.completedMigrations(db) })
            .defaulting(to: [])
        let unperformedMigrations: [KeyedMigration] = sortedMigrations
            .reduce(into: []) { result, next in
                guard !completedMigrations.contains(next.key) else { return }
                
                result.append(next)
            }
        let migrationToDurationMap: [String: TimeInterval] = unperformedMigrations
            .reduce(into: [:]) { result, next in
                result[next.key] = next.migration.minExpectedRunDuration
            }
        let unperformedMigrationDurations: [TimeInterval] = unperformedMigrations
            .map { _, _, migration in migration.minExpectedRunDuration }
        let totalMinExpectedDuration: TimeInterval = migrationToDurationMap.values.reduce(0, +)
        
        self._migrationProgressUpdater.set(to: { targetKey, progress in
            guard let migrationIndex: Int = unperformedMigrations.firstIndex(where: { key, _, _ in key == targetKey }) else {
                return
            }
            
            let completedExpectedDuration: TimeInterval = (
                (migrationIndex > 0 ? unperformedMigrationDurations[0..<migrationIndex].reduce(0, +) : 0) +
                (unperformedMigrationDurations[migrationIndex] * progress)
            )
            let totalProgress: CGFloat = (completedExpectedDuration / totalMinExpectedDuration)
            
            DispatchQueue.main.async {
                onProgressUpdate?(totalProgress, totalMinExpectedDuration)
            }
        })
        
        // Store the logic to run when the migration completes
        let migrationCompleted: (Result<Void, Error>) -> () = { [weak self, migrator, dbWriter] result in
            // Make sure to transition the progress updater to 100% for the final migration (just
            // in case the migration itself didn't update to 100% itself)
            if let lastMigrationKey: String = unperformedMigrations.last?.key {
                self?.migrationProgressUpdater?(lastMigrationKey, 1)
            }
            
            self?.migrationsCompleted = true
            self?._migrationProgressUpdater.set(to: nil)
            
            // Don't log anything in the case of a 'success' or if the database is suspended (the
            // latter will happen if the user happens to return to the background too quickly on
            // launch so is unnecessarily alarming, it also gets caught and logged separately by
            // the 'write' functions anyway)
            switch result {
                case .success: break
                case .failure(DatabaseError.SQLITE_ABORT): break
                case .failure(let error):
                    let completedMigrations: [String] = (try? dbWriter
                        .read { db in try migrator.completedMigrations(db) })
                        .defaulting(to: [])
                    let failedMigrationName: String = migrator.migrations
                        .filter { !completedMigrations.contains($0) }
                        .first
                        .defaulting(to: "Unknown")
                    Log.critical(.migration, "Migration '\(failedMigrationName)' failed with error: \(error)")
            }
            
            onComplete(result)
        }
        
        // if there aren't any migrations to run then just complete immediately (this way the migrator
        // doesn't try to execute on the DBWrite thread so returning from the background can't get blocked
        // due to some weird endless process running)
        guard !unperformedMigrations.isEmpty else {
            migrationCompleted(.success(()))
            return
        }
        
        // If we have an unperformed migration then trigger the progress updater immediately
        if let firstMigrationKey: String = unperformedMigrations.first?.key {
            self.migrationProgressUpdater?(firstMigrationKey, 0)
        }
        
        // Note: The non-async migration should only be used for unit tests
        guard async else { return migrationCompleted(Result(catching: { try migrator.migrate(dbWriter) })) }
        
        migrator.asyncMigrate(dbWriter) { [dependencies] result in
            let finalResult: Result<Void, Error> = {
                switch result {
                    case .failure(let error): return .failure(error)
                    case .success: return .success(())
                }
            }()
            
            // Note: We need to dispatch this to the next run toop to prevent blocking if the callback
            // performs subsequent database operations
            DispatchQueue.global(qos: .userInitiated).async(using: dependencies) {
                migrationCompleted(finalResult)
            }
        }
    }
    
    public func willStartMigration(
        _ db: Database,
        _ migration: Migration.Type,
        _ identifier: TargetMigrations.Identifier
    ) {
        internalCurrentlyRunningMigration = CurrentlyRunningMigration(
            identifier: identifier,
            migration: migration
        )
    }
    
    public func didCompleteMigration() {
        internalCurrentlyRunningMigration = nil
    }
    
    public static func update(
        progress: CGFloat,
        for migration: Migration.Type,
        in target: TargetMigrations.Identifier,
        using dependencies: Dependencies
    ) {
        // In test builds ignore any migration progress updates (we run in a custom database writer anyway)
        guard !SNUtilitiesKit.isRunningTests else { return }
        
        dependencies[singleton: .storage].migrationProgressUpdater?(target.key(with: migration), progress)
    }
    
    // MARK: - Security
    
    private func getDatabaseCipherKeySpec() throws -> Data {
        try dependencies[singleton: .keychain].migrateLegacyKeyIfNeeded(
            legacyKey: "GRDBDatabaseCipherKeySpec",
            legacyService: "TSKeyChainService",
            toKey: .dbCipherKeySpec
        )
        return try dependencies[singleton: .keychain].data(forKey: .dbCipherKeySpec)
    }
    
    private func getOrGenerateDatabaseKeySpec() throws -> Data {
        do {
            var keySpec: Data = try getDatabaseCipherKeySpec()
            defer { keySpec.resetBytes(in: 0..<keySpec.count) }
            
            guard keySpec.count == Storage.SQLCipherKeySpecLength else { throw StorageError.invalidKeySpec }
            
            return keySpec
        }
        catch {
            switch (error, (error as? KeychainStorageError)?.code) {
                case (StorageError.invalidKeySpec, _):
                    // For these cases it means either the keySpec or the keychain has become corrupt so in order to
                    // get back to a "known good state" and behave like a new install we need to reset the storage
                    // and regenerate the key
                    if !SNUtilitiesKit.isRunningTests {
                        // Try to reset app by deleting database.
                        resetAllStorage()
                    }
                    fallthrough
                
                case (_, errSecItemNotFound):
                    // No keySpec was found so we need to generate a new one
                    do {
                        var keySpec: Data = try dependencies[singleton: .crypto].tryGenerate(.randomBytes(Storage.SQLCipherKeySpecLength))
                        defer { keySpec.resetBytes(in: 0..<keySpec.count) } // Reset content immediately after use
                        
                        try dependencies[singleton: .keychain].set(data: keySpec, forKey: .dbCipherKeySpec)
                        return keySpec
                    }
                    catch {
                        Log.error(.storage, "Setting keychain value failed with error: \(error)")
                        Thread.sleep(forTimeInterval: 15)    // Sleep to allow any background behaviours to complete
                        throw StorageError.keySpecCreationFailed
                    }
                    
                default:
                    // Because we use kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly, the keychain will be inaccessible
                    // after device restart until device is unlocked for the first time. If the app receives a push
                    // notification, we won't be able to access the keychain to process that notification, so we should
                    // just terminate by throwing an uncaught exception
                    if dependencies[singleton: .appContext].isMainApp || dependencies[singleton: .appContext].isInBackground {
                        let appState: UIApplication.State = dependencies[singleton: .appContext].reportedApplicationState
                        Log.error(.storage, "CipherKeySpec inaccessible. New install or no unlock since device restart?, ApplicationState: \(appState.name)")
                        
                        // In this case we should have already detected the situation earlier and exited
                        // gracefully (in the app delegate) using isDatabasePasswordAccessible(using:), but we
                        // want to stop the app running here anyway
                        Thread.sleep(forTimeInterval: 5)    // Sleep to allow any background behaviours to complete
                        throw StorageError.keySpecInaccessible
                    }
                    
                    Log.error(.storage, "CipherKeySpec inaccessible; not main app.")
                    Thread.sleep(forTimeInterval: 5)    // Sleep to allow any background behaviours to complete
                    throw StorageError.keySpecInaccessible
            }
        }
    }
    
    // MARK: - File Management
    
    /// In order to avoid the `0xdead10cc` exception we manually track whether database access should be suspended, when
    /// in a suspended state this class will fail/reject all read/write calls made to it. Additionally if there was an existing transaction
    /// in progress it will be interrupted.
    ///
    /// The generally suggested approach is to avoid this entirely by not storing the database in an AppGroup folder and sharing it
    /// with extensions - this may be possible but will require significant refactoring and a potentially painful migration to move the
    /// database and other files into the App folder
    public func suspendDatabaseAccess() {
        guard !isSuspended else { return }
        
        isSuspended = true
        Log.info(.storage, "Database access suspended - cancelling \(currentTasks.count) running task(s).")
        
        /// Before triggering an `interrupt` (which will forcibly kill in-progress database queries) we want to try to cancel all
        /// database tasks to give them a small chance to resolve cleanly before we take a brute-force approach
        currentTasks.forEach { $0.cancel() }
        _currentTasks.performUpdate { _ in [] }
        
        /// Interrupt any open transactions (if this function is called then we are expecting that all processes have finished running
        /// and don't actually want any more transactions to occur)
        dbWriter?.interrupt()
    }
    
    /// This method reverses the database suspension used to prevent the `0xdead10cc` exception (see `suspendDatabaseAccess()`
    /// above for more information
    public func resumeDatabaseAccess() {
        guard isSuspended else { return }
        
        isSuspended = false
        Log.info(.storage, "Database access resumed.")
    }
    
    public func checkpoint(_ mode: Database.CheckpointMode) throws {
        try dbWriter?.writeWithoutTransaction { db in _ = try db.checkpoint(mode) }
    }
    
    public func closeDatabase() throws {
        suspendDatabaseAccess()
        isValid = false
        dbWriter = nil
    }
    
    public func resetAllStorage() {
        isValid = false
        migrationsCompleted = false
        dbWriter = nil
        
        deleteDatabaseFiles()
        do { try deleteDbKeys() } catch { Log.warn(.storage, "Failed to delete database keys.") }
    }
    
    public func reconfigureDatabase() {
        configureDatabase()
    }
    
    public func resetForCleanMigration() {
        // Clear existing content
        resetAllStorage()
        
        // Reconfigure
        reconfigureDatabase()
    }
    
    private func deleteDatabaseFiles() {
        do { try dependencies[singleton: .fileManager].removeItem(atPath: Storage.databasePath) }
        catch { Log.warn(.storage, "Failed to delete database.") }
        do { try dependencies[singleton: .fileManager].removeItem(atPath: Storage.databasePathShm) }
        catch { Log.warn(.storage, "Failed to delete database-shm.") }
        do { try dependencies[singleton: .fileManager].removeItem(atPath: Storage.databasePathWal) }
        catch { Log.warn(.storage, "Failed to delete database-wal.") }
    }
    
    private func deleteDbKeys() throws {
        try dependencies[singleton: .keychain].remove(key: .dbCipherKeySpec)
    }
    
    // MARK: - Logging Functions
    
    enum StorageState {
        case valid(DatabaseWriter)
        case invalid(Error)
        
        var forcedError: Error {
            switch self {
                case .valid: return StorageError.validStorageIncorrectlyHandledAsError
                case .invalid(let error): return error
            }
        }
        
        init(_ storage: Storage?) {
            switch (storage?.isSuspended, storage?.isValid, storage?.dbWriter) {
                case (true, _, _): self = .invalid(StorageError.databaseSuspended)
                case (false, true, .some(let dbWriter)): self = .valid(dbWriter)
                default: self = .invalid(StorageError.databaseInvalid)
            }
        }
        
        fileprivate static func logIfNeeded(_ error: Error, info: Storage.CallInfo) {
            let action: String = (info.isWrite ? "write" : "read")
            
            switch error {
                case DatabaseError.SQLITE_ABORT, DatabaseError.SQLITE_INTERRUPT, DatabaseError.SQLITE_ERROR:
                    let message: String = ((error as? DatabaseError)?.message ?? "Unknown")
                    Log.error(.storage, "Database \(action) failed due to error: \(message) - [ \(info.callInfo) ]")
                
                case StorageError.databaseInvalid:
                    Log.error(.storage, "Database \(action) failed as the database is invalid - [ \(info.callInfo) ]")
                
                case StorageError.databaseSuspended:
                    Log.error(.storage, "Database \(action) failed as the database is suspended - [ \(info.callInfo) ]")
                    
                case StorageError.transactionDeadlockTimeout:
                    Log.critical(.storage, "Database \(action) failed due to a potential synchronous query deadlock timeout - [ \(info.callInfo) ]")
                    
                default: break
            }
        }
        
        fileprivate static func logIfNeeded<T>(_ error: Error, info: Storage.CallInfo) -> T? {
            logIfNeeded(error, info: info)
            return nil
        }
        
        fileprivate static func logIfNeeded<T>(_ error: Error, info: Storage.CallInfo) -> AnyPublisher<T, Error> {
            logIfNeeded(error, info: info)
            return Fail<T, Error>(error: error).eraseToAnyPublisher()
        }
    }
    
    // MARK: - Operations
    
    /// Internal type to wrap the database operation `Task` so it can be cancelled when used with `Combine` (since GRDB doesn't
    /// actually handle publishers publishers)
    final class TaskHolder {
        var task: Task<(), Never>?
    }
    
    private static func track<T>(
        _ db: Database,
        _ info: CallInfo,
        _ operation: @escaping (Database) throws -> T
    ) throws -> T {
        guard info.storage?.isValid == true else { throw StorageError.databaseInvalid }
        guard info.storage?.isSuspended == false else { throw StorageError.databaseSuspended }
        
        let timer: TransactionTimer = TransactionTimer.start(
            duration: Storage.slowTransactionThreshold,
            info: info
        )
        defer { timer.stop() }
        
        // Get the result
        let result: T = try operation(db)
        
        // Update the state flags
        switch info.isWrite {
            case true: info.storage?.hasSuccessfullyWritten = true
            case false: info.storage?.hasSuccessfullyRead = true
        }
        
        return result
    }
    
    /// This function manually performs `read`/`write` operations in either a synchronous or asyncronous way using a semaphore to
    /// block the syncrhonous version because `GRDB` has an internal assertion when using it's built-in synchronous `read`/`write`
    /// functions to prevent reentrancy which is unsupported
    ///
    /// Unfortunately this results in the code getting messy when trying to chain multiple database transactions (even
    /// when using `db.afterNextTransaction`) which is somewhat unintuitive
    ///
    /// The `async` variants don't need to worry about this reentrancy issue so instead we route we use those for all operations instead
    /// and just block the thread when we want to perform a synchronous operation
    ///
    /// **Note:** When running a synchronous operation the result will be returned and `asyncCompletion` will not be called, and
    /// vice-versa for an asynchronous operation
    @discardableResult private static func performOperation<T>(
        _ info: CallInfo,
        _ dependencies: Dependencies,
        _ operation: @escaping (Database) throws -> T,
        _ asyncCompletion: ((Result<T, Error>) -> Void)? = nil
    ) -> (result: Result<T, Error>, task: Task<(), Never>?) {
        /// Ensure we are in a valid state
        let storageState: StorageState = StorageState(info.storage)
        
        guard case .valid(let dbWriter) = storageState else {
            if info.isAsync { asyncCompletion?(.failure(storageState.forcedError)) }
            return (.failure(storageState.forcedError), nil)
        }
        
        /// Setup required variables
        let syncQueue = DispatchQueue(label: "com.session.performOperation.syncQueue")
        let semaphore: DispatchSemaphore = DispatchSemaphore(value: 0)
        var operationResult: Result<T, Error>?
        var operationTask: Task<(), Never>?
        let logErrorIfNeeded: (Result<T, Error>) -> Result<T, Error> = { result in
            switch result {
                case .success: break
                case .failure(let error): StorageState.logIfNeeded(error, info: info)
            }
            
            return result
        }
        
        /// Convenience function to remove duplication
        func completeOperation(with result: Result<T, Error>) {
            syncQueue.sync {
                guard operationResult == nil else { return }
                info.storage?.removeTask(operationTask)
                operationResult = result
                semaphore.signal()
                
                /// For async operations, log and invoke the completion closure.
                if info.isAsync {
                    asyncCompletion?(logErrorIfNeeded(result))
                }
            }
        }
        
        let task: Task<(), Never> = Task {
            return await withThrowingTaskGroup(of: T.self) { group in
                /// Add the task to perform the actual database operation
                group.addTask {
                    let trackedOperation: @Sendable (Database) throws -> T = { db in
                        if dependencies[feature: .forceSlowDatabaseQueries] {
                            Thread.sleep(forTimeInterval: 1)
                        }
                        
                        return try Storage.track(db, info, operation)
                    }
                    
                    return (info.isWrite ?
                        try await dbWriter.write(trackedOperation) :
                        try await dbWriter.read(trackedOperation)
                    )
                }
                
                /// If this is a syncronous task then we want to the operation to timeout to ensure we don't unintentionally
                /// create a deadlock
                if !info.isAsync {
                    group.addTask {
                        let timeoutNanoseconds: UInt64 = UInt64(Storage.transactionDeadlockTimeoutSeconds * 1_000_000_000)
                        
                        /// If the debugger is attached then we want to have a lot of shorter sleep iterations as the clock doesn't get
                        /// paused when stopped on a breakpoint (and we don't want to end up having a bunch of false positive
                        /// database timeouts while debugging code)
                        ///
                        /// **Note:** `isDebuggerAttached` will always return `false` in production builds
                        if isDebuggerAttached() {
                            let numIterations: UInt64 = 50
                            
                            for _ in (0..<numIterations) {
                                try await Task.sleep(nanoseconds: (timeoutNanoseconds / numIterations))
                            }
                        }
                        else if info.isWrite {
                            /// This if statement is redundant **but** it means when we get symbolicated crash logs we can distinguish
                            /// between the database threads which are reading and writing
                            try await Task.sleep(nanoseconds: timeoutNanoseconds)
                        }
                        else {
                            try await Task.sleep(nanoseconds: timeoutNanoseconds)
                        }
                        throw StorageError.transactionDeadlockTimeout
                    }
                }
                
                /// Wait for the first task to finish
                ///
                /// **Note:** THe case where `nextResult` returns `nil` is only meant to happen when the group has no
                /// tasks, so shouldn't be considered a valid case (hence the `invalidQueryResult` fallback)
                let result: Result<T, Error>? = await group.nextResult()
                group.cancelAll()
                completeOperation(with: result ?? .failure(StorageError.invalidQueryResult))
            }
        }
        
        /// Store the task in case we want to
        info.storage?.addTask(task)
        operationTask = task
        
        /// For the `async` operation the returned value should be ignored so just return the `invalidQueryResult` error
        guard !info.isAsync else {
            return (.failure(StorageError.invalidQueryResult), task)
        }
        
        /// Block until we have a result
        semaphore.wait()
        return (logErrorIfNeeded(operationResult ?? .failure(StorageError.transactionDeadlockTimeout)), task)
    }
    
    private func performPublisherOperation<T>(
        _ fileName: String,
        _ functionName: String,
        _ lineNumber: Int,
        isWrite: Bool,
        _ operation: @escaping (Database) throws -> T
    ) -> AnyPublisher<T, Error> {
        let info: CallInfo = CallInfo(self, fileName, functionName, lineNumber, (isWrite ? .asyncWrite : .asyncRead))
        
        switch StorageState(self) {
            case .invalid(let error): return StorageState.logIfNeeded(error, info: info)
            case .valid:
                /// **Note:** GRDB does have `readPublisher`/`writePublisher` functions but it appears to asynchronously
                /// trigger both the `output` and `complete` closures at the same time which causes a lot of unexpected
                /// behaviours (this behaviour is apparently expected but still causes a number of odd behaviours in our code
                /// for more information see https://github.com/groue/GRDB.swift/issues/1334)
                ///
                /// Instead of this we are just using `Deferred { Future {} }` which is executed on the specified scheduler
                /// (which behaves in a much more expected way than the GRDB `readPublisher`/`writePublisher` does)
                /// and hooking that into our `performOperation` function which uses the GRDB async/await functions that support
                /// cancellation (as we want to support cancellation as well)
                let holder: TaskHolder = TaskHolder()
                
                return Deferred { [dependencies] in
                    Future { resolver in
                        let (_, task) = Storage.performOperation(info, dependencies, operation) { result in
                            resolver(result)
                        }
                        holder.task = task
                    }
                }
                .handleEvents(receiveCancel: { [weak self] in
                    holder.task?.cancel()
                    self?.removeTask(holder.task)
                })
                .eraseToAnyPublisher()
        }
    }
    
    // MARK: - Functions
    
    private func addTask(_ task: Task<(), Never>) {
        _currentTasks.performUpdate { $0.inserting(task) }
    }
    
    private func removeTask(_ task: Task<(), Never>?) {
        _currentTasks.performUpdate { $0.removing(task) }
    }
    
    @discardableResult public func write<T>(
        fileName file: String = #file,
        functionName funcN: String = #function,
        lineNumber line: Int = #line,
        updates: @escaping (Database) throws -> T?
    ) -> T? {
        switch Storage.performOperation(CallInfo(self, file, funcN, line, .syncWrite), dependencies, updates) {
            case (.failure, _): return nil
            case (.success(let result), _): return result
        }
    }
    
    open func writeAsync<T>(
        fileName file: String = #file,
        functionName funcN: String = #function,
        lineNumber line: Int = #line,
        updates: @escaping (Database) throws -> T,
        completion: @escaping (Result<T, Error>) -> Void = { _ in }
    ) {
        Storage.performOperation(CallInfo(self, file, funcN, line, .asyncWrite), dependencies, updates, completion)
    }
    
    open func writePublisher<T>(
        fileName: String = #file,
        functionName: String = #function,
        lineNumber: Int = #line,
        updates: @escaping (Database) throws -> T
    ) -> AnyPublisher<T, Error> {
        return performPublisherOperation(fileName, functionName, lineNumber, isWrite: true, updates)
    }
    
    @discardableResult public func read<T>(
        fileName file: String = #file,
        functionName funcN: String = #function,
        lineNumber line: Int = #line,
        _ value: @escaping (Database) throws -> T?
    ) -> T? {
        switch Storage.performOperation(CallInfo(self, file, funcN, line, .syncRead), dependencies, value) {
            case (.failure, _): return nil
            case (.success(let result), _): return result
        }
    }
    
    open func readPublisher<T>(
        fileName: String = #file,
        functionName: String = #function,
        lineNumber: Int = #line,
        value: @escaping (Database) throws -> T
    ) -> AnyPublisher<T, Error> {
        return performPublisherOperation(fileName, functionName, lineNumber, isWrite: false, value)
    }
    
    /// Rever to the `ValueObservation.start` method for full documentation
    ///
    /// - parameter observation: The observation to start
    /// - parameter scheduler: A Scheduler. By default, fresh values are
    ///   dispatched asynchronously on the main queue.
    /// - parameter onError: A closure that is provided eventual errors that
    ///   happen during observation
    /// - parameter onChange: A closure that is provided fresh values
    /// - returns: a DatabaseCancellable
    public func start<Reducer: ValueReducer>(
        _ observation: ValueObservation<Reducer>,
        scheduling scheduler: ValueObservationScheduler = .async(onQueue: .main),
        onError: @escaping (Error) -> Void,
        onChange: @escaping (Reducer.Value) -> Void
    ) -> DatabaseCancellable {
        guard isValid, let dbWriter: DatabaseWriter = dbWriter else {
            onError(StorageError.databaseInvalid)
            return AnyDatabaseCancellable(cancel: {})
        }
        
        return observation.start(
            in: dbWriter,
            scheduling: scheduler,
            onError: onError,
            onChange: onChange
        )
    }
    
    /// Add a database observation
    ///
    /// **Note:** This function **MUST NOT** be called from the main thread
    public func addObserver(_ observer: TransactionObserver?) {
        guard isValid, let dbWriter: DatabaseWriter = dbWriter else { return }
        guard let observer: TransactionObserver = observer else { return }
        
        /// This actually triggers a write to the database so can be blocked by other writes so shouldn't be called on the main thread,
        /// we don't dispatch to an async thread in here because `TransactionObserver` isn't `Sendable` so instead just require
        /// that it isn't called on the main thread
        Log.assertNotOnMainThread()
        dbWriter.add(transactionObserver: observer)
    }
    
    /// Remove a database observation
    ///
    /// **Note:** This function **MUST NOT** be called from the main thread
    public func removeObserver(_ observer: TransactionObserver?) {
        guard isValid, let dbWriter: DatabaseWriter = dbWriter else { return }
        guard let observer: TransactionObserver = observer else { return }
        
        /// This actually triggers a write to the database so can be blocked by other writes so shouldn't be called on the main thread,
        /// we don't dispatch to an async thread in here because `TransactionObserver` isn't `Sendable` so instead just require
        /// that it isn't called on the main thread
        Log.assertNotOnMainThread()
        dbWriter.remove(transactionObserver: observer)
    }
}

// MARK: - Combine Extensions

public extension ValueObservation {
    func publisher(
        in storage: Storage,
        scheduling scheduler: ValueObservationScheduler
    ) -> AnyPublisher<Reducer.Value, Error> where Reducer: ValueReducer {
        guard storage.isValid, let dbWriter: DatabaseWriter = storage.dbWriter else {
            return Fail(error: StorageError.databaseInvalid).eraseToAnyPublisher()
        }
        
        return self.publisher(in: dbWriter, scheduling: scheduler)
            .eraseToAnyPublisher()
    }
}

public extension Publisher where Failure == Error {
    func flatMapStorageWritePublisher<T>(using dependencies: Dependencies, updates: @escaping (Database, Output) throws -> T) -> AnyPublisher<T, Error> {
        return self.flatMap { output -> AnyPublisher<T, Error> in
            dependencies[singleton: .storage].writePublisher(updates: { db in try updates(db, output) })
        }.eraseToAnyPublisher()
    }
    
    func flatMapStorageReadPublisher<T>(using dependencies: Dependencies, value: @escaping (Database, Output) throws -> T) -> AnyPublisher<T, Error> {
        return self.flatMap { output -> AnyPublisher<T, Error> in
            dependencies[singleton: .storage].readPublisher(value: { db in try value(db, output) })
        }.eraseToAnyPublisher()
    }
}

// MARK: - CallInfo

private extension Storage {
    class CallInfo {
        enum Behaviour {
            case syncRead
            case asyncRead
            case syncWrite
            case asyncWrite
        }
        
        weak var storage: Storage?
        let file: String
        let function: String
        let line: Int
        let behaviour: Behaviour
        
        var callInfo: String {
            let fileInfo: String = (file.components(separatedBy: "/").last.map { "\($0):\(line) - " } ?? "")
            
            return "\(fileInfo)\(function)"
        }
        
        var isWrite: Bool {
            switch behaviour {
                case .syncWrite, .asyncWrite: return true
                case .syncRead, .asyncRead: return false
            }
        }
        var isAsync: Bool {
            switch behaviour {
                case .asyncRead, .asyncWrite: return true
                case .syncRead, .syncWrite: return false
            }
        }
        
        init(
            _ storage: Storage?,
            _ file: String,
            _ function: String,
            _ line: Int,
            _ behaviour: Behaviour
        ) {
            self.storage = storage
            self.file = file
            self.function = function
            self.line = line
            self.behaviour = behaviour
        }
    }
}

// MARK: - TransactionTimer

private extension Storage {
    private static let timerQueue = DispatchQueue(label: "\(Storage.queuePrefix)-.transactionTimer", qos: .background)
    
    class TransactionTimer {
        private let info: Storage.CallInfo
        private let start: CFTimeInterval = CACurrentMediaTime()
        private var timer: DispatchSourceTimer? = DispatchSource.makeTimerSource(queue: Storage.timerQueue)
        private var wasSlowTransaction: Bool = false
        
        private init(info: Storage.CallInfo) {
            self.info = info
        }

        static func start(duration: TimeInterval, info: Storage.CallInfo) -> TransactionTimer {
            let result: TransactionTimer = TransactionTimer(info: info)
            result.timer?.schedule(deadline: .now() + .seconds(Int(duration)), repeating: .infinity) // Infinity to fire once
            result.timer?.setEventHandler { [weak result] in
                result?.timer?.cancel()
                result?.timer = nil
                
                let action: String = (info.isWrite ? "write" : "read")
                Log.warn(.storage, "Slow \(action) taking longer than \(Storage.slowTransactionThreshold, format: ".2", omitZeroDecimal: true)s - [ \(info.callInfo) ]")
                result?.wasSlowTransaction = true
            }
            result.timer?.resume()
            
            return result
        }

        func stop() {
            timer?.cancel()
            timer = nil
            
            guard wasSlowTransaction else { return }
            
            let end: CFTimeInterval = CACurrentMediaTime()
            let action: String = (info.isWrite ? "write" : "read")
            Log.warn(.storage, "Slow \(action) completed after \(end - start, format: ".2", omitZeroDecimal: true)s - [ \(info.callInfo) ]")
        }
    }
}

// MARK: - Debug Convenience

public extension Storage {
    static let encKeyFilename: String = "key.enc"
    
    func testAccess(
        databasePath: String,
        encryptedKeyPath: String,
        encryptedKeyPassword: String
    ) throws {
        /// First we need to ensure we can decrypt the encrypted key file
        do {
            var tmpKeySpec: Data = try decryptSecureExportedKey(
                path: encryptedKeyPath,
                password: encryptedKeyPassword
            )
            tmpKeySpec.resetBytes(in: 0..<tmpKeySpec.count)
        }
        catch { return }
        
        /// Then configure the database using the key
        var config = Configuration()

        /// Load in the SQLCipher keys
        config.prepareDatabase { [weak self] db in
            var keySpec: Data = try self?.decryptSecureExportedKey(
                path: encryptedKeyPath,
                password: encryptedKeyPassword
            ) ?? { throw StorageError.invalidKeySpec }()
            defer { keySpec.resetBytes(in: 0..<keySpec.count) } // Reset content immediately after use
            
            // Use a raw key spec, where the 96 hexadecimal digits are provided
            // (i.e. 64 hex for the 256 bit key, followed by 32 hex for the 128 bit salt)
            // using explicit BLOB syntax, e.g.:
            //
            // x'98483C6EB40B6C31A448C22A66DED3B5E5E8D5119CAC8327B655C8B5C483648101010101010101010101010101010101'
            keySpec = try (keySpec.toHexString().data(using: .utf8) ?? { throw StorageError.invalidKeySpec }())
            keySpec.insert(contentsOf: [120, 39], at: 0)    // "x'" prefix
            keySpec.append(39)                              // "'" suffix
            
            try db.usePassphrase(keySpec)
            
            // According to the SQLCipher docs iOS needs the 'cipher_plaintext_header_size' value set to at least
            // 32 as iOS extends special privileges to the database and needs this header to be in plaintext
            // to determine the file type
            //
            // For more info see: https://www.zetetic.net/sqlcipher/sqlcipher-api/#cipher_plaintext_header_size
            try db.execute(sql: "PRAGMA cipher_plaintext_header_size = 32")
        }
        
        // Create the DatabasePool to allow us to connect to the database and mark the storage as valid
        dbWriter = try DatabasePool(path: databasePath, configuration: config)
        isValid = true
    }
    
    func secureExportKey(password: String) throws -> String {
        var keySpec: Data = try getOrGenerateDatabaseKeySpec()
        defer { keySpec.resetBytes(in: 0..<keySpec.count) } // Reset content immediately after use
        
        guard var passwordData: Data = password.data(using: .utf8) else { throw StorageError.generic }
        defer { passwordData.resetBytes(in: 0..<passwordData.count) } // Reset content immediately after use
        
        /// Encrypt the `keySpec` value using a SHA256 of the password provided and a nonce then base64-encode the encrypted
        /// data and save it to a temporary file to share alongside the database
        ///
        /// Decrypt the key via the termincal on macOS by running the command in the project root directory
        /// `swift ./Scropts/DecryptExportedKey.swift {BASE64_CIPHERTEXT} {PASSWORD}`
        ///
        /// Where `BASE64_CIPHERTEXT` is the content of the `key.enc` file and `PASSWORD` is the password provided via the
        /// prompt during export
        let nonce: ChaChaPoly.Nonce = ChaChaPoly.Nonce()
        let hash: SHA256.Digest = SHA256.hash(data: passwordData)
        let key: SymmetricKey = SymmetricKey(data: Data(hash.makeIterator()))
        let sealedBox: ChaChaPoly.SealedBox = try ChaChaPoly.seal(keySpec, using: key, nonce: nonce, authenticating: Data())
        let keyInfoPath: String = "\(dependencies[singleton: .fileManager].temporaryDirectory)/\(Storage.encKeyFilename)"
        let encryptedKeyBase64: String = sealedBox.combined.base64EncodedString()
        try encryptedKeyBase64.write(toFile: keyInfoPath, atomically: true, encoding: .utf8)
        
        return keyInfoPath
    }
    
    func replaceDatabaseKey(path: String, password: String) throws {
        var keySpec: Data = try decryptSecureExportedKey(path: path, password: password)
        defer { keySpec.resetBytes(in: 0..<keySpec.count) } // Reset content immediately after use
        
        try dependencies[singleton: .keychain].set(data: keySpec, forKey: .dbCipherKeySpec)
    }
    
    fileprivate func decryptSecureExportedKey(path: String, password: String) throws -> Data {
        let encKeyBase64: String = try String(contentsOf: URL(fileURLWithPath: path), encoding: .utf8)
        
        guard
            var passwordData: Data = password.data(using: .utf8),
            var encKeyData: Data = Data(base64Encoded: encKeyBase64)
        else { throw StorageError.generic }
        defer {
            // Reset content immediately after use
            passwordData.resetBytes(in: 0..<passwordData.count)
            encKeyData.resetBytes(in: 0..<encKeyData.count)
        }
        
        let hash: SHA256.Digest = SHA256.hash(data: passwordData)
        let key: SymmetricKey = SymmetricKey(data: Data(hash.makeIterator()))
        
        let sealedBox: ChaChaPoly.SealedBox = try ChaChaPoly.SealedBox(combined: encKeyData)
        
        return try ChaChaPoly.open(sealedBox, using: key, authenticating: Data())
    }
}

/// Function to determine if the debugger is attached
///
/// **Note:** Only contains logic when `DEBUG` is defined, otherwise it always returns false
func isDebuggerAttached() -> Bool {
#if DEBUG
    var info = kinfo_proc()
    var size = MemoryLayout<kinfo_proc>.stride
    var mib: [Int32] = [CTL_KERN, KERN_PROC, KERN_PROC_PID, getpid()]
    let sysctlResult = sysctl(&mib, UInt32(mib.count), &info, &size, nil, 0)
    guard sysctlResult == 0 else { return false }
    return (info.kp_proc.p_flag & P_TRACED) != 0
#else
    return false
#endif
}