zeus
69 lines
2.7 KiB
Swift
69 lines
2.7 KiB
Swift
import Dispatch
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/// SchedulingWatchdog makes sure that databases connections are used on correct
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/// dispatch queues, and warns the user with a fatal error whenever she misuses
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/// a database connection.
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///
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/// Generally speaking, each connection has its own dispatch queue. But it's not
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/// enough: users need to use two database connections at the same time:
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/// <https://github.com/groue/GRDB.swift/issues/55>. To support this use case, a
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/// single dispatch queue can be temporarily shared by two or more connections.
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///
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/// - SchedulingWatchdog.makeSerializedQueue(allowingDatabase:) creates a
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/// dispatch queue that allows one database.
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///
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/// It does so by registering one instance of SchedulingWatchdog as a specific
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/// of the dispatch queue, a SchedulingWatchdog that allows that database only.
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///
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/// Later on, the queue can be shared by several databases with the method
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/// inheritingAllowedDatabases(from:execute:). See SerializedDatabase.sync()
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/// for an example.
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///
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/// - preconditionValidQueue() crashes whenever a database is used in an invalid
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/// dispatch queue.
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final class SchedulingWatchdog {
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private static let watchDogKey = DispatchSpecificKey<SchedulingWatchdog>()
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private(set) var allowedDatabases: [Database]
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var databaseObservationBroker: DatabaseObservationBroker?
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private init(allowedDatabase database: Database) {
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allowedDatabases = [database]
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}
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static func allowDatabase(_ database: Database, onQueue queue: DispatchQueue) {
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precondition(queue.getSpecific(key: watchDogKey) == nil)
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let watchdog = SchedulingWatchdog(allowedDatabase: database)
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queue.setSpecific(key: watchDogKey, value: watchdog)
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}
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func inheritingAllowedDatabases<T>(from other: SchedulingWatchdog, execute body: () throws -> T) rethrows -> T {
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let backup = allowedDatabases
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allowedDatabases.append(contentsOf: other.allowedDatabases)
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defer { allowedDatabases = backup }
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return try body()
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}
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static func preconditionValidQueue(
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_ db: Database,
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_ message: @autoclosure() -> String = "Database was not used on the correct thread.",
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file: StaticString = #file,
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line: UInt = #line)
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{
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GRDBPrecondition(allows(db), message(), file: file, line: line)
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}
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/// Returns whether the database argument can be used in the current
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/// dispatch queue.
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static func allows(_ db: Database) -> Bool {
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current?.allows(db) ?? false
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}
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static var current: SchedulingWatchdog? {
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DispatchQueue.getSpecific(key: watchDogKey)
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}
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func allows(_ db: Database) -> Bool {
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allowedDatabases.contains { $0 === db }
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}
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}
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