DatabaseWriter.concurrentRead

[groue]

This pull requests improves the DatabaseWriter protocol by deprecating the readFromCurrentState method, and introducing concurrentRead as a replacement.

protocol DatabaseWriter {
    func concurrentRead<T>(_ block: @escaping (Database) throws -> T) -> Future<T>
}

The problem with the deprecated readFromCurrentState is that it is a synchronous DatabaseQueue method, and an asynchronous DatabasePool method. This makes writing code that target both classes difficult.

concurrentRead, instead, returns a Future value that client code can wait until the concurrent fetch is completed.

concurrentRead aims at helping database observation tools like FetchedRecordsController and RxGRDB: it fetches values from a known database state (right after a transaction has been committed), without blocking the database writer dispatch queue longer than necessary. DatabasePool.concurrentRead, especially, only blocks until snapshot isolation has been established in a concurrent reader.

try writer.writeWithoutTransaction { db in
    // Delete all players
    try Player.deleteAll()
    
    // Count players concurrently
    let future = writer.concurrentRead { db in
        return try Player.fetchCount()
    }
    
    // Insert a player
    try Player(...).insert(db)
    
    // Guaranteed to be zero
    let count = try future.wait()
}