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102 lines
2.9 KiB
Swift
102 lines
2.9 KiB
Swift
import Dispatch
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enum Sealant<R> {
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case pending(Handlers<R>)
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case resolved(R)
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}
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final class Handlers<R> {
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var bodies: [(R) -> Void] = []
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func append(_ item: @escaping(R) -> Void) { bodies.append(item) }
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}
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/// - Remark: not protocol ∵ http://www.russbishop.net/swift-associated-types-cont
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class Box<T> {
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func inspect() -> Sealant<T> { fatalError() }
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func inspect(_: (Sealant<T>) -> Void) { fatalError() }
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func seal(_: T) {}
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}
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final class SealedBox<T>: Box<T> {
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let value: T
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init(value: T) {
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self.value = value
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}
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override func inspect() -> Sealant<T> {
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return .resolved(value)
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}
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}
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class EmptyBox<T>: Box<T> {
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private var sealant = Sealant<T>.pending(.init())
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private let barrier = DispatchQueue(label: "org.promisekit.barrier", attributes: .concurrent)
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override func seal(_ value: T) {
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var handlers: Handlers<T>!
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barrier.sync(flags: .barrier) {
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guard case .pending(let _handlers) = self.sealant else {
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return // already fulfilled!
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}
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handlers = _handlers
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self.sealant = .resolved(value)
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}
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//FIXME we are resolved so should `pipe(to:)` be called at this instant, “thens are called in order” would be invalid
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//NOTE we don’t do this in the above `sync` because that could potentially deadlock
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//THOUGH since `then` etc. typically invoke after a run-loop cycle, this issue is somewhat less severe
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if let handlers = handlers {
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handlers.bodies.forEach{ $0(value) }
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}
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//TODO solution is an unfortunate third state “sealed” where then's get added
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// to a separate handler pool for that state
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// any other solution has potential races
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}
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override func inspect() -> Sealant<T> {
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var rv: Sealant<T>!
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barrier.sync {
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rv = self.sealant
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}
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return rv
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}
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override func inspect(_ body: (Sealant<T>) -> Void) {
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var sealed = false
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barrier.sync(flags: .barrier) {
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switch sealant {
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case .pending:
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// body will append to handlers, so we must stay barrier’d
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body(sealant)
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case .resolved:
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sealed = true
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}
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}
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if sealed {
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// we do this outside the barrier to prevent potential deadlocks
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// it's safe because we never transition away from this state
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body(sealant)
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}
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}
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}
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extension Optional where Wrapped: DispatchQueue {
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@inline(__always)
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func async(flags: DispatchWorkItemFlags?, _ body: @escaping() -> Void) {
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switch self {
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case .none:
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body()
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case .some(let q):
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if let flags = flags {
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q.async(flags: flags, execute: body)
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} else {
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q.async(execute: body)
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}
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}
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}
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}
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