Mark Operator Closures as @Sendable to Prevent Crashes in Swift 6 Isolated Contexts

[AndreiArdelean1]

Short description of the issue:

In Swift 6, closures created in an isolated context automatically inherit the isolation of that context, unless they are marked as @Sendable. Because of this, creating an operation like map, flatMap, distinctUntilChanged, etc. from an isolated context, but calling it from a different thread, causes the call to crash.

Expected outcome:

No crash should happen even without manually marking the closures as @Sendable. All closures passed to operators that could be executed on another thread, should be marked as @Sendable inside the RxSwift framework.

What actually happens:

Unless manually specifying that the closure is Sendable, the code crashes

@preconcurrency import RxSwift

@MainActor
func test() -> any Disposable {
    return Observable<Int>.just(1)
        // any non main thread scheduler
        .observe(on: RxSwift.SerialDispatchQueueScheduler(qos: .background))
        .map({ $0 + 1 })
        .subscribe()
}

Workaround:

@preconcurrency import RxSwift

@MainActor
func test() -> any Disposable {
    return Observable<Int>.just(1)
        // any non main thread scheduler
        .observe(on: RxSwift.SerialDispatchQueueScheduler(qos: .background))
        .map({ @Sendable in $0 + 1 })
        .subscribe()
}

Example fix (Map.swift):

    public func map<Result>(_ transform: @escaping (Element) throws -> Result)
        -> Observable<Result> {

should be:

    public func map<Result>(_ transform: @escaping @Sendable (Element) throws -> Result)
        -> Observable<Result> {

Proposed fix:

• do a find and replace for @escaping and replace it with @escaping @Sendable, as most closures that are escaping should also be Sendable
• make needed classes to conform to the @unchecked Sendable protocol. Most classes already are thread-safe but are not marked as Sendable

RxSwift/RxCocoa/RxBlocking/RxTest version/commit

6.8.0

Platform/Environment

• iOS
• macOS
• tvOS
• watchOS
• playgrounds

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

• easy, 100% repro
• sometimes, 10%-100%
• hard, 2% - 10%
• extremely hard, %0 - 2%

Xcode version:

16.0

⚠️ Fields below are optional for general issues or in case those questions aren't related to your issue, but filling them out will increase the chances of getting your issue resolved. ⚠️

Installation method:

• CocoaPods
• Carthage
• Git submodules

I have multiple versions of Xcode installed:
(so we can know if this is a potential cause of your issue)

• yes (which ones)
• no

Level of RxSwift knowledge:
(this is so we can understand your level of knowledge
and formulate the response in an appropriate manner)

• just starting
• I have a small code base
• I have a significant code base

[danielt1263]

@AndreiArdelean1 Can you submit a PR to that effect?

[AndreiArdelean1]

@danielt1263 I've created pull request #2639 with the changes