https://stackoverflow.com/q/79557802/1271826

TokenManager.swift

private var tokenPub: AnyPublisher<String, Error>?       // make this private to prevent unintended direct access to this publisher … you want to make sure that all access is via your private queue
private let tokenPubQueue = DispatchQueue(               // make this `private`, as it is internal to this type
    label: Bundle.main.bundleIdentifier! + ".tokenQueue"
)

func accessToken(refreshToken: String) -> AnyPublisher<String, Error> {
    return tokenPubQueue.sync {
        if let tokenPub {
            return tokenPub
        }

        let publisher = API.client.publisher(refreshToken)
            .receive(on: tokenPubQueue)
            .handleEvents(receiveCompletion: { [weak self] completion in
                guard let self else { return }

                switch completion {
                case .finished, .failure:
                    self.tokenPub = nil
                }
            })
            .share()
            .eraseToAnyPublisher()

        tokenPub = publisher

        return publisher
    }
}

Also, while I was here, I'd recommend

• giving your synchronization queue a meaningful name that captures the functional purpose of the queue; and
• use if let tokenPub {…} and guard let self else { return } for more concise safe unwrapping of the optionals.