아이덴티티는 저장소다

    /// One data store per identity, named by the identity's id: its cookies
    /// and site storage are in no other identity's reach.
    func store(for identity: IdentityID) -> WKWebsiteDataStore {
        if let store = stores[identity] { return store }
        // The private identity's store lives in memory for this run only.
        let store = identity == .privateSession ? WKWebsiteDataStore.nonPersistent() : WKWebsiteDataStore(forIdentifier: identity.raw)
        stores[identity] = store
        if identity != .privateSession {
            // The cookie store holds its observers weakly; the engine keeps
            // the watcher, and the watcher keeps no store.
            let watcher = CookieWatcher(identity: identity, engine: self)
            store.httpCookieStore.add(watcher)
            cookieWatchers[identity] = watcher
        }
        return store
    }

    @Test func twoIdentitiesShareNothing() async throws {
        let a = IdentityID(), b = IdentityID()

        // The pages live inside this function: a store can be removed only
        // once no web view of its identity exists.
        func exercise() async throws {
            let pageA = makePage(identity: a)
            let pageB = makePage(identity: b)
            let pageA2 = makePage(identity: a)
            for page in [pageA, pageB, pageA2] { await secure(page, "<p>store</p>") }

            let wrote = await js(pageA, "localStorage.setItem('who', 'identity-a'); document.cookie = 'who=identity-a; path=/'; return localStorage.getItem('who') + '|' + document.cookie")
            #expect(wrote == "identity-a|who=identity-a")
            // A second page of the same identity sees both; it is another
            // process, so give the store a moment to hand them over.
            var sameIdentity = ""
            for _ in 0..<60 {
                sameIdentity = await js(pageA2, "return String(localStorage.getItem('who')) + '|' + document.cookie")
                if sameIdentity == "identity-a|who=identity-a" { break }
                try await Task.sleep(for: .milliseconds(50))
            }
            #expect(sameIdentity == "identity-a|who=identity-a")
            let otherIdentity = await js(pageB, "return String(localStorage.getItem('who')) + '|' + document.cookie")
            #expect(otherIdentity == "null|")
            for page in [pageA, pageB, pageA2] { page.close() }
        }
        try await exercise()