reportFullDisplay finishing the wrong span

[brustolin]

Description

There is a problem with the reportFullDisplay API:

• ViewControllerA opens, starts a HTTP request, user clicks a button and opens ViewControllerB.
• ViewControllerB starts an HTTP request.
• ViewControllerA’s HTTP request ends and SentrySDK.reportFullyDisplayed is called.

This will finish TTFD for ViewControllerB, but it is still waiting for its request to respond.

Test inside SentryUIViewControllerPerformanceTrackerTests.swift from #4531 to reproduce the problem:

func test_waitForFullDisplay_NewViewControllerLoaded_BeforeReportTTFD_withBackgroundWork() throws {
    class CustomVC: UIViewController {
        var workSpan: Span?

        override func viewDidAppear(_ animated: Bool) {
            super.viewDidAppear(animated)
            //Start some work, it could be a http request
            workSpan = SentrySDK.span?.startChild(operation: "Work")
        }

        // We will use a function to finish the work
        // but it could be the request response
        func finishWork() {
            workSpan?.finish()
            SentrySDK.reportFullyDisplayed()
        }
    }

    let dateProvider = fixture.dateProvider
    let sut = fixture.getSut()
    let tracker = fixture.tracker
    let firstController = CustomVC()
    let secondController = CustomVC()

    var firstTracer: SentryTracer?
    var secondTracer: SentryTracer?

    sut.enableWaitForFullDisplay = true

    dateProvider.advance(by: 1)
    sut.viewControllerLoadView(firstController) {
        firstController.loadView()
        firstTracer = self.getStack(tracker).first as? SentryTracer
    }

    sut.viewControllerViewDidLoad(firstController) {
        firstController.viewDidLoad()
    }

    dateProvider.advance(by: 1)
    sut.viewControllerViewWillAppear(firstController) {
        firstController.viewWillAppear(false)
    }

    sut.viewControllerViewDidAppear(firstController) {
        firstController.viewDidAppear(false)
    }

    fixture.displayLinkWrapper.normalFrame()

    let firstFullDisplaySpan = try XCTUnwrap(firstTracer?.children.first { $0.operation == "ui.load.full_display" })

    XCTAssertFalse(firstFullDisplaySpan.isFinished)

    XCTAssertEqual(firstTracer?.traceId, SentrySDK.span?.traceId)

    dateProvider.advance(by: 1)
    sut.viewControllerLoadView(secondController) {
        secondController.loadView()
        secondTracer = self.getStack(tracker).first as? SentryTracer
    }

    // The work of the first controller asynchronously ended here, and a new frame was rendered
    // while the second controller hasn’t appeared yet.
    dateProvider.advance(by: 1)
    firstController.finishWork()
    fixture.displayLinkWrapper.normalFrame()

    dateProvider.advance(by: 1)
    sut.viewControllerViewWillAppear(secondController) {
        secondController.viewWillAppear(false)
    }
    sut.viewControllerViewDidAppear(secondController) {
        secondController.viewDidAppear(false)
    }
    fixture.displayLinkWrapper.normalFrame()

    dateProvider.advance(by: 1)
    let timeOfSecondControllerFinishWork = dateProvider.date()
    secondController.finishWork()

    fixture.displayLinkWrapper.normalFrame()

    XCTAssertTrue(firstFullDisplaySpan.isFinished)
    XCTAssertEqual(.deadlineExceeded, firstFullDisplaySpan.status)

    let secondFullDisplaySpan = try XCTUnwrap(secondTracer?.children.first { $0.operation == "ui.load.full_display" })

    XCTAssertEqual(secondFullDisplaySpan.timestamp, timeOfSecondControllerFinishWork)
}