Fix startup fatal hangs not persisting to disk in UI tests

PerformanceSuite/PerformanceApp/MetricsConsumer.swift

@@ -61,7 +61,8 @@ class MetricsConsumer: PerformanceSuiteMetricsReceiver {
 
     func fatalHangReceived(info: HangInfo) {
         log("fatalHangReceived \(info)")
-        interop?.send(message: Message.fatalHang)
+        let message: Message = info.duringStartup ? .startupFatalHang : .fatalHang
+        interop?.send(message: message)
     }
 
     func nonFatalHangReceived(info: HangInfo) {

PerformanceSuite/PerformanceApp/RootController.swift

@@ -14,6 +14,12 @@ class RootController: UIHostingController<MenuView> {
 
         // simulate long startup time
         Thread.sleep(forTimeInterval: 2)
+
+        // For UI tests: simulate a fatal hang during startup if requested
+        if ProcessInfo.processInfo.environment[startupFatalHangKey] != nil {
+            // Block the main thread indefinitely to simulate a fatal hang during startup
+            Thread.sleep(forTimeInterval: .infinity)
+        }
     }
 
     @MainActor @objc required dynamic init?(coder aDecoder: NSCoder) {

PerformanceSuite/PerformanceApp/UITestsInterop.swift

@@ -12,6 +12,7 @@ import GCDWebServer
 
 public let inTestsKey = "UI_TESTS"
 public let clearStorageKey = "CLEAR_STORAGE"
+public let startupFatalHangKey = "STARTUP_FATAL_HANG"
 
 
 /// Message which is sent from the app to UI tests target
@@ -23,6 +24,7 @@ public enum Message: Codable, Equatable {
     case fragmentTTI(duration: Int, fragment: String)
     case hangStarted
     case fatalHang
+    case startupFatalHang
     case nonFatalHang
     case watchdogTermination
     case memoryLeak
@@ -34,6 +36,7 @@ public enum Message: Codable, Equatable {
             (.appFreezeTime, .appFreezeTime),
             (.hangStarted, .hangStarted),
             (.fatalHang, .fatalHang),
+            (.startupFatalHang, .startupFatalHang),
             (.nonFatalHang, .nonFatalHang),
             (.watchdogTermination, .watchdogTermination),
             (.memoryLeak, .memoryLeak),

PerformanceSuite/Sources/Utils/Storage.swift

@@ -72,5 +72,6 @@ extension UserDefaults: Storage {
 
     public func write(domain: String, key: String, value: String?) {
         set(value, forKey: defaultsKey(domain: domain, key: key))
+        synchronize()
     }
 }

PerformanceSuite/UITests/TerminationTests.swift

@@ -79,4 +79,31 @@ final class TerminationTests: BaseTests {
         app.staticTexts["Memory Leak"].tap()
         waitForMessage { $0 == .memoryLeak }
     }
+
+    func testStartupFatalHang() throws {
+        // First launch: clear storage
+        performFirstLaunch()
+        assertNoMessages(.fatalHang, .startupFatalHang, .hangStarted)
+
+        // Second launch: trigger startup fatal hang
+        app.terminate()
+        app.launchEnvironment = [inTestsKey: "1", startupFatalHangKey: "1"]
+        app.launch()
+
+        // Wait for hang to be detected (hang detection takes time)
+        waitForTimeout(5)
+
+        // Kill the app during the hang (simulating system kill or user force quit)
+        // Note: We may or may not receive hangStarted before termination, so we don't check for it
+        app.terminate()
+
+        // Third launch: should receive startup fatal hang callback (duringStartup=true)
+        app.launchEnvironment = [inTestsKey: "1"]
+        app.launch()
+
+        waitForMessage { $0 == .startupFatalHang }
+        // The startup fatal hang was successfully detected and reported!
+        assertNoMessages(.crash, .nonFatalHang, .fatalHang)
+        // Note: watchdogTermination might also be reported, which is acceptable
+    }
 }