RUMM-1615 Allow launch time earlier than `UIApplicationDidBecomeActiv…

…eNotification`

Sources/Datadog/Core/System/LaunchTimeProvider.swift

@@ -14,15 +14,15 @@ import _Datadog_Private
 internal protocol LaunchTimeProviderType {
     /// The app process launch duration (in seconds) measured as the time from loading the first SDK object into memory
     /// to receiving `UIApplication.didBecomeActiveNotification` notification.
-    var launchTime: TimeInterval? { get }
+    var launchTime: TimeInterval { get }
 }
 
 internal class LaunchTimeProvider: LaunchTimeProviderType {
-    var launchTime: TimeInterval? {
+    var launchTime: TimeInterval {
         // Even if __dd_private_AppLaunchTime() is using a lock behind the scenes, TSAN will report a data race if there are no synchronizations at this level.
         objc_sync_enter(self)
         let time = __dd_private_AppLaunchTime()
         objc_sync_exit(self)
-        return time > 0 ? time : nil
+        return time
     }
 }

Sources/Datadog/RUM/RUMMonitor/Scopes/RUMViewScope.swift

@@ -311,7 +311,7 @@ internal class RUMViewScope: RUMScope, RUMContextProvider {
                 crash: nil,
                 error: nil,
                 id: dependencies.rumUUIDGenerator.generateUnique().toRUMDataFormat,
-                loadingTime: dependencies.launchTimeProvider.launchTime?.toInt64Nanoseconds,
+                loadingTime: dependencies.launchTimeProvider.launchTime.toInt64Nanoseconds,
                 longTask: nil,
                 resource: nil,
                 target: nil,

Sources/_Datadog_Private/ObjcAppLaunchHandler.m

@@ -14,6 +14,9 @@
 
 // A Read-Write lock to allow concurrent reads of TimeToApplicationDidBecomeActive, unless the initial (and only) write is locking it.
 static pthread_rwlock_t rwLock;
+// The framework load time  in seconds relative to the absolute reference date of Jan 1 2001 00:00:00 GMT.
+static NSTimeInterval FrameworkLoadTime = 0.0;
+// The time interval between the application starts and it's responsive and accepts touch events.
 static NSTimeInterval TimeToApplicationDidBecomeActive = 0.0;
 
 NS_INLINE NSTimeInterval QueryProcessStartTimeWithFallback(NSTimeInterval fallbackTime) {
@@ -41,14 +44,10 @@ NS_INLINE NSTimeInterval QueryProcessStartTimeWithFallback(NSTimeInterval fallba
     return processStartTime;
 }
 
-NS_INLINE void ComputeTimeToApplicationDidBecomeActiveWithFallback(NSTimeInterval fallbackTime) {
-    if (TimeToApplicationDidBecomeActive > 0) {
-        return;
-    }
-
+NS_INLINE NSTimeInterval ComputeProcessTimeFromStart() {
     NSTimeInterval now = CFAbsoluteTimeGetCurrent();
-    NSTimeInterval processStartTime = QueryProcessStartTimeWithFallback(fallbackTime);
-    TimeToApplicationDidBecomeActive = now - processStartTime;
+    NSTimeInterval processStartTime = QueryProcessStartTimeWithFallback(FrameworkLoadTime);
+    return now - processStartTime;
 }
 
 @interface AppLaunchHandler : NSObject
@@ -58,19 +57,21 @@ @implementation AppLaunchHandler
 
 + (void)load {
     // This is called at the `_Datadog_Private` load time, keep the work minimal
-    NSTimeInterval frameworkLoadTime = CFAbsoluteTimeGetCurrent();
-    id __block token = [NSNotificationCenter.defaultCenter
+    FrameworkLoadTime = CFAbsoluteTimeGetCurrent();
+
+    NSNotificationCenter * __weak center = NSNotificationCenter.defaultCenter;
+    id __block token = [center
                         addObserverForName:UIApplicationDidBecomeActiveNotification
                         object:nil
                         queue:NSOperationQueue.mainQueue
                         usingBlock:^(NSNotification *_){
 
         pthread_rwlock_init(&rwLock, NULL);
         pthread_rwlock_wrlock(&rwLock);
-        ComputeTimeToApplicationDidBecomeActiveWithFallback(frameworkLoadTime);
+        TimeToApplicationDidBecomeActive = ComputeProcessTimeFromStart();
         pthread_rwlock_unlock(&rwLock);
 
-        [NSNotificationCenter.defaultCenter removeObserver:token];
+        [center removeObserver:token];
     }];
 }
 
@@ -79,6 +80,7 @@ + (void)load {
 CFTimeInterval __dd_private_AppLaunchTime() {
     pthread_rwlock_rdlock(&rwLock);
     CFTimeInterval time = TimeToApplicationDidBecomeActive;
+    if (time == 0) time = ComputeProcessTimeFromStart();
     pthread_rwlock_unlock(&rwLock);
     return time;
 }

Sources/_Datadog_Private/include/ObjcAppLaunchHandler.h

@@ -6,4 +6,9 @@
 
 #import <CoreFoundation/CFDate.h>
 
+/// Returns the time interval between startup of the application process and the
+/// `UIApplicationDidBecomeActiveNotification`.
+///
+/// If the `UIApplicationDidBecomeActiveNotification` has not been reached yet,
+/// it returns  time interval between startup of the application process and now.
 CFTimeInterval __dd_private_AppLaunchTime(void);

Tests/DatadogTests/Datadog/Core/System/LaunchTimeProviderTests.swift

@@ -16,7 +16,7 @@ class LaunchTimeProviderTests: XCTestCase {
         var values: [TimeInterval] = []
         (0..<10).forEach { _ in
             Thread.sleep(forTimeInterval: 0.01)
-            values.append(provider.launchTime!)
+            values.append(provider.launchTime)
         }
 
         // Then

Tests/DatadogTests/Datadog/Mocks/CoreMocks.swift

@@ -625,7 +625,7 @@ class DateCorrectorMock: DateCorrectorType {
 }
 
 struct LaunchTimeProviderMock: LaunchTimeProviderType {
-    var launchTime: TimeInterval? = nil
+    var launchTime: TimeInterval = 0
 }
 
 extension UserInfo: AnyMockable, RandomMockable {