Bug 1767063 - make PerformanceEntry.mEpoch narrowing conversion expli…

…cit. r=sefeng

The converted value gets passed into TimingNotification which assigns the value
into a PerformanceEntry (this is the only usage). Since PerformanceEntry is
defined in WebIDL, we could not change its types (which is double for mEpoch) so
we were forced to safely convert the value where we did.

I don't think the existing conversion code had any bugs since we converted a
64-bit signed integer timestamp into uint64_t (safe) into a double (which is
safe for the reasons mentioned in the code comments).

Differential Revision: https://phabricator.services.mozilla.com/D145142

dom/performance/Performance.cpp

@@ -495,7 +495,7 @@ void Performance::LogEntry(PerformanceEntry* aEntry,
 
 void Performance::TimingNotification(PerformanceEntry* aEntry,
                                      const nsACString& aOwner,
-                                     uint64_t aEpoch) {
+                                     const double aEpoch) {
   PerformanceEntryEventInit init;
   init.mBubbles = false;
   init.mCancelable = false;

dom/performance/Performance.h

@@ -175,7 +175,7 @@ class Performance : public DOMEventTargetHelper {
 
   void LogEntry(PerformanceEntry* aEntry, const nsACString& aOwner) const;
   void TimingNotification(PerformanceEntry* aEntry, const nsACString& aOwner,
-                          uint64_t epoch);
+                          const double aEpoch);
 
   void RunNotificationObserversTask();
   void QueueEntry(PerformanceEntry* aEntry);

dom/performance/PerformanceMainThread.cpp

@@ -405,7 +405,7 @@ void PerformanceMainThread::InsertUserEntry(PerformanceEntry* aEntry) {
   MOZ_ASSERT(NS_IsMainThread());
 
   nsAutoCString uri;
-  uint64_t markCreationEpoch = 0;
+  double markCreationEpoch = 0;
 
   if (StaticPrefs::dom_performance_enable_user_timing_logging() ||
       StaticPrefs::dom_performance_enable_notify_performance_timing()) {
@@ -419,7 +419,11 @@ void PerformanceMainThread::InsertUserEntry(PerformanceEntry* aEntry) {
       // If we have no URI, just put in "none".
       uri.AssignLiteral("none");
     }
-    markCreationEpoch = static_cast<uint64_t>(PR_Now() / PR_USEC_PER_MSEC);
+
+    // PR_Now() returns a signed 64-bit integer. Since it represents a
+    // timestamp, only ~32-bits will represent the value which should safely fit
+    // into a double.
+    markCreationEpoch = static_cast<double>(PR_Now() / PR_USEC_PER_MSEC);
 
     if (StaticPrefs::dom_performance_enable_user_timing_logging()) {
       Performance::LogEntry(aEntry, uri);