[macOS] OS::GetCurrentMonotonicTicks should probably use `CLOCK_UPTIME_RAW`.

[knopp]

This would corresponds to other macOS API such as CACurrentMediaTime(). Otherwise we have a problem where DisplayLink callback gives target timestamp in based on CLOCK_UPTIME_RAW, but fml::TimePoint expects CLOCK_MONOTONIC_RAW.

Also the documentation for mach_absolute_time suggests clock_gettime_nsec_np(CLOCK_UPTIME_RAW) as suitable replacement and clock_gettime_nsec_np(CLOCK_UPTIME_RAW) is not mentioned in the prohibited boot time API list.

@rmacnak-google

[cbracken]

@rmacnak-google
For reference, the related patch that switched from mach_absolute_time to clock_gettime_nsec_No problem! which caused the issue:
https://dart-review.googlesource.com/c/sdk/+/348044

[rmacnak-google]

mach_absolute_time and clock_gettime_nsec_np(CLOCK_UPTIME_RAW) access the same clock and so have the same fingerprinting privacy implications.

Dart_TimelineGetTicks/Micros and Timeline.now do not promise any particular clock, only that it is suitable for measuring duration and for use with Dart_RecordTimelineEvent. If an application needs some particular clock, it should access that clock itself instead of assuming Dart timeline APIs use the same clock.

[knopp]

So does CACurrentMediaTime() and yet it's not mentioned in the document. I would assume there is a reason why apple singled out mach_absolute_time in the document and not mentioned either clock_gettime_nsec_np(CLOCK_UPTIME_RAW) or CACurrentMediaTime().

The issue here is that Dart_TimelineGetTicks is used in fml::TimePoint in Flutter, which right now leaks through the embedder API. I opened this issue because I think that maybe the original PR was overly cautious, but if that's not the case we'll need to handle this in the embedder.

[knopp]

I gave this another though - I think the correct solution is to not assume anything about the embedder API clock and always rebase the time, i.e. CACurrentMediaTime() -> FlutterEngineGetCurrentTime().

I still think that maybe CLOCK_MONOTONIC_RAW is bit too defensive since we're using CACurrentMediaTime() anyway in other places of engine and can't really get rid of it, but the engine API doesn't make any guarantees about the clock and embedder should handle that properly.

[a-siva]

@knopp is there anything actionable on the Dart side for this ?

[knopp]

I don't think so. I'll close the issue.