Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 235 insertions, 0 deletions

diff --git a/widget/SystemTimeConverter.h b/widget/SystemTimeConverter.h
new file mode 100644
index 0000000000..4a074d573f
--- /dev/null
+++ b/widget/SystemTimeConverter.h
@@ -0,0 +1,235 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef SystemTimeConverter_h
+#define SystemTimeConverter_h
+
+#include <limits>
+#include <type_traits>
+#include "mozilla/TimeStamp.h"
+
+namespace mozilla {
+
+// Utility class that converts time values represented as an unsigned integral
+// number of milliseconds from one time source (e.g. a native event time) to
+// corresponding mozilla::TimeStamp objects.
+//
+// This class handles wrapping of integer values and skew between the time
+// source and mozilla::TimeStamp values.
+//
+// It does this by using an historical reference time recorded in both time
+// scales (i.e. both as a numerical time value and as a TimeStamp).
+//
+// For performance reasons, this class is careful to minimize calls to the
+// native "current time" function (e.g. gdk_x11_server_get_time) since this can
+// be slow.
+template <typename Time, typename TimeStampNowProvider = TimeStamp>
+class SystemTimeConverter {
+ public:
+  SystemTimeConverter()
+      : mReferenceTime(Time(0)),
+        mReferenceTimeStamp()  // Initializes to the null timestamp
+        ,
+        mLastBackwardsSkewCheck(Time(0)),
+        kTimeRange(std::numeric_limits<Time>::max()),
+        kTimeHalfRange(kTimeRange / 2),
+        kBackwardsSkewCheckInterval(Time(2000)) {
+    static_assert(!std::is_signed_v<Time>, "Expected Time to be unsigned");
+  }
+
+  template <typename CurrentTimeGetter>
+  mozilla::TimeStamp GetTimeStampFromSystemTime(
+      Time aTime, CurrentTimeGetter& aCurrentTimeGetter) {
+    TimeStamp roughlyNow = TimeStampNowProvider::Now();
+
+    // If the reference time is not set, use the current time value to fill
+    // it in.
+    if (mReferenceTimeStamp.IsNull()) {
+      // This sometimes happens when ::GetMessageTime returns 0 for the first
+      // message on Windows.
+      if (!aTime) return roughlyNow;
+      UpdateReferenceTime(aTime, aCurrentTimeGetter);
+    }
+
+    // Check for skew between the source of Time values and TimeStamp values.
+    // We do this by comparing two durations (both in ms):
+    //
+    // i.  The duration from the reference time to the passed-in time.
+    //     (timeDelta in the diagram below)
+    // ii. The duration from the reference timestamp to the current time
+    //     based on TimeStamp::Now.
+    //     (timeStampDelta in the diagram below)
+    //
+    // Normally, we'd expect (ii) to be slightly larger than (i) to account
+    // for the time taken between generating the event and processing it.
+    //
+    // If (ii) - (i) is negative then the source of Time values is getting
+    // "ahead" of TimeStamp. We call this "forwards" skew below.
+    //
+    // For the reverse case, if (ii) - (i) is positive (and greater than some
+    // tolerance factor), then we may have "backwards" skew. This is often
+    // the case when we have a backlog of events and by the time we process
+    // them, the time given by the system is comparatively "old".
+    //
+    // The IsNewerThanTimestamp function computes the equivalent of |aTime| in
+    // the TimeStamp scale and returns that in |timeAsTimeStamp|.
+    //
+    // Graphically:
+    //
+    //                    mReferenceTime              aTime
+    // Time scale:      ........+.......................*........
+    //                          |--------timeDelta------|
+    //
+    //                  mReferenceTimeStamp             roughlyNow
+    // TimeStamp scale: ........+...........................*....
+    //                          |------timeStampDelta-------|
+    //
+    //                                                  |---|
+    //                                         roughlyNow-timeAsTimeStamp
+    //
+    TimeStamp timeAsTimeStamp;
+    bool newer = IsTimeNewerThanTimestamp(aTime, roughlyNow, &timeAsTimeStamp);
+
+    // Tolerance when detecting clock skew.
+    static const TimeDuration kTolerance = TimeDuration::FromMilliseconds(30.0);
+
+    // Check for forwards skew
+    if (newer) {
+      // Make aTime correspond to roughlyNow
+      UpdateReferenceTime(aTime, roughlyNow);
+
+      // We didn't have backwards skew so don't bother checking for
+      // backwards skew again for a little while.
+      mLastBackwardsSkewCheck = aTime;
+
+      return roughlyNow;
+    }
+
+    if (roughlyNow - timeAsTimeStamp <= kTolerance) {
+      // If the time between event times and TimeStamp values is within
+      // the tolerance then assume we don't have clock skew so we can
+      // avoid checking for backwards skew for a while.
+      mLastBackwardsSkewCheck = aTime;
+    } else if (aTime - mLastBackwardsSkewCheck > kBackwardsSkewCheckInterval) {
+      aCurrentTimeGetter.GetTimeAsyncForPossibleBackwardsSkew(roughlyNow);
+      mLastBackwardsSkewCheck = aTime;
+    }
+
+    // Finally, calculate the timestamp
+    return timeAsTimeStamp;
+  }
+
+  void CompensateForBackwardsSkew(Time aReferenceTime,
+                                  const TimeStamp& aLowerBound) {
+    // Check if we actually have backwards skew. Backwards skew looks like
+    // the following:
+    //
+    //        mReferenceTime
+    // Time:      ..+...a...b...c..........................
+    //
+    //     mReferenceTimeStamp
+    // TimeStamp: ..+.....a.....b.....c....................
+    //
+    // Converted
+    // time:      ......a'..b'..c'.........................
+    //
+    // What we need to do is bring mReferenceTime "forwards".
+    //
+    // Suppose when we get (c), we detect possible backwards skew and trigger
+    // an async request for the current time (which is passed in here as
+    // aReferenceTime).
+    //
+    // We end up with something like the following:
+    //
+    //        mReferenceTime     aReferenceTime
+    // Time:      ..+...a...b...c...v......................
+    //
+    //     mReferenceTimeStamp
+    // TimeStamp: ..+.....a.....b.....c..........x.........
+    //                                ^          ^
+    //                          aLowerBound  TimeStamp::Now()
+    //
+    // If the duration (aLowerBound - mReferenceTimeStamp) is greater than
+    // (aReferenceTime - mReferenceTime) then we know we have backwards skew.
+    //
+    // If that's not the case, then we probably just got caught behind
+    // temporarily.
+    if (IsTimeNewerThanTimestamp(aReferenceTime, aLowerBound, nullptr)) {
+      return;
+    }
+
+    // We have backwards skew; the equivalent TimeStamp for aReferenceTime lies
+    // somewhere between aLowerBound (which was the TimeStamp when we triggered
+    // the async request for the current time) and TimeStamp::Now().
+    //
+    // If aReferenceTime was waiting in the event queue for a long time, the
+    // equivalent TimeStamp might be much closer to aLowerBound than
+    // TimeStamp::Now() so for now we just set it to aLowerBound. That's
+    // guaranteed to be at least somewhat of an improvement.
+    UpdateReferenceTime(aReferenceTime, aLowerBound);
+  }
+
+ private:
+  template <typename CurrentTimeGetter>
+  void UpdateReferenceTime(Time aReferenceTime,
+                           const CurrentTimeGetter& aCurrentTimeGetter) {
+    Time currentTime = aCurrentTimeGetter.GetCurrentTime();
+    TimeStamp currentTimeStamp = TimeStampNowProvider::Now();
+    Time timeSinceReference = currentTime - aReferenceTime;
+    TimeStamp referenceTimeStamp =
+        currentTimeStamp - TimeDuration::FromMilliseconds(timeSinceReference);
+    UpdateReferenceTime(aReferenceTime, referenceTimeStamp);
+  }
+
+  void UpdateReferenceTime(Time aReferenceTime,
+                           const TimeStamp& aReferenceTimeStamp) {
+    mReferenceTime = aReferenceTime;
+    mReferenceTimeStamp = aReferenceTimeStamp;
+  }
+
+  bool IsTimeNewerThanTimestamp(Time aTime, TimeStamp aTimeStamp,
+                                TimeStamp* aTimeAsTimeStamp) {
+    Time timeDelta = aTime - mReferenceTime;
+
+    // Cast the result to signed 64-bit integer first since that should be
+    // enough to hold the range of values returned by ToMilliseconds() and
+    // the result of converting from double to an integer-type when the value
+    // is outside the integer range is undefined.
+    // Then we do an implicit cast to Time (typically an unsigned 32-bit
+    // integer) which wraps times outside that range.
+    TimeDuration timeStampDelta = (aTimeStamp - mReferenceTimeStamp);
+    int64_t wholeMillis = static_cast<int64_t>(timeStampDelta.ToMilliseconds());
+    Time wrappedTimeStampDelta = wholeMillis;  // truncate to unsigned
+
+    Time timeToTimeStamp = wrappedTimeStampDelta - timeDelta;
+    bool isNewer = false;
+    if (timeToTimeStamp == 0) {
+      // wholeMillis needs no adjustment
+    } else if (timeToTimeStamp < kTimeHalfRange) {
+      wholeMillis -= timeToTimeStamp;
+    } else {
+      isNewer = true;
+      wholeMillis += (-timeToTimeStamp);
+    }
+    if (aTimeAsTimeStamp) {
+      *aTimeAsTimeStamp =
+          mReferenceTimeStamp + TimeDuration::FromMilliseconds(wholeMillis);
+    }
+
+    return isNewer;
+  }
+
+  Time mReferenceTime;
+  TimeStamp mReferenceTimeStamp;
+  Time mLastBackwardsSkewCheck;
+
+  const Time kTimeRange;
+  const Time kTimeHalfRange;
+  const Time kBackwardsSkewCheckInterval;
+};
+
+}  // namespace mozilla
+
+#endif /* SystemTimeConverter_h */