Particle system: fix infinite loop after running for many hours

The infinite loop was triggered by several issues coincide together.

In short, the direct cause is the particle's born time and lifespan were
represented in 32 bit floats and not precise enough to pass aliveness check as
time grows large.

While the time grows large, the resolution of floating point decreases to the
extent that resolution is even bigger than 2 milliseconds.
Then it will fail to pass the aliveness check. Then, the
dead particles will be treated alive and they are kept inserting into and
popping out of the particles heap, which is similar to a live-lock.

The fix is to separate freeing dead and inserting back alive ones in two
different loops, ensure that the emitter can update time for next frame.

There are still other issues:
1) as the times runs very long, the particle needs several frames's updates
to actually make the states change noticeable, which means animation may
become not so smooth after running for too long (like several days).
May change particle's born/lifespan time to 64 bits in another patch.

2) the particle system's and animation's timers are 32 bit integers,
after 2^31 milliseconds(24.8551348 days), they will overflow. May promote
them to 64 bits in another patch.

3) as the time grows even larger such that the resolution is bigger than 16ms
at 60 hz frame rate, the live-lock may occur again. Because the timer advances/delta
will be not large enough to make dead ones reused.
The next live-lock estimated time is 2^24*16 milliseconds = 3.10689185 days.
The final fixes are 1) and 2)

4) may change the particle system's internal timer be set to arbitrary value
(fast forward to large value) for easier writing autotest for above cases.

Change-Id: I1190c0814c8197876b26dd4182dc4b065dd1ece6
Task-number: QTBUG-64138
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/particles/qquickparticlesystem_p.h b/src/particles/qquickparticlesystem_p.h
index 1e34086b0c..c602183e74 100644
--- a/src/particles/qquickparticlesystem_p.h
+++ b/src/particles/qquickparticlesystem_p.h
@@ -227,6 +227,8 @@ public:
 private:
     int m_size;
     QQuickParticleSystem* m_system;
+    // Only used in recycle() for tracking of alive particles after latest recycling round
+    QVector<QQuickParticleData*> m_latestAliveParticles;
 };
 
 struct Color4ub {