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We currently block the loading thread until the calculated load
time has past and then unblock again by a message sent from the
playback thread. However, because the loading thread itself is not
using a Looper and runs freely, we don't control when the short
calculations on the loader thread that determine how long we have
to wait are happening, and we also don't control how long it takes
to start and stop this thread.
To solve these problems and to make the playback deterministic we
can
1. Send a message on the playback thread to block until the loader
thread has started.
2. Block the playback thread whenever a loading thread is doing its
short calculation of wait times. The playback thread knows when it
can continue because loading either enter a new waiting state for
a simulated load time or loading is finished completely.
3. Also wait on the playback thread until the loader has shut down.
As this is waiting for a message on the playback thread, we can
achieve this by sending messages to ourselves at the current time
until the loader is shut down.
All 3 steps together ensure that the loading thread interaction is
compeltely deterministic when simulating bandwidth profiles with the
BandwidthProfileDataSource. As we need to notify the source before and
after the load started/finished, we also need a small wrapper for the
chunk source when running the playback.
PiperOrigin-RevId: 355810408