Created attachment 248333 [details] [review]
patch which adds the custom downstream sync event

Currently, rtpbin sends out RTP packets on a best-effort basis until it receives an RTCP SR packet. Once that happens, the bin is able to synchronize the packet timestamps. For downstream elements and applications, it is sometimes desirable to know when the SR packet was received (and therefore when rtpbin started to output with synchronization).

To accomplish this, this patch adds an option to rtpbin that enables a new custom in-band downstream event, called "GstRTCPSRReceived". This event is sent when the rtpbin receives an RTCP packet. (In-band, because out-of-band could result in the event arriving downstream before the synchronized buffers do.)

One example use case is when an application wishes to drop buffers until the rtpbin is synchronized. With this patch, the application can hook a probe to an RTP depayloader's src pad, and drop buffers until it sees the GstRTCPSRReceived event. Once this is done, the buffer switches from dropping to passing all buffers.

This is also useful to avoid synchronization problems with an audiosink's internal counters and timestamps.