rtpsession: fix a race condition during the EOS event in gstrtpsession.c.

rtpsession: fix a race condition during the EOS event in gstrtpsession.c.

This patch prevents a possible race condition from taking place between the EOS event handling and rtcp send function/thread.

The condition starts by getting the GST_EVENT_EOS event on the send_rtp_sink pad, which causes two core things to happen -- the event gets pushed down to the send_rtp_src pad and all sessions get marked "bye" prior to completion of the event handler. In another thread the rtp_session_on_timeout function gets called after an expiration of gst_clock_id_wait in the rtcp_thread function. This results in a call to ess->callbacks.send_rtcp(), which is configured as a function pointer to gst_rtp_session_send_rtcp via the RTPSessionCallbacks structure passed to rtp_session_set_callbacks in the gst_rtp_session_init function.

In the race condition, the call to gst_rtp_session_send_rtcp can have the all_sources_bye boolean set to true while GST_PAD_IS_EOS(rtpsession->send_rtp_sink) evaluates to false. This is the result of gst_rtp_session_send_rtcp running before the send_rtp_sink's GST_EVENT_EOS handler completes. The exact point at which this condition occurs is if there's a context switch to the rtcp_thread right after the call to rtp_session_mark_all_bye in the GET_EVENT_EOS handler, but before the handler returns.

Normally, this would not be an issue because the rtcp_thread continues to run and indirectly call gst_rtp_session_send_rtcp. However, the call to rtp_source_reset sets the sent_bye boolean to false, which ends up causing rtp_session_are_all_sources_bye to return false. This gets passed to gst_rtp_session_send_rtcp and the EOS event never gets sent.

The race condition results in the EOS event never getting passed to the rtcp_src pad, which prevents the bin and pipeline from ever completing with EOS.