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rtpmanager/rtsession: data race leading to critical warnings

François Laignel requested to merge fengalin/gstreamer:rtpsession-generate_rtcp-datarace into main

This is a fix for a data race leading to:

GLib-CRITICAL: g_hash_table_foreach: assertion 'version == hash_table->version' failed

Identified sequence:

  • rtp_session_on_timeout acquires the lock on session and proceeds with its processing.
  • rtp_session_process_rtcp is called (debug log : received RTCP packet) and attempts to acquire the lock on session, which is still held by rtp_session_on_timeout.
  • as part of an hash table iterator, rtp_session_on_timeout transitively invokes source_caps which releases the lock on session so as to call session->callbacks.caps.
  • Since rtp_session_process_rtcp was waiting for the lock to be released, it succeeds in acquiring it and proceeds with rtp_session_process_rr which transitively calls g_hash_table_insert via add_source.
  • After source_caps re-acquires the lock and gives the control flow back to rtp_session_on_timeout, the hash table iterator is changed, resulting in the assertion failure.

This commits copies sess->ssrcs[sess->mask_idx] and iterates on the copy so the iterator is not affected by a concurrent change due to the lock being released in the source_caps callback.

Fixes: #2536 (closed)

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