Plan: Restrict RtpTransceiver sender/receiver access to PLAN_B_ONLY

Objective

Restrict access to RtpTransceiver::senders_ and RtpTransceiver::receivers_ such that access to all elements except the first one is only possible via functions marked PLAN_B_ONLY. This aligns the implementation with Unified Plan requirements where only one sender and one receiver are used, while preserving legacy Plan B functionality.

Proposed Changes

1. Header Refactoring (pc/rtp_transceiver.h)

• Deprecate Vector Accessors: Mark senders() and receivers() with the PLAN_B_ONLY macro. This triggers deprecation warnings for any code accessing the full lists, making Plan B dependencies explicit.
• Add Unified Plan Accessors:
  • Add internal_first_sender() and internal_first_receiver() private helpers.
  • These helpers will return the first element (or nullptr) and will be used by Unified Plan paths.

2. Implementation Update (pc/rtp_transceiver.cc)

• Shared Logic Branching: Methods that currently iterate over senders_ or receivers_ (e.g., SetMediaChannels, GetStopSendingAndReceiving, GetDeleteChannelWorkerTask, OnNegotiationUpdate) must be updated:
  • Unified Plan: Use internal_first_... helpers to process only the primary element.
  • Plan B: Wrap iterations in RTC_ALLOW_PLAN_B_DEPRECATION_BEGIN and RTC_ALLOW_PLAN_B_DEPRECATION_END.
• Protect Index Access: Ensure senders_[0] or receivers_[0] are only used in Unified Plan paths (guarded by RTC_DCHECK(unified_plan_)).

3. External Caller Migration

• Update components that iterate over senders/receivers:
  • pc/legacy_stats_collector.cc
  • pc/peer_connection.cc
  • pc/rtc_stats_collector.cc
  • pc/rtp_transmission_manager.cc
• Wrap these iterations in RTC_ALLOW_PLAN_B_DEPRECATION_BEGIN/END to acknowledge the legacy dependency.

Verification Plan

• Compilation: Ensure all targets compile with and without WEBRTC_DEPRECATE_PLAN_B defined.
• Unit Tests: Run pc_unittests and peerconnection_unittests to ensure no regression in both Unified Plan and Plan B modes.