# Plan B Deprecation and Removal This document describes the tooling and process for deprecating and eventually removing Plan B SDP semantics from the WebRTC codebase. ## Build Configuration To support a gradual deprecation, a new GN argument and C++ macro have been introduced. ### GN Argument: `deprecate_plan_b` Defined in `webrtc.gni`, this argument controls whether Plan B specific APIs are marked as deprecated at compile time. - **`false` (default):** Plan B APIs are available without warnings. - **`true`:** Plan B APIs are annotated with `[[deprecated]]`. In WebRTC's default build configuration (which treats warnings as errors), this will cause compilation to fail if any Plan B APIs are used. To enable this in your local build, add the following to your `args.gn`: ```gn deprecate_plan_b = true ``` ### C++ Macro: `PLAN_B_ONLY` Defined in `rtc_base/system/plan_b_only.h`, this macro should be used to annotate functions, classes, or variables that are only needed for Plan B semantics. ```cpp #include "rtc_base/system/plan_b_only.h" PLAN_B_ONLY void MyPlanBSpecificFunction(); ``` - When `deprecate_plan_b` is `true`, `PLAN_B_ONLY` expands to `[[deprecated]]`. - When `deprecate_plan_b` is `false`, `PLAN_B_ONLY` expands to an empty string. ### Suppression Macros To allow existing Plan B code to compile when `deprecate_plan_b = true`, the following macros are provided in `rtc_base/system/plan_b_only.h`: - `RTC_ALLOW_PLAN_B_DEPRECATION_BEGIN()` - `RTC_ALLOW_PLAN_B_DEPRECATION_END()` These macros use compiler pragmas to locally disable deprecation warnings. ## Usage Guidelines ### Annotating New Plan B Code Avoid adding new Plan B code. If it is absolutely necessary, ensure it is annotated with the `PLAN_B_ONLY` macro. ### Identifying Plan B Dependencies By setting `deprecate_plan_b = true`, developers can identify all call sites that still rely on Plan B. This is the primary method for auditing the progress of the migration to Unified Plan. ### Suppressing Deprecation Warnings in Legacy Code Existing Plan B call sites in "glue code" (e.g., `PeerConnection` delegation logic) should be wrapped with the suppression macros to allow the build to pass. ```cpp RTC_ALLOW_PLAN_B_DEPRECATION_BEGIN(); sdp_handler_->RemoveStream(local_stream); RTC_ALLOW_PLAN_B_DEPRECATION_END(); ``` This ensures that the build only fails on *new*, unwrapped Plan B usage, while clearly marking legacy code that requires migration. ### Transitioning to Unified Plan Functions that are compatible with both Plan B and Unified Plan should **not** be annotated. Only those that are functionally incorrect or redundant under Unified Plan (often guarded by `RTC_DCHECK(!IsUnifiedPlan())`) should be marked.