[ObjC] Change default sdpSemantics to NotSpecified.

The default value of sdpSemantics is about to change from PlanB to
UnifiedPlan. In order not to cause subtle bugs by applications that
depend on the default value being PlanB, we are temporarily making the
default NotSpecified. Constructing with NotSpecified causes the C++
layer to crash (https://webrtc-review.googlesource.com/c/src/+/242968).
This is in accordance to the publically announced plans:
https://groups.google.com/u/1/g/discuss-webrtc/c/SdoVP02eUIk

Bug: webrtc:11121
Change-Id: Idbb8fd0f5c224311cf1f25ac2832800124ed14d4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/246060
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35678}

sdk/objc/api/peerconnection/RTCConfiguration.h

@@ -63,8 +63,15 @@ typedef NS_ENUM(NSInteger, RTCEncryptionKeyType) {
 
 /** Represents the chosen SDP semantics for the RTCPeerConnection. */
 typedef NS_ENUM(NSInteger, RTCSdpSemantics) {
+  // TODO(https://crbug.com/webrtc/13528): Remove support for Plan B.
   RTCSdpSemanticsPlanB,
   RTCSdpSemanticsUnifiedPlan,
+  // The default sdpSemantics value is about to change to Unified Plan. During
+  // a short transition period, NotSpecified is used to ensure clients that
+  // don't set sdpSemantics are aware of the change by CHECK-crashing.
+  // TODO(https://crbug.com/webrtc/11121): When the default has changed to
+  // UnifiedPlan, delete NotSpecified.
+  RTCSdpSemanticsNotSpecified,
 };
 
 NS_ASSUME_NONNULL_BEGIN
@@ -161,9 +168,10 @@ RTC_OBJC_EXPORT
  */
 @property(nonatomic, copy, nullable) NSNumber *iceCheckMinInterval;
 
-/** Configure the SDP semantics used by this PeerConnection. Note that the
- *  WebRTC 1.0 specification requires UnifiedPlan semantics. The
- *  RTCRtpTransceiver API is only available with UnifiedPlan semantics.
+/** Configure the SDP semantics used by this PeerConnection. The WebRTC 1.0
+ *  specification requires RTCSdpSemanticsUnifiedPlan semantics and the
+ *  RtpTransceiver API is only available in Unified Plan. RTCSdpSemanticsPlanB
+ *  is being deprecated and will be removed at a future date.
  *
  *  PlanB will cause RTCPeerConnection to create offers and answers with at
  *  most one audio and one video m= section with multiple RTCRtpSenders and
@@ -174,14 +182,18 @@ RTC_OBJC_EXPORT
  *  UnifiedPlan will cause RTCPeerConnection to create offers and answers with
  *  multiple m= sections where each m= section maps to one RTCRtpSender and one
  *  RTCRtpReceiver (an RTCRtpTransceiver), either both audio or both
- *  video. This will also cause RTCPeerConnection) to ignore all but the first a=ssrc
- *  lines that form a Plan B stream.
+ *  video. This will also cause RTCPeerConnection) to ignore all but the first
+ *  a=ssrc lines that form a Plan B stream.
  *
- *  For users who wish to send multiple audio/video streams and need to stay
- *  interoperable with legacy WebRTC implementations or use legacy APIs,
- *  specify PlanB.
+ *  For users who have to interwork with legacy WebRTC implementations, it
+ *  is possible to specify PlanB until the code is finally removed
+ *  (https://crbug.com/webrtc/13528).
  *
- *  For all other users, specify UnifiedPlan.
+ *  The default SdpSemantics value is about to change to UnifiedPlan. During a
+ *  short transition period, NotSpecified is used to ensure clients that don't
+ *  set SdpSemantics are aware of the change by CHECK-crashing.
+ *  TODO(https://crbug.com/webrtc/11121): When the default has changed to
+ *  UnifiedPlan, delete NotSpecified.
  */
 @property(nonatomic, assign) RTCSdpSemantics sdpSemantics;
 

sdk/objc/api/peerconnection/RTCConfiguration.mm

@@ -68,7 +68,7 @@
 - (instancetype)init {
   // Copy defaults.
   webrtc::PeerConnectionInterface::RTCConfiguration config;
-  config.sdp_semantics = webrtc::SdpSemantics::kPlanB_DEPRECATED;
+  config.sdp_semantics = webrtc::SdpSemantics::kNotSpecified;
   return [self initWithNativeConfiguration:config];
 }
 
@@ -525,6 +525,8 @@
       return webrtc::SdpSemantics::kPlanB_DEPRECATED;
     case RTCSdpSemanticsUnifiedPlan:
       return webrtc::SdpSemantics::kUnifiedPlan;
+    case RTCSdpSemanticsNotSpecified:
+      return webrtc::SdpSemantics::kNotSpecified;
   }
 }
 
@@ -535,8 +537,7 @@
     case webrtc::SdpSemantics::kUnifiedPlan:
       return RTCSdpSemanticsUnifiedPlan;
     case webrtc::SdpSemantics::kNotSpecified:
-      RTC_DCHECK_NOTREACHED();
-      return RTCSdpSemanticsUnifiedPlan;
+      return RTCSdpSemanticsNotSpecified;
   }
 }
 
@@ -546,6 +547,8 @@
       return @"PLAN_B";
     case RTCSdpSemanticsUnifiedPlan:
       return @"UNIFIED_PLAN";
+    case RTCSdpSemanticsNotSpecified:
+      return @"NOT_SPECIFIED";
   }
 }
 

sdk/objc/unittests/RTCPeerConnectionTest.mm

@@ -42,6 +42,7 @@
       [[RTC_OBJC_TYPE(RTCIceServer) alloc] initWithURLStrings:urlStrings];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   config.iceServers = @[ server ];
   config.iceTransportPolicy = RTCIceTransportPolicyRelay;
   config.bundlePolicy = RTCBundlePolicyMaxBundle;
@@ -121,6 +122,7 @@
       [[RTC_OBJC_TYPE(RTCIceServer) alloc] initWithURLStrings:urlStrings];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   config.iceServers = @[ server ];
   RTC_OBJC_TYPE(RTCMediaConstraints) *contraints =
       [[RTC_OBJC_TYPE(RTCMediaConstraints) alloc] initWithMandatoryConstraints:@{}
@@ -145,6 +147,7 @@
       [[RTC_OBJC_TYPE(RTCPeerConnectionFactory) alloc] init];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   RTC_OBJC_TYPE(RTCMediaConstraints) *contraints =
       [[RTC_OBJC_TYPE(RTCMediaConstraints) alloc] initWithMandatoryConstraints:@{}
                                                            optionalConstraints:nil];
@@ -175,6 +178,7 @@
       [[RTC_OBJC_TYPE(RTCPeerConnectionFactory) alloc] init];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   RTC_OBJC_TYPE(RTCMediaConstraints) *contraints =
       [[RTC_OBJC_TYPE(RTCMediaConstraints) alloc] initWithMandatoryConstraints:@{}
                                                            optionalConstraints:nil];