Make sure SetSsrc clears non-empty encodings and degradation_preference

This regressed in https://webrtc-review.googlesource.com/c/src/+/442922

Bug: webrtc:500993975
Change-Id: I6400b3bc1f5fbc8f5d809c6cecc4d52e11274669
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462527
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47405}

pc/rtp_sender.cc

diff --git a/pc/rtp_sender.cc b/pc/rtp_sender.cc
index 3edc511..ca1400b 100644
--- a/pc/rtp_sender.cc
+++ b/pc/rtp_sender.cc
@@ -740,41 +740,49 @@
     AddTrackToStats();
   }
 
-  const bool update_parameters =
-      (ssrc_ != 0 && (!init_parameters_.encodings.empty() ||
-                      init_parameters_.degradation_preference.has_value()));
   RtpParameters current_parameters;
   bool params_modified = false;
   worker_thread_->BlockingCall([&, ssrc = ssrc_] {
     RTC_DCHECK_RUN_ON(worker_thread_);
-    if (update_parameters) {
-      RTC_DCHECK(media_channel_);
-      // Get the current parameters, which are constructed from the SDP. The
-      // number of layers in the SDP is currently authoritative to support SDP
-      // munging for Plan-B simulcast with "a=ssrc-group:SIM <ssrc-id>..." lines
-      // as described in RFC 5576. All fields should be default constructed and
-      // the SSRC field set, which we need to copy.
-      current_parameters = media_channel_->GetRtpSendParameters(ssrc);
-      // SSRC 0 has special meaning as "no stream". In this case,
-      // current_parameters may have size 0.
-      RTC_CHECK_GE(current_parameters.encodings.size(),
-                   init_parameters_.encodings.size());
-      for (size_t i = 0; i < init_parameters_.encodings.size(); ++i) {
-        init_parameters_.encodings[i].ssrc =
-            current_parameters.encodings[i].ssrc;
-        init_parameters_.encodings[i].rid = current_parameters.encodings[i].rid;
-        current_parameters.encodings[i] = init_parameters_.encodings[i];
-      }
-      current_parameters.degradation_preference =
-          init_parameters_.degradation_preference;
-      params_modified =
-          media_channel_
-              ->SetRtpSendParameters(ssrc, current_parameters, nullptr)
-              .ok();
-      if (params_modified) {
-        // The parameters may change as they're applied.
+    if (!init_parameters_.encodings.empty() ||
+        init_parameters_.degradation_preference.has_value()) {
+      if (ssrc != 0) {
+        RTC_DCHECK(media_channel_);
+        // Get the current parameters, which are constructed from the SDP. The
+        // number of layers in the SDP is currently authoritative to support SDP
+        // munging for Plan-B simulcast with "a=ssrc-group:SIM <ssrc-id>..."
+        // lines as described in RFC 5576. All fields should be default
+        // constructed and the SSRC field set, which we need to copy.
         current_parameters = media_channel_->GetRtpSendParameters(ssrc);
+        // SSRC 0 has special meaning as "no stream". In this case,
+        // current_parameters may have size 0.
+        RTC_CHECK_GE(current_parameters.encodings.size(),
+                     init_parameters_.encodings.size());
+        for (size_t i = 0; i < init_parameters_.encodings.size(); ++i) {
+          init_parameters_.encodings[i].ssrc =
+              current_parameters.encodings[i].ssrc;
+          init_parameters_.encodings[i].rid =
+              current_parameters.encodings[i].rid;
+          current_parameters.encodings[i] = init_parameters_.encodings[i];
+        }
+        current_parameters.degradation_preference =
+            init_parameters_.degradation_preference;
+        params_modified =
+            media_channel_
+                ->SetRtpSendParameters(ssrc, current_parameters, nullptr)
+                .ok();
+        if (params_modified) {
+          // The parameters may change as they're applied.
+          current_parameters = media_channel_->GetRtpSendParameters(ssrc);
+        }
       }
+      // Clear the `init_parameters_` after they have been applied to the
+      // media channel. This prevents stale values from being used in
+      // subsequent calls to `SetSsrc`, which could happen if `SetSsrc` is
+      // called multiple times on the same sender. See
+      // https://issues.webrtc.org/issues/500993975 for details.
+      init_parameters_.encodings.clear();
+      init_parameters_.degradation_preference = std::nullopt;
     }
 
     // While we're on the worker thread, attach the frame decryptor, transformer