Bug 1657394: Test for cases where we have a fallback bundle transport. r=mjf

Differential Revision: https://phabricator.services.mozilla.com/D87955

media/webrtc/signaling/gtest/jsep_session_unittest.cpp

@@ -2557,21 +2557,40 @@ TEST_P(JsepSessionTest, RenegotiationAnswererDisablesBundleTransport) {
   ASSERT_EQ(0U, ot0->mTransport.mComponents);
   ASSERT_EQ(0U, at0->mTransport.mComponents);
 
+  // With bundle-policy "balanced", this ends up being somewhat complicated.
+  // The first m-section of each type is _not_ marked bundle-only,
+  // but subsequent m-sections of that type are. This means that if there is
+  // more than one type, there is more than one transport, which means there's
+  // a fallback when the bundle-tag is disabled (the first such transport). We
+  // should be using that fallback when it exists.
+  Maybe<size_t> fallbackTransport;
+  for (size_t level = 1; level != types.size(); ++level) {
+    if (types[level] != types[0]) {
+      fallbackTransport = Some(level);
+      break;
+    }
+  }
+
   for (size_t i = 1; i < newOffererTransceivers.size(); ++i) {
     JsepTransceiver* ot = GetTransceiverByLevel(newOffererTransceivers, i);
     JsepTransceiver* at = GetTransceiverByLevel(newAnswererTransceivers, i);
-    JsepTransceiver* ot1 = GetTransceiverByLevel(newOffererTransceivers, 1);
-    JsepTransceiver* at1 = GetTransceiverByLevel(newAnswererTransceivers, 1);
+    // If there is no fallback, the bundle level will be left pointing at the
+    // dead transport at index 0.
+    size_t expectedBundleLevel = fallbackTransport.valueOr(0);
+    JsepTransceiver* otWithTransport =
+        GetTransceiverByLevel(newOffererTransceivers, expectedBundleLevel);
+    JsepTransceiver* atWithTransport =
+        GetTransceiverByLevel(newAnswererTransceivers, expectedBundleLevel);
     ASSERT_TRUE(ot->HasBundleLevel());
     ASSERT_TRUE(at->HasBundleLevel());
-    ASSERT_EQ(1U, ot->BundleLevel());
-    ASSERT_EQ(1U, at->BundleLevel());
+    ASSERT_EQ(expectedBundleLevel, ot->BundleLevel());
+    ASSERT_EQ(expectedBundleLevel, at->BundleLevel());
     // TODO: When creating an answer where we have rejected the bundle
     // transport, we do not do a good job of creating a sensible SDP. Mainly,
     // when we remove the rejected mid from the bundle group, we can leave a
     // bundle-only mid as the first one when others are available.
-    ASSERT_TRUE(Equals(ot1->mTransport, ot->mTransport));
-    ASSERT_TRUE(Equals(at1->mTransport, at->mTransport));
+    ASSERT_TRUE(Equals(otWithTransport->mTransport, ot->mTransport));
+    ASSERT_TRUE(Equals(atWithTransport->mTransport, at->mTransport));
   }
 }