Reject the descriptions that attempt to change the order of m= section in current local description.

webrtc/pc/peerconnectioninterface_unittest.cc

Index: webrtc/pc/peerconnectioninterface_unittest.cc
diff --git a/webrtc/pc/peerconnectioninterface_unittest.cc b/webrtc/pc/peerconnectioninterface_unittest.cc
index c23a79d597b4fe8bdaec83edbd867df52ef3f3f5..6cc6e81abcc54a81511b83f37e22d216b5728dbf 100644
--- a/webrtc/pc/peerconnectioninterface_unittest.cc
+++ b/webrtc/pc/peerconnectioninterface_unittest.cc
@@ -1241,6 +1241,35 @@ class PeerConnectionInterfaceTest : public testing::Test {
     return false;
   }
 
+  void ReorderMediaSections(SessionDescriptionInterface* session_description) {

[Taylor Brandstetter, 2017/09/14 00:41:47]

nit: This could be simplified by using std::reverse. It then would be such a
small amount of code it could go in the test itself:

std::reverse(offer->description()->contents().begin(),
             offer->description()->contents().end());
std::reverse(offer->description()->transport_infos().begin(),
             offer->description()->transport_infos().end());

+    if (!session_description) {
+      return;
+    }
+    auto desc = session_description->description();
+    RTC_DCHECK(desc);
+    RTC_DCHECK(desc->contents().size() == desc->transport_infos().size());
+    // Reorder the ContentInfo.
+    if (desc->contents().empty()) {
+      return;
+    }
+    size_t num_msections = desc->contents().size();
+    auto first_content_info = desc->contents()[0];
+    for (size_t i = 1; i < num_msections; ++i) {
+      desc->contents()[i - 1] = desc->contents()[i];
+    }
+    desc->contents()[num_msections - 1] = first_content_info;
+
+    // Reorder the TransportInfo.
+    if (desc->transport_infos().empty()) {
+      return;
+    }
+    auto first_transport_info = desc->transport_infos()[0];
+    for (size_t i = 1; i < num_msections; ++i) {
+      desc->transport_infos()[i - 1] = desc->transport_infos()[i];
+    }
+    desc->transport_infos()[num_msections - 1] = first_transport_info;
+  }
+
   std::unique_ptr<rtc::VirtualSocketServer> vss_;
   rtc::AutoSocketServerThread main_;
   rtc::scoped_refptr<FakeAudioCaptureModule> fake_audio_capture_module_;
@@ -3818,6 +3847,33 @@ TEST_F(PeerConnectionInterfaceTest, CreateAnswerWithoutRemoteDescription) {
   EXPECT_TRUE(DoCreateAnswer(&answer, nullptr));
 }
 
+// Test that an error is returned if a description is applied that doesn't
+// respect the order of existing media sections.
+TEST_F(PeerConnectionInterfaceTest,
+       MediaSectionOrderEnforcedForSubsequentOffers) {
+  CreatePeerConnection();
+  FakeConstraints constraints;
+  constraints.SetMandatoryReceiveAudio(true);
+  constraints.SetMandatoryReceiveVideo(true);
+  std::unique_ptr<SessionDescriptionInterface> offer;
+  ASSERT_TRUE(DoCreateOffer(&offer, &constraints));
+  EXPECT_TRUE(DoSetRemoteDescription(offer.release()));
+
+  std::unique_ptr<SessionDescriptionInterface> answer;
+  ASSERT_TRUE(DoCreateAnswer(&answer, nullptr));
+  EXPECT_TRUE(DoSetLocalDescription(answer.release()));
+
+  // An remote offer with different m=line order would be rejected.

[Taylor Brandstetter, 2017/09/14 00:41:47]

nit: "A" instead of "An", and "should" instead of "would"

+  ASSERT_TRUE(DoCreateOffer(&offer, &constraints));
+  ReorderMediaSections(offer.get());
+  EXPECT_FALSE(DoSetRemoteDescription(offer.release()));
+
+  // A subsequent local offer with different m=line order would be rejected.
+  ASSERT_TRUE(DoCreateOffer(&offer, &constraints));
+  ReorderMediaSections(offer.get());
+  EXPECT_FALSE(DoSetLocalDescription(offer.release()));
+}
+
 class PeerConnectionMediaConfigTest : public testing::Test {
  protected:
   void SetUp() override {