add support for RTCConfiguration.alwaysNegotiateDataChannels

which always negotiates datachannels and puts them first in the SDP.
Spec:
  w3c/webrtc-extensions#242

Author: fippo

src/aiortc/rtcconfiguration.py

@@ -64,3 +64,6 @@ class RTCConfiguration:
 
     bundlePolicy: RTCBundlePolicy = RTCBundlePolicy.BALANCED
     "The media-bundling policy to use when gathering ICE candidates."
+
+    alwaysNegotiateDataChannels: bool = False
+    "Whether to always negotiate data channels in the SDP."

src/aiortc/rtcpeerconnection.py

@@ -702,6 +702,22 @@ def get_media_section(
         def next_mline_index() -> int:
             return len(description.media)
 
+        def add_datachannel_section() -> None:
+            self.__sctp_mline_index = next_mline_index()
+            description.media.append(
+                create_media_description_for_sctp(
+                    self.__sctp, legacy=self._sctpLegacySdp, mid=allocate_mid(mids)
+                )
+            )
+
+        # JSEP specifies that it should only be added after audio/video m-sections
+        # but the `alwaysNegotiateDataChannels` option overrides that:
+        # https://w3c.github.io/webrtc-extensions/#always-negotiating-datachannels
+        # This implies that it will be used for negotiating BUNDLE.
+        if not self.__sctp and self.__configuration.alwaysNegotiateDataChannels:
+            self.__createSctpTransport()
+            add_datachannel_section()
+
         for transceiver in filter(
             lambda x: x.mid is None and not x.stopped, self.__transceivers
         ):
@@ -714,13 +730,12 @@ def next_mline_index() -> int:
                     mid=allocate_mid(mids),
                 )
             )
-        if self.__sctp and self.__sctp.mid is None:
-            self.__sctp_mline_index = next_mline_index()
-            description.media.append(
-                create_media_description_for_sctp(
-                    self.__sctp, legacy=self._sctpLegacySdp, mid=allocate_mid(mids)
-                )
-            )
+        if (
+            self.__sctp
+            and self.__sctp.mid is None
+            and not self.__configuration.alwaysNegotiateDataChannels
+        ):
+            add_datachannel_section()
 
         bundle = sdp.GroupDescription(semantic="BUNDLE", items=[])
         for media in description.media:

tests/test_rtcpeerconnection.py

@@ -5471,3 +5471,34 @@ async def test_bundlepolicy_transports_max_bundle(self) -> None:
             transceiver1.receiver.transport.transport.iceGatherer.getLocalParameters(),
             pc.sctp.transport.transport.iceGatherer.getLocalParameters(),
         )
+
+    @asynctest
+    async def test_always_negotiate_datachannels(self) -> None:
+        pc = RTCPeerConnection(RTCConfiguration(alwaysNegotiateDataChannels=True))
+        pc.addTransceiver("audio")
+        offer = await pc.createOffer()
+        parsed = SessionDescription.parse(offer.sdp)
+        self.assertEqual(len(parsed.media), 2)
+        self.assertEqual("application", parsed.media[0].kind)
+        self.assertEqual("audio", parsed.media[1].kind)
+
+    @asynctest
+    async def test_always_negotiate_datachannels_subsequent(self) -> None:
+        pc1 = RTCPeerConnection(RTCConfiguration(alwaysNegotiateDataChannels=True))
+        pc2 = RTCPeerConnection()
+
+        pc1.addTransceiver("audio")
+        await pc1.setLocalDescription()
+        await pc2.setRemoteDescription(pc1.localDescription)
+        await pc2.setLocalDescription()
+        await pc1.setRemoteDescription(pc2.localDescription)
+
+        for pc in [pc1, pc2]:
+            parsed = SessionDescription.parse(pc.localDescription.sdp)
+            self.assertEqual(len(parsed.media), 2)
+
+            pc.createDataChannel("test")
+            offer = await pc.createOffer()
+            parsed = SessionDescription.parse(offer.sdp)
+            self.assertEqual(len(parsed.media), 2)
+