Fenced frames: disableUntrustedNetwork() for WebTransport.

This CL prevents WebTransport connections from being created in frames
with network access revoked, and closes existing WebTransport
connections if a frame loses network access.

Change-Id: Iacb3eff60c8bbd4a154d3f9390f37ad966eafcb0
Bug: 385155392
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6113689
Reviewed-by: Xiaochen Zhou <xiaochenzh@chromium.org>
Commit-Queue: Liam Brady <lbrady@google.com>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1404463}

webtransport/handlers/token-count.py

@@ -0,0 +1,35 @@
+"""
+This is a WebTransport handler that reads tokens sent from client streams and
+tracks how many times the token was sent. When a client sends a token, the
+server will send the total token count back to the client.
+"""
+streams_dict = {}
+
+def session_established(session):
+    # When a WebTransport session is established, a bidirectional stream is
+    # created by the server, which is used to echo back stream data from the
+    # client.
+    session.create_bidirectional_stream()
+
+
+def stream_data_received(session,
+                         stream_id: int,
+                         data: bytes,
+                         stream_ended: bool):
+    count = session.stash.take(data) or 0
+    count += 1
+    session.stash.put(key=data, value=count)
+    # If a stream is unidirectional, create a new unidirectional stream and echo
+    # the token count on that stream.
+    if session.stream_is_unidirectional(stream_id):
+        if (session.session_id, stream_id) not in streams_dict.keys():
+            new_stream_id = session.create_unidirectional_stream()
+            streams_dict[(session.session_id, stream_id)] = new_stream_id
+        session.send_stream_data(streams_dict[(session.session_id, stream_id)],
+                                 str(count).encode())
+        if (stream_ended):
+          del streams_dict[(session.session_id, stream_id)]
+        return
+    # Otherwise (e.g. if the stream is bidirectional), echo back the token count
+    # on the same stream.
+    session.send_stream_data(stream_id, str(count).encode())