Repository: matrix-org/synapse

Issue:
Unhandled error in Deferred: BodyExceededMaxSize
We're seeing a few of these in the logs from the federation sender:
```
2021-01-12 21:41:24,475 - twisted - 254 - CRITICAL - None - Unhandled error in Deferred:
2021-01-12 21:41:24,485 - twisted - 254 - CRITICAL - None - 
synapse.http.client.BodyExceededMaxSize
```



Discussion:
it looks like we should be catching `BodyExceededMaxSize`, so maybe it's something like: we are aborting the request, but failing to cancel it: it then goes on in the background until we get a huge response?
or possibly: we *are* correctly catching the `BodyExceededMaxSize`, but it is being thrown again into a second deferred:

```python
    def dataReceived(self, data: bytes) -> None:
        self.stream.write(data)
        self.length += len(data)
        if self.max_size is not None and self.length >= self.max_size:
            self.deferred.errback(BodyExceededMaxSize())
            self.deferred = defer.Deferred()
            self.transport.loseConnection()
```

... replacing `self.deferred` in that way seems risky. Calling `loseConnection` does not, afaik, guarantee that `dataReceived` will not be called again.
This is probably happening more now due to checking the well known size. (I had noticed there's a few servers that are offline and redirect to large 404 pages...) I'll take a look at this.

Resolving patch:
diff --git a/synapse/http/client.py b/synapse/http/client.py
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -766,14 +766,24 @@ def __init__(
         self.max_size = max_size
 
     def dataReceived(self, data: bytes) -> None:
+        # If the deferred was called, bail early.
+        if self.deferred.called:
+            return
+
         self.stream.write(data)
         self.length += len(data)
+        # The first time the maximum size is exceeded, error and cancel the
+        # connection. dataReceived might be called again if data was received
+        # in the meantime.
         if self.max_size is not None and self.length >= self.max_size:
             self.deferred.errback(BodyExceededMaxSize())
-            self.deferred = defer.Deferred()
             self.transport.loseConnection()
 
     def connectionLost(self, reason: Failure) -> None:
+        # If the maximum size was already exceeded, there's nothing to do.
+        if self.deferred.called:
+            return
+
         if reason.check(ResponseDone):
             self.deferred.callback(self.length)
         elif reason.check(PotentialDataLoss):


Test patch:
diff --git a/tests/http/federation/test_matrix_federation_agent.py b/tests/http/federation/test_matrix_federation_agent.py
--- a/tests/http/federation/test_matrix_federation_agent.py
+++ b/tests/http/federation/test_matrix_federation_agent.py
@@ -1095,7 +1095,7 @@ def test_well_known_cache_with_temp_failure(self):
         # Expire both caches and repeat the request
         self.reactor.pump((10000.0,))
 
-        # Repated the request, this time it should fail if the lookup fails.
+        # Repeat the request, this time it should fail if the lookup fails.
         fetch_d = defer.ensureDeferred(
             self.well_known_resolver.get_well_known(b"testserv")
         )
@@ -1130,7 +1130,7 @@ def test_well_known_too_large(self):
             content=b'{ "m.server": "' + (b"a" * WELL_KNOWN_MAX_SIZE) + b'" }',
         )
 
-        # The result is sucessful, but disabled delegation.
+        # The result is successful, but disabled delegation.
         r = self.successResultOf(fetch_d)
         self.assertIsNone(r.delegated_server)
 
diff --git a/tests/http/test_client.py b/tests/http/test_client.py
new file mode 100644
--- /dev/null
+++ b/tests/http/test_client.py
@@ -0,0 +1,101 @@
+#  Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+from io import BytesIO
+
+from mock import Mock
+
+from twisted.python.failure import Failure
+from twisted.web.client import ResponseDone
+
+from synapse.http.client import BodyExceededMaxSize, read_body_with_max_size
+
+from tests.unittest import TestCase
+
+
+class ReadBodyWithMaxSizeTests(TestCase):
+    def setUp(self):
+        """Start reading the body, returns the response, result and proto"""
+        self.response = Mock()
+        self.result = BytesIO()
+        self.deferred = read_body_with_max_size(self.response, self.result, 6)
+
+        # Fish the protocol out of the response.
+        self.protocol = self.response.deliverBody.call_args[0][0]
+        self.protocol.transport = Mock()
+
+    def _cleanup_error(self):
+        """Ensure that the error in the Deferred is handled gracefully."""
+        called = [False]
+
+        def errback(f):
+            called[0] = True
+
+        self.deferred.addErrback(errback)
+        self.assertTrue(called[0])
+
+    def test_no_error(self):
+        """A response that is NOT too large."""
+
+        # Start sending data.
+        self.protocol.dataReceived(b"12345")
+        # Close the connection.
+        self.protocol.connectionLost(Failure(ResponseDone()))
+
+        self.assertEqual(self.result.getvalue(), b"12345")
+        self.assertEqual(self.deferred.result, 5)
+
+    def test_too_large(self):
+        """A response which is too large raises an exception."""
+
+        # Start sending data.
+        self.protocol.dataReceived(b"1234567890")
+        # Close the connection.
+        self.protocol.connectionLost(Failure(ResponseDone()))
+
+        self.assertEqual(self.result.getvalue(), b"1234567890")
+        self.assertIsInstance(self.deferred.result, Failure)
+        self.assertIsInstance(self.deferred.result.value, BodyExceededMaxSize)
+        self._cleanup_error()
+
+    def test_multiple_packets(self):
+        """Data should be accummulated through mutliple packets."""
+
+        # Start sending data.
+        self.protocol.dataReceived(b"12")
+        self.protocol.dataReceived(b"34")
+        # Close the connection.
+        self.protocol.connectionLost(Failure(ResponseDone()))
+
+        self.assertEqual(self.result.getvalue(), b"1234")
+        self.assertEqual(self.deferred.result, 4)
+
+    def test_additional_data(self):
+        """A connection can receive data after being closed."""
+
+        # Start sending data.
+        self.protocol.dataReceived(b"1234567890")
+        self.assertIsInstance(self.deferred.result, Failure)
+        self.assertIsInstance(self.deferred.result.value, BodyExceededMaxSize)
+        self.protocol.transport.loseConnection.assert_called_once()
+
+        # More data might have come in.
+        self.protocol.dataReceived(b"1234567890")
+        # Close the connection.
+        self.protocol.connectionLost(Failure(ResponseDone()))
+
+        self.assertEqual(self.result.getvalue(), b"1234567890")
+        self.assertIsInstance(self.deferred.result, Failure)
+        self.assertIsInstance(self.deferred.result.value, BodyExceededMaxSize)
+        self._cleanup_error()