TLSUpgradeProto: don't set multiple results for an event

In the case of a misbehaving server, the client may receive more than
one byte in separate data_received() invocations from the server. While
we can't do much sane with this, we should handle it gracefully and not
crash with asyncio.InvalidStateError when trying to set another result
on the event.

Fixes MagicStack#729

asyncpg/_testbase/__init__.py

@@ -13,6 +13,7 @@
 import logging
 import os
 import re
+import socket
 import textwrap
 import time
 import traceback
@@ -525,3 +526,41 @@ def connect_standby(cls, **kwargs):
             kwargs
         )
         return pg_connection.connect(**conn_spec, loop=cls.loop)
+
+
+class InstrumentedServer:
+    """
+    A socket server for testing.
+    It will write each item from `data`, and wait for the corresponding event
+    in `received_events` to notify that it was received before writing the next
+    item from `data`.
+    """
+    def __init__(self, data, received_events):
+        assert len(data) == len(received_events)
+        self._data = data
+        self._server = None
+        self._received_events = received_events
+
+    async def _handle_client(self, _reader, writer):
+        for datum, received_event in zip(self._data, self._received_events):
+            writer.write(datum)
+            await writer.drain()
+            await received_event.wait()
+
+        writer.close()
+        await writer.wait_closed()
+
+    async def start(self):
+        """Start the server."""
+        self._server = await asyncio.start_server(self._handle_client, 'localhost', 0)
+        assert self._server.sockets
+        sock = self._server.sockets[0]
+        addr, port = sock.getsockname()
+        return {
+            'host': addr,
+            'port': port,
+        }
+
+    def stop(self):
+        """Stop the server."""
+        self._server.close()

asyncpg/connect_utils.py

@@ -714,6 +714,11 @@ def __init__(self, loop, host, port, ssl_context, ssl_is_advisory):
         self.ssl_is_advisory = ssl_is_advisory
 
     def data_received(self, data):
+        if self.on_data.done():
+            # Only expect to receive one byte here; ignore unsolicited further
+            # data.
+            return
+
         if data == b'S':
             self.on_data.set_result(True)
         elif (self.ssl_is_advisory and

tests/test_connect.py

@@ -7,6 +7,7 @@
 
 import asyncio
 import contextlib
+import copy
 import gc
 import ipaddress
 import os
@@ -17,11 +18,13 @@
 import stat
 import tempfile
 import textwrap
+import time
 import unittest
 import unittest.mock
 import urllib.parse
 import warnings
 import weakref
+from unittest import mock
 
 import asyncpg
 from asyncpg import _testbase as tb
@@ -1989,6 +1992,58 @@ async def test_prefer_standby_picks_master_when_standby_is_down(self):
         await con.close()
 
 
+class TestMisbehavingServer(tb.TestCase):
+    """Tests for client connection behaviour given a misbehaving server."""
+
+    async def test_tls_upgrade_extra_data_received(self):
+        data = [
+            # First, the server writes b"S" to signal it is willing to perform
+            # SSL
+            b"S",
+            # Then, the server writes an unsolicted arbitrary byte afterwards
+            b"N",
+        ]
+        data_received_events = [asyncio.Event() for _ in data]
+
+        # Patch out the loop's create_connection so we can instrument the proto
+        # we return.
+        old_create_conn = self.loop.create_connection
+
+        async def _mock_create_conn(*args, **kwargs):
+            transport, proto = await old_create_conn(*args, **kwargs)
+            old_data_received = proto.data_received
+
+            num_received = 0
+
+            def _data_received(*args, **kwargs):
+                nonlocal num_received
+                # Call the original data_received method
+                ret = old_data_received(*args, **kwargs)
+                # Fire the event to signal we've received this datum now.
+                data_received_events[num_received].set()
+                num_received += 1
+                return ret
+
+            proto.data_received = _data_received
+
+            # To deterministically provoke the race we're interested in for
+            # this regression test, wait for all data to be received before
+            # returning from create_connection().
+            await data_received_events[-1].wait()
+            return transport, proto
+
+        server = tb.InstrumentedServer(data, data_received_events)
+        conn_spec = await server.start()
+
+        # The call to connect() should raise a ConnectionResetError as the
+        # server will close the connection after writing all the data.
+        with (mock.patch.object(self.loop, "create_connection", side_effect=_mock_create_conn),
+              self.assertRaises(ConnectionResetError)):
+            await pg_connection.connect(**conn_spec, ssl=True, loop=self.loop)
+
+        server.stop()
+
+
 def _get_connected_host(con):
     peername = con._transport.get_extra_info('peername')
     if isinstance(peername, tuple):