avoid refcycles in tracebacks from happy eyeballs exceptions

docs/versionhistory.rst

@@ -20,6 +20,10 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
   current Python version on several of its methods, and made the ``is_junction`` method
   unavailable on Python versions earlier than 3.12
   (`#794 <https://github.com/agronholm/anyio/issues/794>`_)
+- Fixed connect_tcp producing cyclic references in tracebacks
+  when raising exceptions (`#809 <https://github.com/agronholm/anyio/pull/809>`_)
+  (PR by @graingert)
+
 
 **4.6.0**
 

pyproject.toml

@@ -55,7 +55,8 @@ test = [
     """\
     uvloop >= 0.21.0b1; platform_python_implementation == 'CPython' \
     and platform_system != 'Windows'\
-    """
+    """,
+    "ephemeral-port-reserve >= 1.1.4",
 ]
 doc = [
     "packaging",

src/anyio/_core/_sockets.py

@@ -211,20 +211,23 @@ async def try_connect(remote_host: str, event: Event) -> None:
             target_addrs = [(socket.AF_INET, addr_obj.compressed)]
 
     oserrors: list[OSError] = []
-    async with create_task_group() as tg:
-        for i, (af, addr) in enumerate(target_addrs):
-            event = Event()
-            tg.start_soon(try_connect, addr, event)
-            with move_on_after(happy_eyeballs_delay):
-                await event.wait()
-
-    if connected_stream is None:
-        cause = (
-            oserrors[0]
-            if len(oserrors) == 1
-            else ExceptionGroup("multiple connection attempts failed", oserrors)
-        )
-        raise OSError("All connection attempts failed") from cause
+    try:
+        async with create_task_group() as tg:
+            for i, (af, addr) in enumerate(target_addrs):
+                event = Event()
+                tg.start_soon(try_connect, addr, event)
+                with move_on_after(happy_eyeballs_delay):
+                    await event.wait()
+
+        if connected_stream is None:
+            cause = (
+                oserrors[0]
+                if len(oserrors) == 1
+                else ExceptionGroup("multiple connection attempts failed", oserrors)
+            )
+            raise OSError("All connection attempts failed") from cause
+    finally:
+        oserrors = []
 
     if tls or tls_hostname or ssl_context:
         try:

tests/test_sockets.py

@@ -18,6 +18,7 @@
 from threading import Thread
 from typing import Any, NoReturn, TypeVar, cast
 
+import ephemeral_port_reserve
 import psutil
 import pytest
 from _pytest.fixtures import SubRequest
@@ -125,6 +126,16 @@ def check_asyncio_bug(anyio_backend_name: str, family: AnyIPAddressFamily) -> No
             pytest.skip("Does not work due to a known bug (39148)")
 
 
+if sys.version_info <= (3, 11):
+
+    def no_other_refs() -> list[object]:
+        return [sys._getframe(1)]
+else:
+
+    def no_other_refs() -> list[object]:
+        return []
+
+
 _T = TypeVar("_T")
 
 
@@ -307,6 +318,29 @@ def serve() -> None:
         server_sock.close()
         assert client_addr[0] == expected_client_addr
 
+    @pytest.mark.skipif(
+        sys.implementation.name == "pypy",
+        reason=(
+            "gc.get_referrers is broken on PyPy see "
+            "https://github.com/pypy/pypy/issues/5075"
+        ),
+    )
+    async def test_happy_eyeballs_refcycles(self) -> None:
+        """
+        Test derived from https://github.com/python/cpython/pull/124859
+        """
+        ip = "127.0.0.1"
+        port = ephemeral_port_reserve.reserve(ip=ip)
+        exc = None
+        try:
+            async with await connect_tcp(ip, port):
+                pass
+        except OSError as e:
+            exc = e.__cause__
+
+        assert isinstance(exc, OSError)
+        assert gc.get_referrers(exc) == no_other_refs()
+
     @pytest.mark.parametrize(
         "target, exception_class",
         [