gh-84808: socket.connect_ex: Handle negative errno (GH-122304)

POSIX allows errno to be negative.
Even though all currently supported platforms have non-negative errno,
relying on a quirk like that would make Python less portable.

Misc/NEWS.d/next/Library/2024-09-06-10-17-54.gh-issue-84808.ION67Z.rst

@@ -0,0 +1,3 @@
+Fix error handling in :py:class:`~socket.socket` method
+:py:func:`~socket.socket.connect_ex` on platforms where
+:c:data:`errno` can be negative.

Modules/socketmodule.c

@@ -3421,6 +3421,18 @@ sock_connect_impl(PySocketSockObject *s, void* Py_UNUSED(data))
     return 1;
 }
 
+/* Common functionality for socket.connect and socket.connect_ex.
+ *
+ * If *raise* is set:
+ * - On success, return 0.
+ * - On any failure, return -1 with an exception set.
+ * If *raise* is zero:
+ * - On success, return 0.
+ * - On connect() failure, return errno (without an exception set)
+ * - On other error, return -1 with an exception set.
+ *
+ *   Note that -1 is a valid errno value on some systems.
+ */
 static int
 internal_connect(PySocketSockObject *s, struct sockaddr *addr, int addrlen,
                  int raise)
@@ -3505,8 +3517,10 @@ sock_connect(PySocketSockObject *s, PyObject *addro)
     }
 
     res = internal_connect(s, SAS2SA(&addrbuf), addrlen, 1);
-    if (res < 0)
+    if (res < 0) {
+        assert(PyErr_Occurred());
         return NULL;
+    }
 
     Py_RETURN_NONE;
 }
@@ -3536,8 +3550,9 @@ sock_connect_ex(PySocketSockObject *s, PyObject *addro)
     }
 
     res = internal_connect(s, SAS2SA(&addrbuf), addrlen, 0);
-    if (res < 0)
+    if (res == -1 && PyErr_Occurred()) {
         return NULL;
+    }
 
     return PyLong_FromLong((long) res);
 }