gh-160622: Fix socket recv() and recvfrom() with MSG_TRUNC

With MSG_TRUNC, recv() and recvfrom() can return the real length of
the datagram, which may be larger than the requested size. Since
PyBytesWriter_FinishWithSize() now rejects a size larger than the
allocated one, this raised ValueError after the datagram had already
been consumed. Return only the bytes actually received instead.

Author: ViderStar

Lib/test/test_socket.py

@@ -3286,6 +3286,26 @@ def testRecvFromNegative(self):
     def _testRecvFromNegative(self):
         self.cli.sendto(MSG, 0, (HOST, self.port))
 
+    # gh-160622: with MSG_TRUNC, recv() and recvfrom() must not return
+    # more bytes than requested, even if the datagram was longer.
+    @requireAttrs(socket, "MSG_TRUNC")
+    @unittest.skipIf(sys.platform == "win32", "Windows raises WSAEMSGSIZE")
+    def testRecvTrunc(self):
+        msg = self.serv.recv(2, socket.MSG_TRUNC)
+        self.assertEqual(msg, MSG[:2])
+
+    def _testRecvTrunc(self):
+        self.cli.sendto(MSG, 0, (HOST, self.port))
+
+    @requireAttrs(socket, "MSG_TRUNC")
+    @unittest.skipIf(sys.platform == "win32", "Windows raises WSAEMSGSIZE")
+    def testRecvFromTrunc(self):
+        msg, addr = self.serv.recvfrom(2, socket.MSG_TRUNC)
+        self.assertEqual(msg, MSG[:2])
+
+    def _testRecvFromTrunc(self):
+        self.cli.sendto(MSG, 0, (HOST, self.port))
+
 
 @unittest.skipUnless(HAVE_SOCKET_UDPLITE,
           'UDPLITE sockets required for this test.')

Misc/NEWS.d/next/Library/2026-10-25-19-04-57.gh-issue-160622.7n56jv.rst

@@ -0,0 +1,4 @@
+:meth:`socket.socket.recv` and :meth:`socket.socket.recvfrom` no longer
+raise :exc:`ValueError` and lose the datagram when called with
+``MSG_TRUNC`` and the datagram is longer than *bufsize*. Only the bytes
+actually received are returned now.

Modules/socketmodule.c

@@ -4066,6 +4066,12 @@ _socket_socket_recv_impl(PySocketSockObject *s, Py_ssize_t recvlen,
         PyBytesWriter_Discard(writer);
         return NULL;
     }
+    /* With MSG_TRUNC, recv() can return the real length of the datagram,
+       which may be larger than the buffer.  Only recvlen bytes were
+       written. */
+    if (outlen > recvlen) {
+        outlen = recvlen;
+    }
 
     return PyBytesWriter_FinishWithSize(writer, outlen);
 }
@@ -4242,6 +4248,10 @@ _socket_socket_recvfrom_impl(PySocketSockObject *s, Py_ssize_t recvlen,
         PyBytesWriter_Discard(writer);
         goto error;
     }
+    /* See the comment in _socket_socket_recv_impl(). */
+    if (outlen > recvlen) {
+        outlen = recvlen;
+    }
 
     PyObject *buf = PyBytesWriter_FinishWithSize(writer, outlen);
     if (buf == NULL) {