GeezerVison

With MSG_TRUNC, Linux recv(2)/recvfrom(2) return the real length of the datagram, which can be larger than the buffer. Since gh-157226, PyBytesWriter_FinishWithSize() rejects a size larger than the allocated one, so sock.recv(n, MSG_TRUNC) and sock.recvfrom(n, MSG_TRUNC) raise ValueError after the datagram has already been consumed.

This clamps the returned length to the requested size, so only the bytes actually received are returned (as suggested in gh-69121). recv_into() and recvfrom_into() are unchanged: they still return the value from the system call as is, which is the way to get the full datagram length.

Tests: added testRecvTrunc and testRecvFromTrunc to BasicUDPTest. On a Linux debug build they fail on main with ValueError: size larger than allocated size and pass with this change; full test_socket passes on Linux and macOS.

• Issue: socket.recv() and recvfrom() with MSG_TRUNC raise ValueError and lose the datagram since 3.15.1 #160622