When ReadConsole is canceled by CancelSynchronousIo [1], for some reason the call still succeeds. It's probably related to the hack that maps STATUS_ALERTED to ERROR_OPERATION_ABORTED when a console read is interrupted by Ctrl+C.The problem is that, when canceled, ReadConsole doesn't update the value of lpNumberOfCharsRead. Thus in read_console_w inModules/_io/winconsoleio.c, the value of `n` is a random number that gets added to `readlen`, which is subsequently used to index into `buf`. The problem is the same for `n_read` in _PyOS_WindowsConsoleReadline.For example, in 3.6:    import sys, ctypes, threading    kernel32 = ctypes.WinDLL('kernel32')    hMain = kernel32.OpenThread(1, 0, kernel32.GetCurrentThreadId())    t = threading.Timer(30, kernel32.CancelSynchronousIo, (hMain,))    t.start()    sys.stdin.readline()    Breakpoint 0 hit    KERNELBASE!ReadConsoleW:    00007ffc`fb558200 4053            push    rbx    0:000> pt    KERNELBASE!GetImmersiveColorTypeFromName+0x49d2:    00007ffc`fb5d2672 c3              ret    0:000> r rax    rax=0000000000000001    0:000> ?? @$teb->LastErrorValue == 995    bool true    0:000> gu    python36!read_console_w+0x8b:    00000000`6e43a483 85c0            test    eax,eax    0:000> ?? n    unsigned long 0xefdc39e8    0:000> g    (1154.11fc): Access violation - code c0000005 (first chance)    First chance exceptions are reported before any exception handling.    This exception may be expected and handled.    python36!read_console_w+0x11f:    00000000`6e43a517 66833a0a        cmp     word ptr [rdx],0Ah                                            ds:000001e8`cfb72c7e=????If the value of `n` is initialized to (DWORD)-1, then checking for a failed or canceled call could be implemented as follows:    if (!res || (n == (DWORD)-1 && GetLastError() ==        ERROR_OPERATION_ABORTED)) {        err = GetLastError();        break;    }[1]:https://msdn.microsoft.com/en-us/library/ms684958

Oops, I pasted the MSDN link for ReadConsole instead of CancelSynchronousIo [1].[1]:https://msdn.microsoft.com/en-us/library/aa363794

New changesetce75df3031c86b78311b1ad76c39c0b39d7d7424 by Steve Dower (ValeriyaSinevich) in branch 'master':bpo-30237: Output error when ReadConsole is canceled by CancelSynchronousIo. (GH-7911)https://github.com/python/cpython/commit/ce75df3031c86b78311b1ad76c39c0b39d7d7424

New changesetc3af73d580888b4d444264e79dce6ec0818522cd by Steve Dower (Miss Islington (bot)) in branch '3.7':bpo-30237: Output error when ReadConsole is canceled by CancelSynchronousIo. (GH-7911)https://github.com/python/cpython/commit/c3af73d580888b4d444264e79dce6ec0818522cd

New changeset28bbbdabb1e3601047530febac1b05b7b89dc65e by Steve Dower in branch '3.6':bpo-30237: Output error when ReadConsole is canceled by CancelSynchronousIo. (GH-7911)https://github.com/python/cpython/commit/28bbbdabb1e3601047530febac1b05b7b89dc65e

PR 7911 was merged and backported. Should this issue be closed?

Yep, thanks for the ping!