[io] _WindowsConsoleIO misbehaves when Ctrl+C is ignored

[eryksun]

BPO	28166
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba, @ZackerySpytz, @ValeriyaSinevich
PRs	bpo-28166: Continue to read after Ctrl+C if SIGINT was ignored #8344 gh-72353: WindowsConsoleIO misbehavior when Ctrl+C is ignored #17976

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-09-15.08:10:25.219>
labels = ['interpreter-core', 'type-bug', '3.8', '3.9', 'expert-IO', '3.10', 'OS-windows']
title = 'WindowsConsoleIO misbehavior when Ctrl+C is ignored'
updated_at = <Date 2021-03-20.07:45:47.180>
user = 'https://github.com/eryksun'

bugs.python.org fields:

activity = <Date 2021-03-20.07:45:47.180>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core', 'Windows', 'IO']
creation = <Date 2016-09-15.08:10:25.219>
creator = 'eryksun'
dependencies = []
files = []
hgrepos = []
issue_num = 28166
keywords = ['patch']
message_count = 3.0
messages = ['276530', '321979', '322003']
nosy_count = 7.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'ZackerySpytz', 'valer']
pr_nums = ['8344', '17976']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28166'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

Linked PRs

• gh-72353: Continue reading from the console after ignored Ctrl+C #155434
• [3.15] gh-72353: Continue reading from the console after ignored Ctrl+C (GH-155434) #156589
• [3.14] gh-72353: Continue reading from the console after ignored Ctrl+C (GH-155434) #156590
• [3.13] gh-72353: Continue reading from the console after ignored Ctrl+C (GH-155434) #156591

[eryksun]

_PyOS_WindowsConsoleReadline should continue to read when interrupted by Ctrl+C if SIGINT is ignored or the handler doesn't raise an exception. Currently it breaks out of the read loop, which looks like an EOF:

    >>> import signal
    >>> signal.signal(signal.SIGINT, signal.SIG_IGN)
    <built-in function default_int_handler>
    >>> input()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    EOFError 

Also, in this case Ctrl+C quits the REPL.

Similarly sys.stdin.buffer.raw should retry a read. Instead a read(1) returns an empty string and readall() raises an exception:

    >>> sys.stdin.buffer.raw.read(1)
    b''

    >>> sys.stdin.buffer.raw.read()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: [WinError 87] The parameter is incorrect

[ValeriyaSinevich]

I posted a PR but the problem with it is that all the input before the ignored Ctrl+C is lost.

[eryksun]

all the input before the ignored Ctrl+C is lost

The console host process doesn't know that Python is ignoring Ctrl+C, so it cancels the read and flushes the input buffer. For now, we have to accept this as a limitation of relying on the high-level console API (i.e. ReadConsoleW with the default settings), which implements command-line editing, aliases and history completely in the console host process. Restarting the read with a flushed input buffer is better than raising EOFError or exiting the REPL.

In the future, Python's console readline implementation could hook deeper into the operation by temporarily disabling the console's ENABLE_PROCESSED_INPUT mode and using the pInputControl parameter to handle ^C and ^H (backspace). This would be fairly involved but still an imperfect solution because ^C will replace whichever character is under the cursor (fine if the cursor is at the end of the line, but otherwise pretty clunky). The only way to have complete control is to use the low-level ReadConsoleInput function. pyreadline implements this via ctypes.