gh-69573: Check the return value of _putch() and _putwch() in msvcrt

[gianghungtien]

msvcrt.putch() and msvcrt.putwch() discarded the value returned by the underlying _putch() / _putwch() CRT functions, so a failed write was silently ignored. This has been the case since the module was written, and it is inconsistent with ungetch() / ungetwch() in the same module, which already check their return value and raise OSError.

The most common failure mode is a process with no console attached (for example running under pythonw.exe, or a child created with DETACHED_PROCESS). In that situation _putch() returns EOF and _putwch() returns WEOF without writing anything, and the caller had no way to notice.

Why not just PyErr_SetFromErrno()

Copying the existing ungetch() pattern verbatim would have been wrong here. I checked what the UCRT actually reports on this failure, in a process started without a console:

_putwch  ret=65535 (WEOF)  errno=0  GetLastError=0
_putch   ret=-1    (EOF)   errno=0  GetLastError=0

The CRT short-circuits on the missing console handle and records no error information at all, so PyErr_SetFromErrno() on its own would produce a misleading [Errno 0] exception. The fix therefore uses the errno-based message when an error code is available, and falls back to a generic OSError("write to console failed") when it is not.

For reference, _ungetch() succeeds without a console (it only fills a CRT-internal pushback buffer), which is why that function never exposed this problem.

Tests

• test_putch_without_console runs a child process with the DETACHED_PROCESS creation flag (no console at all) and asserts that both functions raise OSError. It needs no gui resource, so it runs by default. Verified that it fails against an unpatched build (msvcrt.putch() did not raise OSError) and passes with the fix.
• The existing test_putch / test_putwch could previously never fail, since the functions ignored all errors. Now that they can raise, they are guarded with a requires_console skip so they do not spuriously fail when the test suite itself is run without a console.

PCbuild\build.bat -p x64 -c Release compiles clean (0 warnings), and python -m test test_msvcrt passes (14 run, 3 skipped for the gui resource).

Compatibility

This turns a silent no-op into a raised exception, which is a behaviour change. It matches what the issue asks for and what the module already does elsewhere; putch()/putwch() are documented only as writing to the console, and code that relied on the silent failure was not writing anything anyway. Happy to adjust the scope or the error message if reviewers prefer something different.

The getch() / getche() / getwch() / getwche() family has the same underlying problem, but changing those affects return values rather than just error reporting, so I left them out to keep this change reviewable. Glad to follow up separately if wanted.

• Issue: msvcrt_putch/msvcrt_putwch don't check the return value of _putch/_putwch #69573

[read-the-docs-community]

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