bpo-42044: Write all bytes to the console in unbuffered mode on Windows

Note: a fix for issue11395 started to write bytes capped at 32k and mandated
the caller to handle partial writes, but when the text io flush was called,
it only does only a single call in unbuffered mode. This made it so that the
bytes unwritten were actually lost (and not printed) in the process.
The reason for that was historical to support older versions of Windows. As
of Windows 10, this is no longer needed and thus given issue42044, the number
of bytes written to the console is no longer capped.

Author: fabioz

Misc/NEWS.d/next/Windows/2021-06-11-15-13-08.bpo-42044.Jnky9u.rst

@@ -0,0 +1,2 @@
+When the io is flushed in unbuffered mode in Windows, all bytes are now
+properly written to the console.

Modules/_io/winconsoleio.c

@@ -978,16 +978,6 @@ _io__WindowsConsoleIO_write_impl(winconsoleio *self, Py_buffer *b)
 
     Py_BEGIN_ALLOW_THREADS
     wlen = MultiByteToWideChar(CP_UTF8, 0, b->buf, len, NULL, 0);
-
-    /* issue11395 there is an unspecified upper bound on how many bytes
-       can be written at once. We cap at 32k - the caller will have to
-       handle partial writes.
-       Since we don't know how many input bytes are being ignored, we
-       have to reduce and recalculate. */
-    while (wlen > 32766 / sizeof(wchar_t)) {
-        len /= 2;
-        wlen = MultiByteToWideChar(CP_UTF8, 0, b->buf, len, NULL, 0);
-    }
     Py_END_ALLOW_THREADS
 
     if (!wlen)
@@ -998,6 +988,11 @@ _io__WindowsConsoleIO_write_impl(winconsoleio *self, Py_buffer *b)
     Py_BEGIN_ALLOW_THREADS
     wlen = MultiByteToWideChar(CP_UTF8, 0, b->buf, len, wbuf, wlen);
     if (wlen) {
+        /* Note that it's possible that this would fail to print big strings
+         * on Windows 7 (before Windows 8 LPC-based pseudo-files were used
+         * for I/O and would be limited to a 64 KiB heap).
+         * As Windows 7 is currently unsupported, that's Ok.
+         */
         res = WriteConsoleW(handle, wbuf, wlen, &n, NULL);
         if (res && n < wlen) {
             /* Wrote fewer characters than expected, which means our