python
diff --git a/‎Lib/test/test_winconsoleio.py
Lines changed: 23 additions & 0 deletions b/‎Lib/test/test_winconsoleio.py
Lines changed: 23 additions & 0 deletions
diff --git a/‎Misc/NEWS.d/next/Library/2024-09-13-18-24-27.gh-issue-124008.XaiPQx.rst
Lines changed: 2 additions & 0 deletions b/‎Misc/NEWS.d/next/Library/2024-09-13-18-24-27.gh-issue-124008.XaiPQx.rst
Lines changed: 2 additions & 0 deletions
diff --git a/‎Modules/_io/winconsoleio.c
Lines changed: 90 additions & 28 deletions b/‎Modules/_io/winconsoleio.c
Lines changed: 90 additions & 28 deletions
@@ -142,6 +142,29 @@ def test_write_empty_data(self):
         with ConIO('CONOUT$', 'w') as f:
             self.assertEqual(f.write(b''), 0)
 
+    @requires_resource('console')
+    def test_write(self):
+        testcases = []
+        with ConIO('CONOUT$', 'w') as f:
+            for a in [
+                b'',
+                b'abc',
+                b'\xc2\xa7\xe2\x98\x83\xf0\x9f\x90\x8d',
+                b'\xff'*10,
+            ]:
+                for b in b'\xc2\xa7', b'\xe2\x98\x83', b'\xf0\x9f\x90\x8d':
+                    testcases.append(a + b)
+                    for i in range(1, len(b)):
+                        data = a + b[:i]
+                        testcases.append(data + b'z')
+                        testcases.append(data + b'\xff')
+                        # incomplete multibyte sequence
+                        with self.subTest(data=data):
+                            self.assertEqual(f.write(data), len(a))
+            for data in testcases:
+                with self.subTest(data=data):
+                    self.assertEqual(f.write(data), len(data))
+
     def assertStdinRoundTrip(self, text):
         stdin = open('CONIN$', 'r')
         old_stdin = sys.stdin
 
@@ -0,0 +1,2 @@
+Fix possible crash (in debug build), incorrect output or returning incorrect
+value from raw binary ``write()`` when writing to console on Windows.
@@ -135,19 +135,67 @@ char _PyIO_get_console_type(PyObject *path_or_fd) {
 }
 
 static DWORD
-_find_last_utf8_boundary(const char *buf, DWORD len)
+_find_last_utf8_boundary(const unsigned char *buf, DWORD len)
 {
-    /* This function never returns 0, returns the original len instead */
-    DWORD count = 1;
-    if (len == 0 || (buf[len - 1] & 0x80) == 0) {
-        return len;
-    }
-    for (;; count++) {
-        if (count > 3 || count >= len) {
+    for (DWORD count = 1; count < 4 && count <= len; count++) {
+        unsigned char c = buf[len - count];
+        if (c < 0x80) {
+            /* No starting byte found. */
             return len;
         }
-        if ((buf[len - count] & 0xc0) != 0x80) {
-            return len - count;
+        if (c >= 0xc0) {
+            if (c < 0xe0 /* 2-bytes sequence */ ? count < 2 :
+                c < 0xf0 /* 3-bytes sequence */ ? count < 3 :
+                c < 0xf8 /* 4-bytes sequence */)
+            {
+                /* Incomplete multibyte sequence. */
+                return len - count;
+            }
+            /* Either complete or invalid sequence. */
+            return len;
+        }
+    }
+    /* Either complete 4-bytes sequence or invalid sequence. */
+    return len;
+}
+
+/* Find the number of UTF-8 bytes that corresponds to the specified number of
+ * wchars.
+ * I.e. find x <= len so that MultiByteToWideChar(CP_UTF8, 0, s, x, NULL, 0) == n.
+ *
+ * WideCharToMultiByte() cannot be used for this, because the UTF-8 -> wchar
+ * conversion is not reversible (invalid UTF-8 byte produces \ufffd which
+ * will be converted back to 3-bytes UTF-8 sequence \xef\xbf\xbd).
+ * So we need to use binary search.
+ */
+static DWORD
+_wchar_to_utf8_count(const unsigned char *s, DWORD len, DWORD n)
+{
+    DWORD start = 0;
+    while (1) {
+        DWORD mid = 0;
+        for (DWORD i = len / 2; i <= len; i++) {
+            mid = _find_last_utf8_boundary(s, i);
+            if (mid != 0) {
+                break;
+            }
+            /* The middle could split the first multibytes sequence. */
+        }
+        if (mid == len) {
+            return start + len;
+        }
+        if (mid == 0) {
+            mid = len > 1 ? len - 1 : 1;
+        }
+        DWORD wlen = MultiByteToWideChar(CP_UTF8, 0, s, mid, NULL, 0);
+        if (wlen <= n) {
+            s += mid;
+            start += mid;
+            len -= mid;
+            n -= wlen;
+        }
+        else {
+            len = mid;
         }
     }
 }
@@ -563,8 +611,10 @@ read_console_w(HANDLE handle, DWORD maxlen, DWORD *readlen) {
     int err = 0, sig = 0;
 
     wchar_t *buf = (wchar_t*)PyMem_Malloc(maxlen * sizeof(wchar_t));
-    if (!buf)
+    if (!buf) {
+        PyErr_NoMemory();
         goto error;
+    }
 
     *readlen = 0;
 
@@ -622,6 +672,7 @@ read_console_w(HANDLE handle, DWORD maxlen, DWORD *readlen) {
             Py_UNBLOCK_THREADS
             if (!newbuf) {
                 sig = -1;
+                PyErr_NoMemory();
                 break;
             }
             buf = newbuf;
@@ -645,8 +696,10 @@ read_console_w(HANDLE handle, DWORD maxlen, DWORD *readlen) {
     if (*readlen > 0 && buf[0] == L'\x1a') {
         PyMem_Free(buf);
         buf = (wchar_t *)PyMem_Malloc(sizeof(wchar_t));
-        if (!buf)
+        if (!buf) {
+            PyErr_NoMemory();
             goto error;
+        }
         buf[0] = L'\0';
         *readlen = 0;
     }
@@ -824,8 +877,10 @@ _io__WindowsConsoleIO_readall_impl(winconsoleio *self)
     bufsize = BUFSIZ;
 
     buf = (wchar_t*)PyMem_Malloc((bufsize + 1) * sizeof(wchar_t));
-    if (buf == NULL)
+    if (buf == NULL) {
+        PyErr_NoMemory();
         return NULL;
+    }
 
     while (1) {
         wchar_t *subbuf;
@@ -847,6 +902,7 @@ _io__WindowsConsoleIO_readall_impl(winconsoleio *self)
                                          (bufsize + 1) * sizeof(wchar_t));
             if (tmp == NULL) {
                 PyMem_Free(buf);
+                PyErr_NoMemory();
                 return NULL;
             }
             buf = tmp;
@@ -1022,43 +1078,49 @@ _io__WindowsConsoleIO_write_impl(winconsoleio *self, PyTypeObject *cls,
         len = (DWORD)b->len;
 
     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;
+    const DWORD max_wlen = 32766 / sizeof(wchar_t);
+    /* UTF-8 to wchar ratio is at most 3:1. */
+    len = Py_MIN(len, max_wlen * 3);
+    while (1) {
         /* Fix for github issues gh-110913 and gh-82052. */
         len = _find_last_utf8_boundary(b->buf, len);
         wlen = MultiByteToWideChar(CP_UTF8, 0, b->buf, len, NULL, 0);
+        if (wlen <= max_wlen) {
+            break;
+        }
+        len /= 2;
     }
     Py_END_ALLOW_THREADS
 
-    if (!wlen)
-        return PyErr_SetFromWindowsErr(0);
+    if (!wlen) {
+        return PyLong_FromLong(0);
+    }
 
     wbuf = (wchar_t*)PyMem_Malloc(wlen * sizeof(wchar_t));
+    if (!wbuf) {
+        PyErr_NoMemory();
+        return NULL;
+    }
 
     Py_BEGIN_ALLOW_THREADS
     wlen = MultiByteToWideChar(CP_UTF8, 0, b->buf, len, wbuf, wlen);
     if (wlen) {
         res = WriteConsoleW(handle, wbuf, wlen, &n, NULL);
+#ifdef Py_DEBUG
+        if (res) {
+#else
         if (res && n < wlen) {
+#endif
             /* Wrote fewer characters than expected, which means our
              * len value may be wrong. So recalculate it from the
-             * characters that were written. As this could potentially
-             * result in a different value, we also validate that value.
+             * characters that were written.
              */
-            len = WideCharToMultiByte(CP_UTF8, 0, wbuf, n,
-                NULL, 0, NULL, NULL);
-            if (len) {
-                wlen = MultiByteToWideChar(CP_UTF8, 0, b->buf, len,
-                    NULL, 0);
-                assert(wlen == len);
-            }
+            len = _wchar_to_utf8_count(b->buf, len, n);
         }
     } else
         res = 0;