CI validation only (do not merge): win32 COM0 device path

serial/serialwin32.py

@@ -13,13 +13,31 @@
 
 # pylint: disable=invalid-name,too-few-public-methods
 import ctypes
+import re
 import time
 from serial import win32
 
 import serial
 from serial.serialutil import SerialBase, SerialException, to_bytes, PortNotOpenError, SerialTimeoutException
 
 
+LEGACY_DOS_DEVICE_NAMES = frozenset('COM{}'.format(number) for number in range(1, 10))
+COM_PORT_NAME = re.compile('COM[0-9]+', re.IGNORECASE)
+
+
+def device_path(port):
+    r"""Return the name that Windows resolves to the given serial port.
+
+    Windows resolves only the bare names COM1 through COM9 as MS-DOS device
+    aliases. No other serial port has an alias, so COM0 as well as COM10 and
+    above must be named in the "\\.\COMx" device namespace format; a bare name
+    would be resolved as an ordinary relative file system path instead.
+    """
+    if COM_PORT_NAME.fullmatch(port) and port.upper() not in LEGACY_DOS_DEVICE_NAMES:
+        return '\\\\.\\' + port
+    return port
+
+
 class Serial(SerialBase):
     """Serial port implementation for Win32 based on ctypes."""
 
@@ -41,18 +59,8 @@ def open(self):
             raise SerialException("Port must be configured before it can be used.")
         if self.is_open:
             raise SerialException("Port is already open.")
-        # the "\\.\COMx" format is required for devices other than COM1-COM8
-        # not all versions of windows seem to support this properly
-        # so that the first few ports are used with the DOS device name
-        port = self.name
-        try:
-            if port.upper().startswith('COM') and int(port[3:]) > 8:
-                port = '\\\\.\\' + port
-        except ValueError:
-            # for like COMnotanumber
-            pass
         self._port_handle = win32.CreateFile(
-            port,
+            device_path(self.name),
             win32.GENERIC_READ | win32.GENERIC_WRITE,
             0,  # exclusive access
             None,  # no security

test/test_serialwin32.py

@@ -0,0 +1,66 @@
+# This file is part of pySerial - Cross platform serial port support for Python
+#
+# SPDX-License-Identifier:    BSD-3-Clause
+
+"""
+Test the Win32 serial port naming.
+"""
+
+import os
+
+import pytest
+
+if os.name == "nt":
+    from serial import serialwin32, win32
+
+pytestmark = pytest.mark.skipif(os.name != "nt", reason="Windows only")
+
+
+@pytest.mark.parametrize(
+    "port, expected",
+    (
+        ("COM1", "COM1"),
+        ("COM8", "COM8"),
+        ("COM9", "COM9"),
+        ("com3", "com3"),
+        ("COM0", r"\\.\COM0"),
+        ("com0", r"\\.\com0"),
+        ("COM00", r"\\.\COM00"),
+        ("COM01", r"\\.\COM01"),
+        ("COM10", r"\\.\COM10"),
+        ("COM255", r"\\.\COM255"),
+        (r"\\.\COM1", r"\\.\COM1"),
+        (r"\\.\COM0", r"\\.\COM0"),
+        ("COM", "COM"),
+        ("COMnotanumber", "COMnotanumber"),
+        ("/dev/ttyS0", "/dev/ttyS0"),
+    ),
+)
+def test_device_path(port, expected):
+    """Verify that every port without an MS-DOS device alias is prefixed."""
+
+    assert serialwin32.device_path(port) == expected
+
+
+@pytest.mark.parametrize(
+    "port, expected",
+    (
+        pytest.param("COM0", r"\\.\COM0", id="device-namespace"),
+        pytest.param("COM1", "COM1", id="ms-dos-alias"),
+    ),
+)
+def test_open_names_the_device(monkeypatch, port, expected):
+    """Verify that `open()` hands the resolvable name to CreateFile."""
+
+    names = []
+
+    def create_file(name, *args):
+        names.append(name)
+        return win32.INVALID_HANDLE_VALUE
+
+    monkeypatch.setattr(win32, "CreateFile", create_file)
+
+    with pytest.raises(serialwin32.SerialException):
+        serialwin32.Serial(port)
+
+    assert names == [expected]