Correct the watchdog poll mask note and use the public get_osfhandle

The null device reports POLLIN (plus POLLOUT under the default event
mask), never POLLERR; the migration note taught the wrong mask. The
Windows handle rebind now uses msvcrt.get_osfhandle, the documented
public API, instead of pywin32's underscore-private equivalent, which
also drops the win32file import. The migration entry also gains the
one API-visible change: a second concurrent stdio_server() raises
RuntimeError.

Author: maxisbey

docs/migration.md

@@ -1866,15 +1866,20 @@ during a session now sees the diversions, not the wire.
 
 One pattern needs migrating: watchdog threads that watch fd 0 to detect a vanished
 client (a POSIX-specific pattern; `select.poll` does not exist on Windows). The null
-device does not behave like the old pipe: it never reports `POLLHUP`, and it reports
-readable immediately and permanently (`POLLIN|POLLERR` from `poll()` on Linux, ready
-from `select()`, and macOS can report `POLLNVAL` for devices). A watcher waiting for
-`POLLHUP` is silently disarmed; a watcher that treats any event as "client gone" now
-fires at startup instead of never. Watch the parent process instead: on POSIX, exit
+device does not behave like the old pipe: it never reports `POLLHUP` or `POLLERR`,
+and it reports readable immediately and permanently (`POLLIN` from `poll()` on Linux,
+plus `POLLOUT` under the default event mask; ready from `select()`; and macOS can
+report `POLLNVAL` for devices). A watcher waiting for `POLLHUP` or `POLLERR` is
+silently disarmed; a watcher that treats any event as "client gone" now fires at
+startup instead of never. Watch the parent process instead: on POSIX, exit
 when `os.getppid()` changes, which happens when the client dies because orphaned
 processes are reparented. That works on both v1 and v2 and does not depend on
 descriptor layout.
 
+Also new: a second concurrent `stdio_server()` on the process's default streams now
+raises `RuntimeError` instead of silently contending for stdin, a configuration that
+never worked (there is one stdin).
+
 Also worth knowing: a child process that streams large output to its inherited
 stdout now streams it into the client's stderr channel. Capture output you do not
 want in the client's logs, and be aware that a client which never drains its stderr

src/mcp/os/win32/utilities.py

@@ -17,16 +17,17 @@
 
 # Windows-specific imports for Job Objects
 if sys.platform == "win32":
+    import msvcrt
+
     import pywintypes
     import win32api
     import win32con
-    import win32file
     import win32job
 else:
     # Type stubs for non-Windows platforms
     win32api = None
     win32con = None
-    win32file = None
+    msvcrt = None
     win32job = None
     pywintypes = None
 
@@ -40,11 +41,11 @@ def rebind_std_handle_to_fd(fd: int) -> None:
     Raises:
         OSError: The slot could not be set.
     """
-    if sys.platform != "win32" or not win32api or not win32file or not pywintypes:
+    if sys.platform != "win32" or not win32api or not msvcrt or not pywintypes:
         return
     std_ids = {0: win32api.STD_INPUT_HANDLE, 1: win32api.STD_OUTPUT_HANDLE, 2: win32api.STD_ERROR_HANDLE}
     try:
-        win32api.SetStdHandle(std_ids[fd], win32file._get_osfhandle(fd))
+        win32api.SetStdHandle(std_ids[fd], msvcrt.get_osfhandle(fd))
     except pywintypes.error as exc:
         # Normalized so callers' OSError-based best-effort handling covers it.
         raise OSError(f"SetStdHandle failed for fd {fd}") from exc