[issue37517] Improve error messages for Windows reserved file names

[Eryk Sun]

Eryk Sun  added the comment:

> if the operation succeeds, no error/message is displayed

Some errors pass silently and may be confusing later on. For example, 
CreateDirectoryW calls NtCreateFile with the disposition FILE_CREATE and the 
option FILE_DIRECTORY_FILE (i.e. the call *must* create a new directory), but 
some non-filesystem devices ignore this and let the request succeed. For 
example:

>>> os.mkdir('C:/Temp/nul')
>>> os.mkdir('C:/Temp/conin$')

Both calls 'succeed' but don't actually create a directory:

>>> os.path.exists(r'\\?\C:\Temp\nul')
False
>>> os.path.exists(r'\\?\C:\Temp\conin$')
False

> either we need to search the string for the special names or find 
> an API that will clarify it

GetFullPathNameW is a library function that shares the implementation that's 
used to normalize paths in a create or open context. 

If the path does not start with \\?\, then we resolve it via GetFullPathNameW. 
If it becomes a device path, or if the final component changes (e.g. a trailing 
dot or space is removed), then we can parenthetically include the resolved 
path. For example:

os.stat('spam.'):

FileNotFoundError: [WinError 2] The system cannot find the file specified: 
'spam.' [resolved path: 'C:\\Temp\\spam']

os.startfile('con'):

OSError: [WinError 1200] The specified device name is invalid: 'con' 
[resolved path: '.\\con']

os.open('C:/Temp/lpt9', 0):

FileNotFoundError: [Errno 2] No such file or directory: 'C:/Temp/lpt9' 
[resolved path: '.\\lpt9']

For os.open() and open(), we should switch to using 
PyErr_SetExcFromWindowsErrWithFilenameObject instead of 
PyErr_SetFromErrnoWithFilenameObject. _doserrno is valid in this context, so we 
should be using it anyway to retain the more specific winerror value. Then we 
only have to special case reserved names in the Windows-only function 
PyErr_SetExcFromWindowsErrWithFilenameObjects.

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keywords: +3.6regression

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[issue37517] Improve error messages for Windows reserved file names

[Steve Dower]

Steve Dower  added the comment:

Presumably this is linked to issue37515 (why not just repurpose that one?), but 
I'm inclined to think this is okay provided:

* if the operation succeeds, no error/message is displayed
* if the operation fails, we only update the error message if one of the 
special filenames is the target

So probably this ought to be a special case in 
PyErr_SetFromWindowsErrWithFilename(). And either we need to search the string 
for the special names or find an API that will clarify it (GetFileAttributes?).

I definitely do not want every error message suggesting that this edge case may 
be the cause, and I don't want to prevent people using these names or purpose 
for their actual uses.

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[issue37517] Improve error messages for Windows reserved file names

[Eryk Sun]

Eryk Sun  added the comment:

> Perhaps Windows builds can check for reserved file names and give a
> more descriptive error message in the event of IO error?

An operation on a reserved DOS device name can also succeed with unexpected 
results. For example, a script may unintentionally write to the active console 
screen buffer, "conout$":

>>> open('C:/conout$::. .::.dat', 'w').write('spam\n')
spam
5 

There's also the issue of normalization that removes trailing spaces and dots 
from the final path component. All paths get normalized, except for device 
paths that begin with exactly "\\?\" (i.e. extended paths) in a create or open 
context. For example, say a script creates a file with the reserved name "spam. 
. .":

>>> open(r'\\?\C:\Temp\spam. . .', 'w').close()

Then later, it generically uses os.walk('C:/Temp'), without the "\\?\" prefix, 
and tries to remove the file:

>>> os.remove('C:/Temp/spam. . .')
Traceback (most recent call last):
  File "", line 1, in 
FileNotFoundError: [WinError 2] The system cannot find the file specified: 
'C:/Temp/spam. . .'

Without an extended path, "spam. . ." gets normalized as "spam". The script 
would need to use os.walk(r'\\?\C:\Temp'). Should we special case this error as 
well to suggest using an extended path?

> Eryksun also mentions two reserved names which Microsoft apparently 
> does not document: "CONIN$" and "CONOUT$".

The system's behavior with these two names depends on the Windows version. In 
Windows 7 and earlier, "CONIN$" and "CONOUT$" are special cased by CreateFileW, 
and only when it's just the bare names (case insensitive) without trailing 
colons, spaces, or an extension, and never in a directory. In Windows 8+, as 
part of updating the internal console implementation to use an I/O device (i.e. 
"\Device\ConDrv"), "CONIN$" and "CONOUT$" were added to the system runtime 
library's list of DOS devices, so they behave the same as other DOS device 
names, including "NUL", "CON", "AUX", "PRN", "COM<1-9>", and "LPT<1-9>". This 
change is undocumented.

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[issue37517] Improve error messages for Windows reserved file names

[Steven D'Aprano]

Change by Steven D'Aprano :


--
type:  -> enhancement

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[issue37517] Improve error messages for Windows reserved file names

[Steven D'Aprano]

New submission from Steven D'Aprano :

See #37515.

Perhaps Windows builds can check for reserved file names and give a more 
descriptive error message in the event of IO error?

(Eryksun also mentions two reserved names which Microsoft apparently does not 
document: "CONIN$" and "CONOUT$".)

--
components: Windows
messages: 347456
nosy: CarK, SilentGhost, eryksun, paul.moore, steve.dower, steven.daprano, 
tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Improve error messages for Windows reserved file names
versions: Python 3.8

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