[issue25492] subprocess with redirection fails after FreeConsole

[Eryk Sun]

Changes by Eryk Sun :


--
resolution:  -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> subprocess failing in GUI applications on Windows

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[issue25492] subprocess with redirection fails after FreeConsole

[George Prekas]

New submission from George Prekas:

Under Windows, if I do FreeConsole and then subprocess.call with redirected 
stdin or stdout or stderr, then subprocess.call fails. The only solution is 
either DO NOT redirect any handle OR to redirect ALL of them. The problem lies 
in function _get_handles at subprocess.py:810 (Python 2.7.10), which does:
* GetStdHandle
* DuplicateHandle via _make_inheritable.

DuplicateHandle fails if called on stdin, stdout or stderr after FreeConsole.

$ cygstart /mnt/c/utils/Python35/python.exe testcase.py fails
$ cygstart /mnt/c/utils/Python35/python.exe testcase.py works1
$ cygstart /mnt/c/utils/Python35/python.exe testcase.py works2
$ cygstart /mnt/c/utils/Python35/python.exe testcase.py works3

--
components: Library (Lib)
files: testcase.py
messages: 253547
nosy: George Prekas
priority: normal
severity: normal
status: open
title: subprocess with redirection fails after FreeConsole
type: behavior
versions: Python 2.7, Python 3.5
Added file: http://bugs.python.org/file40868/testcase.py

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[issue25492] subprocess with redirection fails after FreeConsole

[Zachary Ware]

Changes by Zachary Ware :


--
components: +Windows
nosy: +eryksun, paul.moore, steve.dower, tim.golden, zach.ware

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[issue25492] subprocess with redirection fails after FreeConsole

[eryksun]

eryksun added the comment:

For me, fails() lives up to its name in Windows 7, but it doesn't fail in 
Windows 10. It shouldn't fail in Windows 8, either. 

In Windows 8+ the console interface is implemented using a kernel device. 
Console handles reference virtual files on the ConDrv device, such as Connect, 
Input, and Output. The example doesn't fail because the I/O handles (Input and 
Output) are valid even if the main console handle (Connect) has been closed by 
calling FreeConsole. 

OTOH, prior to Windows 8, console I/O handles don't reference kernel objects. 
You may have noticed that the old API tags the values by setting the lower 2 
bits (e.g. 3, 7, 11). This enables redirecting actions on console I/O handles 
to functions in the console host process (i.e. conhost.exe in Win 7, and 
csrss.exe in XP/Vista). For example, for a regular kernel object, the 
DuplicateHandle API is serviced by the system call NtDuplicateObject. But for a 
console handle, Windows instead sends the request to the console LPC port, to 
be dispatched to SrvDuplicateHandle in the console. Obviously this can't work 
after the client has called FreeConsole. 

Prior to Windows 8, this invalid-handle error can also be the result of running 
pythonw.exe from a console application such as python.exe or cmd.exe. See issue 
3905. In this case Windows is copying the values of the parent's standard 
handles into the pythonw.exe process parameters, but they're meaningless values 
since pythonw.exe doesn't attach to a console automatically. (You could of 
course manually call AllocConsole or AttachConsole.) The new implementation in 
Windows 8 and 10 is smart enough to initialize the standard handles to 0 in 
this case. Thus in Windows 10 Terry's example in msg220218 doesn't raise and 
exception, and p.stdout.read() == b'32\r\n'.

A workaround for Python 3.4+ on older Windows versions would be to check 
os.get_handle_inheritable, which calls WinAPI GetHandleInformation [1]:

ERROR_INVALID_HANDLE = 0x0006

if stdin is None:
p2cread = _winapi.GetStdHandle(_winapi.STD_INPUT_HANDLE)
try:
os.get_handle_inheritable(p2cread)
except OSError as e:
if e.winerror != ERROR_INVALID_HANDLE:
raise
p2cread = None
if p2cread is None:
p2cread, _ = _winapi.CreatePipe(None, 0)
p2cread = Handle(p2cread)
_winapi.CloseHandle(_)

For 2.7, GetHandleInformation could be added to _subprocess. But then it may as 
well be added to Python 3's _winapi as well. 

[1]: https://msdn.microsoft.com/en-us/library/ms724329

--
versions: +Python 3.4, Python 3.6

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[issue25492] subprocess with redirection fails after FreeConsole

[eryksun]

eryksun added the comment:

I forgot to first check whether p2cread is None:

ERROR_INVALID_HANDLE = 0x0006

if stdin is None:
p2cread = _winapi.GetStdHandle(_winapi.STD_INPUT_HANDLE)
if p2cread is not None:
try:
os.get_handle_inheritable(p2cread)
except OSError as e:
if e.winerror != ERROR_INVALID_HANDLE:
raise
p2cread = None
if p2cread is None:
p2cread, _ = _winapi.CreatePipe(None, 0)
p2cread = Handle(p2cread)
_winapi.CloseHandle(_)

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