Problems with native Unix domain sockets on Win 10/2019

[Michael McMahon]

On 24/09/2020 12:26, Ken Brown wrote:
> On 9/23/2020 7:25 AM, Michael McMahon via Cygwin wrote:
>> Hi,
>>
>> I searched for related issues but haven't found anything.
>>
>> I am having some trouble with Windows native Unix domain sockets
>> (a recent feature in Windows 10 and 2019 server) and Cygwin.
>> I think I possibly know the cause since I had to investigate a similar
>> looking issue on another platform built on Windows.
>>
>> The problem is that cygwin commands don't seem to recognise native Unix
>> domain sockets correctly. For example, the socket "foo.sock" should
>> have the same ownership and similar permissions to other files
>> in the example below:
>>
>> $ ls -lrt
>> total 2181303
>>
>> -rw-r--r--  1 mimcmah      None             1259   Sep 23 10:22 test.c
>> -rwxr-xr-x  1 mimcmah      None             3680   Sep 23 10:22 test.obj
>> -rwxr-xr-x  1 mimcmah      None             121344 Sep 23 10:22 test.exe
>> -rw-r-----  1 Unknown+User Unknown+Group         0 Sep 23 10:23 foo.sock
>> -rw-r--r--  1 mimcmah      None             144356 Sep 23 10:27 check.ot
>>
>> A bigger problem is that foo.sock can't be deleted with the cygwin "rm"
>> command.
>>
>> $ rm -f foo.sock
>> rm: cannot remove 'foo.sock': Permission denied
>>
>> $ chmod 777 foo.sock
>> chmod: changing permissions of 'foo.sock': Permission denied
>>
>> $ cmd /c del foo.sock
>>
>> But, native Windows commands are okay, as the third example shows.
>>
>> I think the problem may relate to the way native Unix domain sockets are
>> implemented in Windows and the resulting special handling required.
>> They are implemented as NTFS reparse points and when opening them
>> with CreateFile, you need to specify the FILE_FLAG_OPEN_REPARSE_POINT
>> flag. Otherwise, you get an ERROR_CANT_ACCESS_FILE. There are other
>> complications unfortunately, which I'd be happy to discuss further.
>>
>> But, to reproduce it, you can compile the attached code snippet
>> which creates foo.sock in the current directory. Obviously, this
>> only works on recent versions of Windows 10 and 2019 server.
> 
> Cygwin doesn't currently support native Windows AF_UNIX sockets, as 
> you've discovered.  See
> 
>    
> https://urldefense.com/v3/__https://cygwin.com/pipermail/cygwin/2020-June/245088.html__;!!GqivPVa7Brio!P7lIFI4rYAtWh8_DtCbRCxT-M_E4vwQ0qwzQ0p656T73BpJ0jbUkLI_bXdA6mmSL9lJcSQ$ 
> 
> for the current state of AF_UNIX sockets on Cygwin, including the 
> possibility of using native Windows AF_UNIX sockets on systems that 
> support them.
> 
> If all you want is for Cygwin to recognize such sockets and allow you to 
> apply rm, chmod, etc., I don't think it would be hard to add that 
> capability.  But I doubt if that's all you want.
> 
> Further discussion of this will have to wait until Corinna is available.
> 

Thanks for the info. It's mainly about recognition of sockets for
regular commands. Since these objects can exist on Windows filesystems
now, potentially created by any kind of Windows application,
it would be great if Cygwin could handle them, irrespective of whether
the Cygwin development environment does. Though that sounds like a
good idea too.

- Michael