sftp breakage on Win32

[John A Meinel]

David Allouche wrote:
> On Wed, 2005-11-30 at 12:12 -0500, Matt Lavin wrote:
>> After catching up with bzr.dev on my Windows machine I ran into the 
>> following problem using sftp on Windows.  To work around the issue I 
>> just removed the 'close_fds' argument, but I doubt that it the best 
>> solution.  I checked the Python docs and it doesn't mention that the 
>> argument is not supported on Windows.
> 
> From memory, when I looked quite closely at how subprocess handled
> process spawning and compared it to twisted, and asked Robert Collins
> for extra information:
> 
>       * On win32, fds are not inherited, you have to pass them
>         explicitly across to the subprocess. So close_fds is implicit on
>         win32.
>       * That's probably one reason why twisted will close all fds not
>         explicitely passed to the subprocess, which makes process
>         spawning in twisted quite needlessly expensive (it iterate over
>         all 32k entries of the fd space).
>       * The subprocess module does not provide support for passing
>         anything but stdio fds. Except when using the  preexec callback
>         which is not supported on windows either because subprocesses
>         there are created by spawn, not by fork-exec.
> 
> In my opinion, the subprocess module sucks seriously in that particular
> respect, but it's still clearly the least broken process spawning
> facility available in the standard library.
> 
> So, if my understanding of the situation is correct, and you care about
> close_fds, you need to duplicate the os.name check done in the
> subprocess module and pass close_fds set close_fds to true if and only
> if that test says it's not windows.
> 
> However, you probably do not need to care about close_fds because, as
> far as I know, the number of open fds on a file only makes a difference
> for pipes (i/o on a pipe may block instead of failing if some subprocess
> has a stray fd to this pipe).
> 
> Caveats, that's all from memory, and I do not really know anything about
> windows process and fd handling.

Thanks for more info. I did do a check for "sys.platform" which sets
close_fds as appropriate.

I agree we probably don't really care about them. I think the main
reason is just for security. Just to have as streamlined a subprocess as
we can.

John
=:->


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