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Subject: Re: Retrieving per-process environment block?
To: cygwin AT cygwin DOT com
References: <CAOTD34ZFH5E3r3AuDOXctss46e1hoU2f9pwE1mt4L674J2Ak_A AT mail DOT gmail DOT com> <20161117140012 DOT GA23664 AT calimero DOT vinschen DOT de> <CAOTD34Y9TRq2Qq8Mn2awTf9SCgz0qnbBa-a117pkSEvz9gaHKQ AT mail DOT gmail DOT com>
From: cyg Simple <cygsimple AT gmail DOT com>
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Date: Tue, 29 Nov 2016 09:11:38 -0500
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On 11/29/2016 8:26 AM, Erik Bray wrote:
> On Thu, Nov 17, 2016 at 3:00 PM, Corinna Vinschen
> <corinna-cygwin AT cygwin DOT com> wrote:
>> On Nov 17 14:30, Erik Bray wrote:
>>> Hi all,
>>>
>>> For a quick bit of background, I'm working on porting the highly
>>> useful psutil [1] Python library to Cygwin.  This has proved an
>>> interesting exercise, as much of the functionality of psutil works on
>>> Cygwin through existing POSIX interfaces, and a handful of
>>> Linux-specific interfaces as well.  But there are some bits that
>>> simply don't map at all.
>>>
>>> The one I'm struggling with right now is retrieving Cygwin environment
>>> variables for a process (under inspection--i.e. not listing a
>>> process's environment from within that process which is obviously
>>> trivial).
>>>
>>> I've looked at every route I could conceive of but as far as I can
>>> tell this is currently impossible.  That's fine for now--I simply
>>> disable that functionality in psutil.  But it is unfortunate, though,
>>> since the information is there.
>>>
>>> There are a couple avenues I could see to this.  The most "obvious"
>>> (to me) being to implement /proc/<pid>/environ.
>>>
>>> I would be willing to provide a patch for this if it would be
>>> accepted.  Is there some particular non-obvious hurdle to this that it
>>> hasn't been implemented?  Obviously there are security
>>> implications--the /proc/<pid>/environ should only be readable to the
>>> process's owner, but that is already within Cygwin's capabilities, and
>>> works for other /proc files.
>>
>> Patch welcome.  Implementing this should be fairly straightforward.
>> The only hurdle is winsup/CONTRIBUTORS ;)
> 
> Thanks--I went to go work on this finally but it turns out not to be
> straightforward after all, as the process's environment is not shared
> in any way between processes.
> 
> I could do this, if each process kept a copy of its environment block
> in shared memory, which would in turn have to be updated every time
> the process's environment is updated.  But I don't know what the
> impact of that would be performance-wise.
> 
> Any advice?
> 

Sounds like a job for a thread that wakes every X time units to check
the contents of the environment.  Or is there a notification API that
could be used to wake the thread?  I know there is a disk change
notification API for this; maybe one for environment changes as well.

-- 
cyg Simple

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