windows code to detect if a process is still alive

[John Arbash Meinel]

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On 04/14/2011 09:22 AM, Voelker, Bernhard wrote:
> Martin Pool wrote:
> 
>> For detection of stale lock files, I want to detect whether a given
>> process id on the local machine still corresponds to a live process.
>> On unix, kill(pid, 0) will do this.  What's the best way to do this
>> from inside Python on Windows?
> 
> I don't know too much about Python, but as (C code) fopen("file", "w") locks
> a file for others automatically on Windows, doesn't Python just use
> that internally for locking?
> 
> Have a nice day,
> Berny

It locks it from being renamed, but any # of processes can open a file
for writing concurrently.

You can get into OS level locking, but that opens up a host of new bugs.
Plus, it doesn't work remotely. (You can't ask for an OS lock over ftp
or sftp, for example.)

Also, it is really hard to be a little bit cooperative with OS level
locks. In bzr, most Read locks are no-ops that just enable caching,
since our data structures on disk can be added to without violating
consistency of the older data.  OS level locks won't actually let you
have a Read lock and a Write lock. You also tend to get bad error messages.

For example, if I hold an OS lock, and your process wants to access it,
we get a "Cannot do that", but we don't get the information about what
user holds the lock, how long they've held it, etc. (hence the 'info' file.)

John
=:->
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