[python-win32] Python service hangs in Dispatch when frozen with py2exe

[Mark Hammond]

On 2/06/2010 3:42 AM, Don Dwiggins wrote:
> The object is part of a DLL written in VB6. It's currently single
> threaded -- apartment threading is the only other option in the IDE.

Right - Apartment threading might give better results.

>>
>> What is your main thread doing? Can you make it run a message loop to
>> test (ie, call pythoncom.PumpMessages() in that main thread?)
>
> Hmm, I'll have to look at that. I'm running under a Twisted reactor, and
> I'm not sure without looking how the reactor sets up the thread. I'm not
> sure how (or why) PumpMessages would work here -- there's nothing going
> on in the object that would require a Windows message loop.

The Windows message loop is used by the COM marshalling process.  IIRC, 
the first thread to initialize COM in a process is the thread in which 
single-threaded objects will always end up being called in.  If a 
different thread creates the object, COM uses Windows messages to 
marshall all calls back to that main thread.  IOW, your second thread 
makes a call - even to create the object - which results in that thread 
sending a windows message to the main thread to act on the request.

What this probably means in practice is that twisted needs to use a 
reactor which calls MsgWaitForMultipleObjects() and runs a message loop 
when the function detects a new message is in the queue.  I'm not sure 
if there is an existing reactor which does this.

IIRC, apartment threading may fix this - the second thread can call 
CoInit indicating it is a new apartment, which should mean that no 
marshalling will happen between that thread and the new 
apartment-threaded object.  As mentioned above though, if the object 
reports it is "single threaded", calls will *always* be marshalled to 
the main thread and will always require a message loop to work correctly.

HTH,

Mark