Python Signal Emitter Race Condition Question

[Havoc Pennington]

Thiago Macieira wrote:
> Jim Kytola wrote:
>> For full disclosure, I have another script that listens for
>> NameOwnerChanged and when it finds the org.mozilla.thunderbird.DBus
>> change, connects a callback function to the NewMail signal - so by
>> successful run, I mean the callback function gets fired. It's certainly
>> possible that my problem is on that side, but it does seem strange that
>> I'm seeing the NewMail signal before the GetNameOwner method call in
>> the bad run.
> 
> This is your problem: you're waiting for the application to register.
> 
> Don't.
> 
> Connect the signal as soon as the listener starts. When the application 
> starts, register and emits the signal, your listener will have already 
> set everything up. In fact, it won't even need to listen for the 
> NameOwnerChanged signal.
> 

To add to this a bit, whenever you connect to a signal *after* the 
emitter exists you need to use this pattern:

  1) connect to signal
  2) request the current state that would be notified about by the signal

(in that order)

Any API with signals must have a way to do 2) or it's broken, because an 
app can't connect race-free.

But Thiago is certainly right that you need not wait for the bus name to 
exist before you match for signals on it.

Havoc