Initially the concern is only for outbound notifications. However, there will be inbound notifications. For inbound the library can provide an fdfor DC to select() on so that any execution of Condor code by thelibrary would be coming from a Condor controlled thread of control. Thelibrary would not call into any Condor code from a thread it controls.
It seems to me that the library would need to be designed specifically with this in mind. For example, if the Condor controlled thread is reading from something or writing to it, what guarantees do I have that the library isn't also reading or writing to it? I think you'd need to provide lock and unlock functions to be called from within Condor to guard against this, or find some other way to synchronize -- in other words, the library would need to be designed to link against a single threaded application.
Have you discussed this with the authors/maintainers of the library? I can't imagine that this is the first time anyone's ever asked this.
Take a look at some of the GAHP implementations. I think we'd need a pretty similar interface. Or, as Todd implies, we could make a GAHP for the threaded messaging library.
-Ian