We have a set of DDS adapters written in C++ with necessary callbacks and pub/sub message functions to send data back and forth across messaging buses between two pieces of software.
The maintainers of the adapters are only interested in maintaining callbacks and functions they use for their own purposes. But our team runs our own tool using these adapters and we need one additional callback and pub/sub message function that the upstream maintainers do not.
Until now, we've been using a fork of their upstream, where we chop the functionality we do not need, add the functionality we do need, and then instantiate both sets of adapters to manage their respective communications over the buses.
The problem that's increasingly apparent is the divergence of the structural code, e.g. a certain adapter class is modified by the maintainers to have an entirely new class structure, albeit with the same data members for the time being. Our old version is probably going to work...until it doesn't.
What git strategy could we use on our modified set of adapters to both keep the structural code up to date with the upstream, while also keeping our local changes and additional functionality?