backporting and dual-licensing

[Shriramana Sharma <samjnaa@gmail.com>]

Say a person X writes a library libfoo. He licenses the library out 
under both the GPL and a commercial licence.

A person Y uses libfoo under the GPL. He goes and does a lot of 
improvements in the library since it is under the GPL. Now the modified 
version of libfoo is copyrighted by both X and Y.

So X would not be able to import such improvements into the main 
distribution of libfoo since then he would not be able to dual-licence 
it - specifically he would not be able to licence it commercially, not 
owning it entirely. He must either take Y into his business or obtain a 
waiver from Y by one-time payment or whatever.

My question is: What would be considered a big enough 
difference/modification that X would need Y's permission for backporting 
the changes?

1. If Y fixes a bug (of whatever severity) with a patch?

2. If Y makes improvements to the library by optimizing some algorithms?

3. If Y adds new functionality to the library?

4. If Y just restructures the library in a more efficient manner.

Shriramana Sharma.