This is kind of the crux of a long history of these licensing arguments. That dynamic linking is problematic because it potentially creates a derived work. And then you might distribute that derived work.

But shelling out to something or in some cases having an automated script create or download the derived work automatically is sometimes acceptable.

It’s a bit of a silly line in some ways. But it’s a long story.

I'm skeptical that there's a coherent legal distinction to be drawn between using a dynamic library with dlopen etc and running a similar executable with arguments. It's functionally identical usage of some public interface.

Agreed, I am very skeptical too. I have a long time believed that dynamic linking vs running an executable with arguments will be regarded the same in a court. So either both are derivative works or neither.

My personal belief is that a court will only care about if your code is a wrapper (e.g, libgpg) or if it is its independently useful and just uses the library (e.g. PostgreSQL using libreadline) for a small part of its operation. But I am not a lawyer and this has not been tested in court as far as I know.

They don't seem to claim there is a distinction on the mechanics there? Rather their distinction seems to be "are these part of the same overall program, or two separate ones", however you measure that: https://www.gnu.org/licenses/gpl-faq.html#GPLPlugins

By claiming that using dlopen to load some code makes your program a derivative work of the library it's using, but shelling out to it doesn't, they're implicitly making a claim as to the distinction of the mechanics.

This has always been my perspective as well, but it goes against long-established norms—for example, it's for exactly this reason that the LGPL was created. I don't think either theory has been thoroughly tested in court, though. See https://sfconservancy.org/blog/2016/feb/25/zfs-and-linux/ for a countervailing position from the SFC and FSF.

License holder grants you permission to do Foo, but not Bar. For example, it may be legal to show a film at site A (e.g. Netflix), but not at site B (e.g. YouTube), because the license holder said so. Functionality of both sites are almost identical.

There's also interpretation that merely linking with compatible interface would be infringing even without distributing two parts together (curiously, used by Stallman): https://news.ycombinator.com/item?id=26606328

addr2line already does this, and it's exactly what TFA is making use of to get the speedup.

Oops. Literally the only line that I didn't read. I thought the author was providing a patch to link against libbfd for users to manually apply (which afaik wouldn't violate the licenses as long as no one redistributes the patched binary).