C API to pin domain to C thread ?

[nojb]

(This was discussed during the WG1 meeting today 22/11/2021, and the conclusion was that there is some design work to do.)

The question is how to design an API that would allow creating a domain "pinned" to an existing C thread, from C. One use-case is when the main program is written in C or some other language and this main program wants to call back into OCaml to do some computation (potentially short lived) and return the result to the main program. As domain creation is relatively costly, the idea would be that the C threads (and thus the domain pinned to them) would be long-lived, and not create a new domain for each call back into OCaml.

Of course, the actual solution may involve a slightly different approach and/or abstraction. @avsm mentioned also another use-case, namely interfacing with the Grand Central Dispatch API on macOS.