I am working on some Go (golang) bindings[1] for the LMDB library and I have some interest in exposing the functionality of mdb_set_compare (and mdb_set_dupsort). But it is proving difficult and I have a question about the function(s).

Calling mdb_set_compare from the Go runtime is challenging. Using C APIs with callbacks comes with restrictions[2][3]. I believe it impossible to bind these functions way that is flexible, as one would expect. A potential change to LMDB that would make binding drastically easier is having MDB_cmp_func to take a third "context" argument with type void*. Then a binding could safely use an arbitrary Go function for comparisons.

Is it possible for future versions of LMDB to add a third argument to the MDB_cmp_func signature? Otherwise would it be acceptable for a variant API to be added using a different function type, one accepting three arguments?

Thanks for the consideration.

- Bryan

[1] Go bindings --
[2] Cgo pointer restrictions --
[3] Cgo documentation --