Without context (I'm not familiar with nginx' code) the blatant subtraction of pointers was scary, too.

As everyone is of course no doubt aware, subtraction of pointers is only valid in C if you can guarantee that the pointers point to the same "object" (or at most 1 character past the end of an object).

I would prefer an interface using a size_t count of available space, rather than passing an end-pointer around.

Again, this is based solely on reading the patch, so it might be perfectly fine. Considering nginx' reputation, I guess it's academic.