Usual reason for mistakes like this is: someone was debugging something then (partially) forgot to clean up before committing.

Once that is done it's incredibly easy for these kind of bugs to go un-noticed for a long time.

Assuming you don't have a test for it, sure. And don't fuzz everything on a regular basis.

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.

It's kind of implied by the parameters that they point to the same object; they're named "pos" and "end" after all. But I agree, a count variable is nicer.

The subtraction also has a (very theoretical) problem that its result must fit in ptrdiff_t (signed), and it would produce undefined behavior when you have a huge object whose size doesn't fit in ptrdiff_t (but does in size_t, by definition).

It's a common practice to use two pointers when parsing some stream data buffer. If you will use a pointer to the start of the buffer and the size of an available data to parse, then you would need to change two variables instead of one on every parsing stage. So you have more opportunity to make a mistake.

And why leave in a #if when patching too?

I assume the reason was to keep diff as minimal as possible. Note that it should apply to all mentioned versions without problems.

Keeps the changes to a minimum, reducing the risk of effecting anything else. (quicker + safer)