Just my two cents. :-)
I appreciate the effort to reduce redundancy in the code..

There are some things to consider here:

• check_curl should replace check_http, so then there will be no more shared code
• check_curl is a copy of check_http intentionally: keep the new one as close as possible to the old one and don't touch the old one not to create a breaking plugin - that was the premise while developing check_curl.

There is a counter-argument if we would go in the direction that instead of having a check_curl we would have a

• check_http based on libcurl
• check_ftp, also based on libcurl
  etc.
  Then having some common shared code would be a good idea..

i intend add support for certificate authentication (-J and -K) to check_tcp and check_smtp in the near future where i intend to use the same validation code again; this code is merely in preparation for that. sorry for not stating my full intentions with this MR in the first place.

Ah, that's different then. :-)

I guess I should have responded here earlier. Sorry for that.

I don't want to touch check_http or create any connections from it to other code. I consider it to be dead and don't want put any work into it or create later work by somehow connecting it closer to other code.

That being said, I am totally for shared functions, if they make sense otherwise.
As for this concrete PR I would reject it on the basis of not wanting to touch check_http in any meaningful way.

But feel free to combine the changes here with the changes to check_tcp and check_smtp if that makes more sense.