add Host header to requests submitted by Client.fromHttpService if the request URI is absolute

[bpholt]

Addresses #1872 in the easy case where the request URI is absolute.

[ChristopherDavenport]

_ <- Alternative[Option].guard(req.headers.get(Host).isEmpty)

[bpholt]

@ChristopherDavenport I don't understand the difference here. Can you explain why Alternative[Option].guard is preferred?

[ChristopherDavenport]

One is implemented in terms of flatMap the other in terms of withFilter. I just see this as utilizing non-functional language features which has the possibility for issues in the future.

I'm sure people may disagree here.

[jmcardon]

No comment on cats fp guard syntax (I don't care for it but I don't hate it), however, instead of the withfilter guard, you can always just

req.uri.host match {
  case Some(host) if req.headers.get(Host).isEmpty => 
     req.withHeaders(req.headers.put((Host(host.value, req.uri.port)))
  case _ => req
}

That's microscopically more efficient (no fold call + no withFilter)

[rossabaker]

What is the functional objection to withFilter on a type that has a reasonable empty? I agree that @jmcardon's version saves a few nanoseconds, and I agree that withFilter on Future is regrettable, but I don't have a problem with what @bpholt wrote.

[rossabaker]

Did you also want to set the secure flag if the scheme is https? That was mentioned on Gitter, and sounded like a smart idea. That's another thing that a real client backend does for free.

[ChristopherDavenport]

I'm not sure I have a solid objection to withFilter on option other than a lack of trust in withFilter. As such this is an improvement, but if we want the secure flag I agree with that change as well.

[bpholt]

Updated with @jmcardon's pattern match suggestion.

I'm going to leave the secure flag alone, as it's not easily changed for this usage.