Substitution string has changed

[mlibbey]

In ATS <=7, $f was the original host header. It isn't anymore.

[zwoop]

$f ? Regardless, I'm not sure we should change the docs, before understanding why this was changed.

[mlibbey]

This is the behavior of 5 released versions -- shouldn't we document its actual behavior? (I'd be fine with reverting this if our 8 release (8.0.7) finds and changes it back to the pre-8 behavior.

[bryancall]

[approve ci clang-format]

[zwoop]

Yeh, we "broke" this behavior... Remapping now happens before plugins are called.

[zwoop]

Cherry-picked to 8.0.x

[gtenev]

@mlibbey, @zwoop, could not look into this fast enough to squeeze in my twopence.

Plugin execution behavior changed in core after PR #4964 according to what was discussed on dev mailing list about "rewriting before instead of after only the first plugin (original behavior) or after all plugins are run (unsuccessful attempt to fix)".

The documentation for the old 5 releases was not correct even before PR #4964. It was correct only if the regex_remap plugin was the first plugin in the remap rule where it would get the original (source) request. If the plugin was 2nd in the remap rule $h would have returned the target hostname even for those old releases. This is the inconsistency PR #4964 fixes.

After PR #4964 the regex_remap behavior changed for the case when regex_remap is the first plugin but not for the rest of the cases. If we want to keep the old documentation true the plugin would need to be fixed (I already fixed a couple of other plugins at the time of PR #4964 ). The new behavior makes sense to me too so we can just update the docs in this case.

I also have a comment about the wording in this change. Instead of host as used in the "to" portion of the remap rule I would use something like the target or remapped request host header since it can be confused with $t which is the "to" portion of the remap rule in remap.config file and does not come from the target (remapped) request which can be different from what is written in the file itself (similarly it can be confused with $f if we decide to fix the plugin so $h would return the original request's host header)

HTH! Please let me know if more information needed!