Fix x-forwarded-prefix annotation

[alexkursell]

What this PR does / why we need it: With the now mandatory use of regexps in paths when using rewrite-target, the x-forwarded-prefix annotation is broken.

Before 0.22, setting the annotation to true would set the x-forwarded-prefix header value to the path value. However, now that the path always includes a capture group, doing this no longer makes sense.

This PR changes the type of the x-forwarded-prefix annotation from bool to string, and changes things so that the value of the x-forwarded-prefix header is just equal to the value of the annotation (if the annotation value exists and is non-empty).

This is actually the behaviour that is explained by the docs (see here). My guess is that when this was changed to copying the path value into the header, the docs were never updated.

Which issue this PR fixes: fixes #3733, #3670, #3132

This would be a breaking change, so we should probably think about how migrating would work, as well as if there is a better way to do this.

[aledbf]

so we should probably think about how migrating would work, as well as if there is a better way to do this.

@alexkursell can we use the new lint plugin feature to also provide examples of migration?

[alexkursell]

so we should probably think about how migrating would work, as well as if there is a better way to do this.

@alexkursell can we use the new lint plugin feature to also provide examples of migration?

@aledbf We can add a lint that warns on the use of x-forwarded-prefix with boolean values that links to this issue. Once the lint command is merged I'll add the lint to this PR.

[alexkursell]

@aledbf I've added a lint for this change:

✗ example-ingress1
  - The x-forwarded-prefix annotation value is a boolean instead of a string
      Lint added for version 0.24.0
      https://github.com/kubernetes/ingress-nginx/issues/3786

[ElvinEfendi]

/approve

[aledbf]

/lgtm

[aledbf]

@alexkursell thanks!

[k8s-ci-robot]

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[Sudharma]

if I have ingress with path as below (with re-write path nginx.ingress.kubernetes.io/rewrite-target: /$1) and with x-forward-prefix annotation as true. For my service with path /path/api shall result in the correct forward prefix /path/api instead of /path/?(.*) if I am correct

paths:
         - path: path/?(.*)
           backend:
             serviceName: test 
             servicePort:  8086 

[alexkursell]

@Sudharma with this change the x-forwarded-prefix header is just set to whatever the annotation value is. So if you have nginx.ingress.kubernetes.io/x-forwarded-prefix: /foo/bar then the value of the header would always be /foo/bar, for any path that you access via that ingress.

[Sudharma]

well but thats more hard-coded, Is it not possible to have it /foo/$1/ which is like rewrite? because If I have multiple backends and each has its own angular app and would want to have the suffix accordingly.

[alexkursell]

Is it not possible to have it /foo/$1/ which is like rewrite?

This is not possible right now. If you need a different header value for different backends you will need to create different ingresses.