networking: add prefix_rewrite and uri_regex_rewrite to HTTPRedirect

[liamawhite]

Summary

HTTPRedirect.uri performs a full path replacement and has no prefix-aware variant. Envoy's RedirectAction has supported prefix_rewrite and regex_rewrite for years, but Istio has never exposed them, making it impossible to perform redirects that preserve the path suffix (e.g. example.com/foo/bar → foo.example.com/bar).

This was requested in istio/istio#47500, istio/istio#47777, and istio/istio#52521, all of which were closed as stale.

This PR adds two new fields to HTTPRedirect inside a oneof path_rewrite_specifier:

• prefix_rewrite — replaces the matched route prefix, leaving the rest of the path intact. The route must use a prefix URI match.
• uri_regex_rewrite — RE2 regex rewrite with capture group substitution (reuses the existing RegexRewrite message).

Both are mutually exclusive with the existing uri field (enforced by validation in istio/istio). The legacy uri field is unchanged.

Examples

Strip a prefix and change host (example.com/foo/bar → foo.example.com/bar):

- match:
  - uri:
      prefix: /foo/
  redirect:
    authority: foo.example.com
    prefix_rewrite: /
    redirectCode: 301

Replace a prefix (example.com/foo/bar → example.com/baz/bar):

- match:
  - uri:
      prefix: /foo
  redirect:
    prefix_rewrite: /baz
    redirectCode: 301

Regex rewrite — canonicalize account-scoped paths (example.com/accounts/12345/orders → example.com/orders/12345):

- match:
  - uri:
      prefix: /accounts/
  redirect:
    uri_regex_rewrite:
      match: "^/accounts/([^/]+)/(.*)$"
      rewrite: "/\\2/\\1"
    redirectCode: 301

Full path replacement (existing behaviour, unchanged):

- match:
  - uri:
      prefix: /foo
  redirect:
    uri: /bar
    redirectCode: 301

Related

• Support Prefix Rewrite and Regex Rewrite for HTTP Redirections istio#47500
• Redirect support rewrite the path with match prefix instead of full path  istio#47777
• Allow request rewrite before redirect istio#52521

[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[keithmattix]

I'm not sure why we shouldn't do this under the banner of gateway API (/cc @Stevenjin8). The fields will likely exist on VS since that's how we translate, but ideally Gateway API is the scope

[Stevenjin8]

Fromt the title though, this is for http redirect, not path rewrites, so they are slightly different things.

[liamawhite]

IIRC prefix rewrite for redirect exists in the gateway API but Istio implements it using a hack to get round the fact its not in the virtual service API.

i.e. Some metadata gets added during translation that allows the gateway API to pass this through to the thing generating the XDS configuration without it existing in the VirtualService API.