feat: add username to possible auth header

[bartjkdp]

This change allows system administrators to pass an additional header to the backend containing the username of the requester.

backends:
  haal-centraal-brp:
    type: REST
    baseUrl: http://localhost:8051/api/brp/v1
    auth:
      header:
        X-Username: ${REQUEST_USERNAME}

Summary by CodeRabbit

• New Features
  • Header templates can now include the authenticated username when forwarding to backends.
  • Configuration substitution supports per-request overrides for dynamic values.
  • Passthrough paths continue to behave unchanged (no override supplied).
  • Unknown substitution variables now remain in-place instead of being emptied.

[coderabbitai]

Caution

Review failed

The pull request is closed.

Walkthrough

AuthorizationResponse adds a Username field. EnvSubst gains an overrides map parameter and now prefers overrides over environment variables, leaving unknown placeholders intact. main.go updates EnvSubst calls: passthrough/basic paths pass nil; non-passthrough passes overrides with REQUEST_USERNAME set from AuthorizationResponse.Username and logs backend headers after substitution.

Changes

Cohort / File(s)	Summary
Proxy auth and header substitution cmd/filter-proxy/main.go	Added Username to AuthorizationResponse. Updated EnvSubst call sites to the new signature; passthrough/basic calls pass nil; non-passthrough header substitution supplies map[string]string{"REQUEST_USERNAME": authorizationResponse.Username}. Added debug log of backendRequest.Header after substitution.
Environment substitution utility internal/utils/utils.go	Changed signature to EnvSubst(input string, additionalReplacements map[string]string) string. Substitution now checks additionalReplacements first, then OS env vars; if no match, leaves ${VAR} unchanged. Minor internal refactor and import ordering tweak.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant C as Client
  participant P as Filter Proxy
  participant A as Auth Service
  participant B as Backend

  C->>P: Incoming request
  P->>A: Authorize(request)
  A-->>P: AuthorizationResponse { Username }
  note over P: Build backend headers\nFor each value: EnvSubst(value, overrides)\n- passthrough/basic: overrides = nil\n- non-passthrough: overrides = { REQUEST_USERNAME: Username }
  P-->>B: Forward request with substituted headers
  B-->>P: Backend response
  P-->>C: Return response

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• feat: add username to possible auth header #54 — Modifies the same authorization/header substitution paths and EnvSubst override usage; likely a closely related change.

Poem

I hop through code with eager paws,
Tucking usernames into headers' laws.
If envs are absent, I leave the mark,
Subst by map or the system's spark.
Forward I go — a quiet, clever hare. 🐇

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📥 Commits

Reviewing files that changed from the base of the PR and between 50b8ca4 and ef51441.

📒 Files selected for processing (2)

• cmd/filter-proxy/main.go (3 hunks)
• internal/utils/utils.go (2 hunks)

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• Commit unit tests in branch feat/add-username-to-filter-proxy

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