feat: add exit-transformer plugin

[AlinsRan]

Summary

This PR introduces the exit-transformer plugin and extends core/response.lua with an exit callback mechanism to support it.

Description

The exit-transformer plugin intercepts APISIX exit responses (plugin rejections, route-not-found, auth failures, etc.) and transforms them using user-defined Lua functions before the response is sent to the client.

Use cases:

• Normalize error response format across all upstream and gateway errors
• Remap status codes (e.g. translate 401 → 200 for legacy clients)
• Inject custom headers or bodies into error responses
• Apply uniform error envelope format across all routes

Example

The following example wraps all error responses in a JSON envelope {"error": <original body>} and forces the status code to 200:

curl http://127.0.0.1:9180/apisix/admin/routes/1 \
  -H "X-API-KEY: $admin_key" -X PUT -d '{
  "uri": "/api/*",
  "plugins": {
    "key-auth": {},
    "exit-transformer": {
      "functions": [
        "return function(code, body, headers)\n  return 200, [[{\"error\":]] .. (body or \"null\") .. [[}]], headers\nend"
      ]
    }
  },
  "upstream": {
    "type": "roundrobin",
    "nodes": { "127.0.0.1:1980": 1 }
  }
}'

A request without a valid API key would now receive HTTP 200 with {"error":"Missing API key in request"} instead of the default HTTP 401.

Changes

apisix/core/response.lua

Two additions to support the callback mechanism:

1. exit_insert_callback(func, conf) — registers a per-request callback in ngx.ctx. Each callback has the signature (code, body, headers, conf) → (code, body, headers).

2. resp_exit with callback path — when callbacks are registered, resp_exit builds a structured (code, body, headers) representation from the variadic arguments, runs all callbacks in order, then sends the transformed response. The original variadic fast path is preserved when no callbacks are present, maintaining full backward compatibility.

apisix/plugins/exit-transformer.lua

New plugin. In the rewrite phase, each configured Lua function is registered as an exit callback. Functions are cached by an LRU cache to avoid repeated load() calls.

Files changed

• apisix/core/response.lua — add exit_insert_callback and callback path in resp_exit
• apisix/plugins/exit-transformer.lua — plugin implementation
• t/plugin/exit-transformer.t — test cases
• docs/en/latest/plugins/exit-transformer.md — English documentation
• docs/zh/latest/plugins/exit-transformer.md — Chinese documentation
• apisix/cli/config.lua — register plugin in default list
• docs/en/latest/config.json / docs/zh/latest/config.json — add to sidebar