feat: support registering custom variable

apisix/core/ctx.lua

@@ -215,11 +215,19 @@ do
                 key = sub_str(key, 9)
                 val = get_parsed_graphql()[key]
 
-            elseif apisix_var_names[key] then
-                val = ngx.ctx.api_ctx and ngx.ctx.api_ctx[key]
-
             else
-                val = get_var(key, t._request)
+                local getter = apisix_var_names[key]
+                if getter then
+                    if getter == true then
+                        val = ngx.ctx.api_ctx and ngx.ctx.api_ctx[key]
+                    else
+                        -- the getter is registered by ctx.register_var
+                        val = getter(ngx.ctx.api_ctx)
+                    end
+
+                else
+                    val = get_var(key, t._request)
+                end
             end
 
             if val ~= nil and not no_cacheable_var_names[key] then
@@ -239,6 +247,18 @@ do
         end,
     }
 
+function _M.register_var(name, getter)
+    if type(getter) ~= "function" then
+        error("the getter of registered var should be a function")
+    end
+
+    if apisix_var_names[name] then
+        error(name .. " is registered")
+    end
+
+    apisix_var_names[name] = getter
+end
+
 function _M.set_vars_meta(ctx)
     local var = tablepool.fetch("ctx_var", 0, 32)
     if not var._cache then

docs/en/latest/plugin-develop.md

@@ -32,10 +32,11 @@ title: Plugin Develop
 - [implement the logic](#implement-the-logic)
   - [conf parameter](#conf-parameter)
   - [ctx parameter](#ctx-parameter)
-- [Register public API](#register-public-api)
-- [Register control API](#register-control-api)
+- [register public API](#register-public-api)
+- [register control API](#register-control-api)
+- [register custom variable](#register-custom-variable)
 - [write test case](#write-test-case)
-  - [Attach the test-nginx execution process:](#attach-the-test-nginx-execution-process)
+  - [attach the test-nginx execution process:](#attach-the-test-nginx-execution-process)
 
 This documentation is about developing plugin in Lua. For other languages,
 see [external plugin](./external-plugin.md).
@@ -388,7 +389,7 @@ function _M.access(conf, ctx)
 end
 ```
 
-## Register public API
+## register public API
 
 A plugin can register API which exposes to the public. Take jwt-auth plugin as an example, this plugin registers `GET /apisix/plugin/jwt/sign` to allow client to sign its key:
 
@@ -411,7 +412,7 @@ end
 Note that the public API is exposed to the public.
 You may need to use [interceptors](plugin-interceptors.md) to protect it.
 
-## Register control API
+## register control API
 
 If you only want to expose the API to the localhost or intranet, you can expose it via [Control API](./control-api.md).
 
@@ -441,6 +442,28 @@ end
 
 If you don't change the default control API configuration, the plugin will be expose `GET /v1/plugin/example-plugin/hello` which can only be accessed via `127.0.0.1`.
 
+## register custom variable
+
+We can use variables in many places of APISIX. For example, customizing log format in http-logger, using it as the key of `limit-*` plugins. In some situations, the builtin variables are not enough. Therefore, APISIX allows developers to register their variables globally, and use them as normal builtin variables.
+
+For instance, let's register a variable called `a6_labels_zone` to fetch the value of the `zone` label in a route:
+
+```
+local core = require "apisix.core"
+
+core.ctx.register_var("a6_labels_zone", function(ctx)
+    local route = ctx.matched_route and ctx.matched_route.value
+    if route and route.labels then
+        return route.labels.zone
+    end
+    return nil
+end)
+```
+
+After that, any get operation to `$a6_labels_zone` will call the registered getter to fetch the value.
+
+Note that the custom variables can't be used in features that depend on the Nginx directive, like `access_log_format`.
+
 ## write test case
 
 For functions, write and improve the test cases of various dimensions, do a comprehensive test for your plugin! The
@@ -488,7 +511,7 @@ Additionally, there are some convenience testing endpoints which can be found [h
 
 Refer the following [document](how-to-build.md#Step-4-Run-Test-Cases) to setup the testing framework.
 
-### Attach the test-nginx execution process:
+### attach the test-nginx execution process:
 
 According to the path we configured in the makefile and some configuration items at the front of each __.t__ file, the
 framework will assemble into a complete nginx.conf file. "__t/servroot__" is the working directory of Nginx and start the

docs/zh/latest/plugin-develop.md

@@ -33,6 +33,7 @@ title: 插件开发
   - [ctx 参数](#ctx-参数)
 - [注册公共接口](#注册公共接口)
 - [注册控制接口](#注册控制接口)
+- [注册自定义变量](#注册自定义变量)
 - [编写测试用例](#编写测试用例)
   - [附上 test-nginx 执行流程](#附上-test-nginx-执行流程)
 
@@ -361,6 +362,28 @@ end
 
 如果你没有改过默认的 control API 配置，这个插件暴露的 `GET /v1/plugin/example-plugin/hello` API 只有通过 `127.0.0.1` 才能访问它。
 
+## 注册自定义变量
+
+我们可以在APISIX的许多地方使用变量。例如，在 http-logger 中自定义日志格式，用它作为 `limit-*` 插件的键。在某些情况下，内置的变量是不够的。因此，APISIX允许开发者在全局范围内注册他们的变量，并将它们作为普通的内置变量使用。
+
+例如，让我们注册一个叫做 `a6_labels_zone` 的变量来获取路由中 `zone` 标签的值。
+
+```
+local core = require "apisix.core"
+
+core.ctx.register_var("a6_labels_zone", function(ctx)
+    local route = ctx.matched_route and ctx.matched_route.value
+    if route and route.labels then
+        return route.labels.zone
+    end
+    return nil
+end)
+```
+
+此后，任何对 `$a6_labels_zone` 的获取操作都会调用注册的获取器来获取数值。
+
+注意，自定义变量不能用于依赖 Nginx 指令的功能，如 `access_log_format`。
+
 ## 编写测试用例
 
 针对功能，完善各种维度的测试用例，对插件做个全方位的测试吧！插件的测试用例，都在 __t/plugin__ 目录下，可以前去了解。

t/core/ctx2.t

@@ -316,3 +316,63 @@ Content-Type: application/x-www-form-urlencoded
 --- error_code: 404
 --- response_body
 {"error_msg":"404 Route Not Found"}
+
+
+
+=== TEST 15: register custom variable
+--- config
+    location /t {
+        content_by_lua_block {
+            local t = require("lib.test_admin").test
+            local code, body = t('/apisix/admin/routes/1',
+                 ngx.HTTP_PUT,
+                 [=[{
+                        "upstream": {
+                            "nodes": {
+                                "127.0.0.1:1980": 1
+                            },
+                            "type": "roundrobin"
+                        },
+                        "plugins": {
+                            "serverless-pre-function": {
+                                "phase": "rewrite",
+                                "functions" : ["return function(conf, ctx) ngx.say('find ctx.var.a6_labels_zone: ', ctx.var.a6_labels_zone) end"]
+                            }
+                        },
+                        "uri": "/hello",
+                        "labels": {
+                            "zone": "Singapore"
+                        }
+                }]=]
+                )
+
+            if code >= 300 then
+                ngx.status = code
+            end
+            ngx.say(body)
+        }
+    }
+
+
+
+=== TEST 16: hit
+--- config
+    location /t {
+        content_by_lua_block {
+            local http = require "resty.http"
+            local core = require "apisix.core"
+            core.ctx.register_var("a6_labels_zone", function(ctx)
+                local route = ctx.matched_route and ctx.matched_route.value
+                if route and route.labels then
+                    return route.labels.zone
+                end
+                return nil
+            end)
+            local uri = "http://127.0.0.1:" .. ngx.var.server_port .. "/hello"
+            local httpc = http.new()
+            local res = assert(httpc:request_uri(uri))
+            ngx.print(res.body)
+        }
+    }
+--- response_body
+find ctx.var.a6_labels_zone: Singapore