feat: adds support for nginx variables in service_name param

apisix/core/utils.lua

@@ -461,5 +461,35 @@ function _M.check_tls_bool(fields, conf, plugin_name)
     end
 end
 
+---
+-- Checks if a string is an nginx variable.
+-- An nginx variable starts with the '$' character.
+--
+-- @function core.utils.is_nginx_variable
+-- @tparam string str The string to check
+-- @treturn boolean true if the string starts with '$', false otherwise
+-- @usage
+-- local utils = require("apisix.core.utils")
+--
+-- -- Usage examples:
+-- local is_var = utils.is_nginx_variable("$host")     -- true
+-- local is_var = utils.is_nginx_variable("host")      -- false
+-- local is_var = utils.is_nginx_variable("${host}")   -- true
+-- local is_var = utils.is_nginx_variable("\\$host")   -- false
+--
+-- -- Usage in APISIX context:
+-- if utils.is_nginx_variable(up_conf.service_name) then
+--     -- Handle as nginx variable
+-- else
+--     -- Handle as regular service name
+-- end
+function _M.is_nginx_variable(str)
+    if not str or type(str) ~= "string" then
+        return false
+    end
+
+    -- Check if the string starts with '$' and it's not escaped
+    return str:sub(1, 1) == "$" and (str:sub(1, 2) ~= "\\$")
+end
 
 return _M

apisix/upstream.lua

@@ -297,7 +297,12 @@ function _M.set_by_route(route, api_ctx)
             return 503, err
         end
 
-        local new_nodes, err = dis.nodes(up_conf.service_name, up_conf.discovery_args)
+        local service_name = up_conf.service_name
+        if core.utils.is_nginx_variable(up_conf.service_name) then
+            service_name = core.utils.resolve_var(up_conf.service_name, api_ctx.var)
+        end
+
+        local new_nodes, err = dis.nodes(service_name, up_conf.discovery_args)
         if not new_nodes then
             return HTTP_CODE_UPSTREAM_UNAVAILABLE, "no valid upstream node: " .. (err or "nil")
         end

t/core/utils.t

@@ -393,3 +393,33 @@ res:nil
 res:5
 res:12
 res:7
+
+
+
+=== TEST X: is_nginx_variable
+--- config
+    location /t {
+        content_by_lua_block {
+            local is_nginx_variable = require("apisix.core.utils").is_nginx_variable
+
+            local cases = {
+                {str = "$host", expected = true},
+                {str = "host", expected = false},
+                {str = "${host}", expected = true},
+                {str = "\\$host", expected = false},
+                {str = nil, expected = false},
+                {str = 123, expected = false},
+                {str = "", expected = false},
+            }
+
+            for _, case in ipairs(cases) do
+                local res = is_nginx_variable(case.str)
+                assert(res == case.expected,
+                       string.format("case %s failed: got %s, expected %s",
+                                   case.str, res, case.expected))
+            end
+        }
+    }
+--- request
+GET /t
+--- response_body