Override the body before sending it to upstream

[vaibhavsw]

Description

I have written a transformer that transforms the HTTP request body in serverless-pre-function in the before-proxy phase

"serverless-pre-function": {
      "disable": false,
      "functions": [
        "return function(conf, ctx) local core = require(\"apisix.core\")  local httpc = require(\"resty.http\").new() \n ngx.req.read_body() \n local res, err = httpc:request_uri(\"http://host.docker.internal:9191/transform\", {method = \"POST\", body = ngx.req.get_body_data(), headers = { [\"Content-Type\"] = \"application/json\", [\"Upstream\"] = core.request.header(ctx, \"Upstream\"), [\"Transformer\"] = core.request.header(ctx, \"Transformer\")}}) ngx.req.set_body_data(res.body) end"
      ],
      "phase": "before_proxy"
    }

However, when this is executed, it is throwing below error.

2022/09/14 06:50:39 [error] 46#46: *1287423 lua entry thread aborted: runtime error: [string "return function(conf, ctx) local core = requi..."]:3: attempt to call field 'set_body_data' (a string value)
stack traceback:
coroutine 0:
	[string "return function(conf, ctx) local core = requi..."]: in function 'func'
	/usr/local/apisix/apisix/plugins/serverless/init.lua:84: in function 'phase_func'
	/usr/local/apisix/apisix/plugin.lua:897: in function 'common_phase'
	/usr/local/apisix/apisix/init.lua:555: in function 'http_access_phase'
	access_by_lua(nginx.conf:284):2: in main chunk, client: 172.18.0.1, server: _, request: "POST /card/balance HTTP/1.1", host: "127.0.0.1:9080"

The expectation is to override the client request body with the transformed request body after accessing the external service with resty.http and then that must be sent to the upstream.

Environment

• APISIX version (run apisix version): 2.15
• Operating system (run uname -a): MacOS
• OpenResty / Nginx version (run openresty -V or nginx -V): nginx version: openresty/1.21.4.1
• etcd version, if relevant (run curl http://127.0.0.1:9090/v1/server_info): 3.4.15
• APISIX Dashboard version, if relevant:
• Plugin runner version, for issues related to plugin runners:
• LuaRocks version, for installation issues (run luarocks --version): 3.8.0

[vaibhavsw]

I was able to solve this by using

ngx.req.init_body() ngx.req.append_body(res.body) ngx.req.finish_body()

[tzssangglass]

Since this question is not related to the project, I turned it into a discussion.