help request: Why does ngx.req.set_uri_args fail after proxy rewrite is enabled?

[kingmouse-yx]

Description

I have a requirement now. I need to parse some data in the request body, splice it behind the URL, and then forward it to the upstream.
Original request：
url：http://192.168.2.11:9080/userinfo
body：

{
	"userId":"12345",
	"type":"01"
}

final request:
http://192.168.2.11:9080/userinfo?userId=12345&type=01

This seems to be a very simple requirement, so I get the message in the rewrite phase and use gx.req.set_uri_args.
Part code:

local req_body_json = core.json.decode(req_body)
ngx.req.set_uri_args(req_body_json)

This test is completely OK. It can really meet this requirement. But I found that if the proxy rewrite is enabled (modify the path or modify the request method, etc.), the request to the downstream URL is an error. The data in the body is not spliced behind the url. This can be seen through the log.
I checked the implementation logic of the proxy rewrite. I found that in the proxy rewrite, it is by modifying ctx.var.upstream_ uri to modify the url.

So I made a judgment in my code, as follows:

local req_body_json = core.json.decode(req_body)
ngx.req.set_uri_args(req_body_json)

local ctx_uri = ctx.var.upstream_uri;
if ctx_uri and #ctx_uri > 0 then
    ctx.var.upstream_uri = ctx_uri .. '?' .. ctx.var.args
end

After this processing, this requirement can be realized whether the proxy rewrite is enabled or not.
But I don't quite understand why apisix handles it in this way. I think ngx.req.set_uri_args is more convenient

Environment

• APISIX version (run apisix version): 2.13.0-alpine
• Operating system (run uname -a): Darwin kingmouse.local 21.4.0 Darwin Kernel Version 21.4.0: Mon Feb 21 20:36:53 PST 2022; root:xnu-8020.101.4~2/RELEASE_ARM64_T8101 arm64
• OpenResty / Nginx version (run openresty -V or nginx -V): openresty/1.19.3.1
• etcd version, if relevant (run curl http://127.0.0.1:9090/v1/server_info): 3.4.0
• APISIX Dashboard version, if relevant: 2.10.1
• Plugin runner version, for issues related to plugin runners:
• LuaRocks version, for installation issues (run luarocks --version):

[tzssangglass]

Because the variable upstream_uri is the one that will be forwarded upstream, see:

apisix/apisix/cli/ngx_tpl.lua

Line 705 in 2800a06

proxy_pass $upstream_scheme://apisix_backend$upstream_uri;

proxy-rewrite chooses to modify this variable directly in order to ensure that rewrites are valid.