ngx.exec() can not pass the data of post quest body to the upstream server

Hello, This is a problem i meet, That is the ngx.exec() can not pass the data of post quest body to the upstream server.
below is my code in content_by_lua*:

local method = ngx.req.get_method()
if method == 'POST' then
local ct = ngx.var.content_type
if string.find(ct, "multipart/form-data;", 1 ,true) ~= nil then
-- '@client' is a named locations in nginx confg
return ngx.exec('@client')
end
end

the upstream server can get the post request, and the http header is correct，but this is no body data, so what can i do to resovle the problem?? sorry to my English...

Start with the content_by_lua* snippet and the ngx.exec('@client') entry point, then inspect the related nginx configuration and upstream request handling. Reproduce the multipart/form-data POST and compare the headers and body received upstream; done means the missing-body behavior is explained and a verified resolution or documented limitation is established.