Nginx content_by_lua_block deletes request body? - nginx

Question

I wanted to make some changes on existing nginx configuration. My previous configuration was like this:
location /serviceprovider {
proxy_pass http://localhost:8080;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_redirect off;
}
Then I added content_by_lua_block as below:
location /serviceprovider {
proxy_pass http://localhost:8080;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_redirect off;
content_by_lua_block{
ngx.req.read_body()
body = ngx.req.get_body_data()
file = io.open("/etc/nginx/test", "w")
file:write(body)
file:close()
}
}
My /serviceprovider actually accepts JSON request in POST. When I send request after adding lua block I get error message "Invalid JSON: Empty string", but It writes request body to file successfully.
I think this lua block deletes request body after writing it to file. I only want that location block worked as usual but in addition to write request body to a file.