Missing Content-Length header when using Nginx + Gzip + Unicorn - ruby-on-rails

Question

I don't know why http response is missed "Content-Length header" when I use gzip in nginx, i'm really getting stuck,please somebody help me, thank you so much!
this is my config file,
nginx.conf
user nobody nobody ;
worker_processes 8;
events {
worker_connections 1024;
accept_mutex on; # "on" if nginx worker_processes > 1
use epoll; # enable for Linux 2.6+
}
http {
include mime.types;
default_type application/octet-stream;
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';
upstream backend-unicorn {
server unix:/tmp/unicorn_app.sock fail_timeout=0;
#server localhost:5000;
}
#access_log logs/access.log main;
sendfile on;
keepalive_timeout 100;
gzip on;
gzip_static on;
gzip_proxied any;
gzip_http_version 1.1;
gzip_comp_level 6;
gzip_min_length 1100;
gzip_buffers 16 8k;
gzip_types text/plain application/zip text/css application/x-javascript text/xml application/xml application/xml+rss text/javascript image$
server {
listen 8080;
server_name misago;
access_log /var/log/nginx/unicorn.access.log main;
client_max_body_size 64M;
location /uploads/ {
root /usr/local/rails_apps/me_management_tool/current/public/uploads;
break;
}
error_page 500 502 503 504 /50x.html;
location = /50x.html {
root html;
}
}
}

Answer 1

I don't know why http response is missed "Content-Length header" when I use gzip in nginx
Because in this case the length of content is unknown at the moment of sending headers. Nginx cannot know how well the content will be compressed.

Answer 2

Below is a performant Nginx solution to add x-file-size header to Gzip responses. Full discussion here: https://github.com/AnthumChris/fetch-progress-indicators/issues/13
location / {
## Nginx Lua module must be installed https://docs.nginx.com/nginx/admin-guide/dynamic-modules/lua/
## https://github.com/openresty/lua-nginx-module#header_filter_by_lua
header_filter_by_lua_block {
function file_len(file_name)
local file = io.open(file_name, "r")
if (file == nil) then return -1 end
local size = file:seek("end")
file:close()
return size
end
ngx.header["X-File-Size"] = file_len(ngx.var.request_filename);
}
}