always_forward_body=true 未按预期工作

[颜博]

我的nginx有以下配置（它将父请求转发并镜像到/pgsql，/mongo locations，并接收/pgsql的响应结果）

```conf
        http {
            ......
            log_subrequest on;
            proxy_buffering off;
            proxy_request_buffering off;
            ......
        }

        location /test
        {
            error_log /data/logs/error.log debug;
            lua_need_request_body on;
            content_by_lua_block {
                local reqBody = ngx.req.get_body_data()
                local reqUri = ngx.var.uri or ""
                local resPg, resMongo = ngx.location.capture_multi( {
                                {"/pgsql".. reqUri, { method = ngx.HTTP_POST, body = reqBody , args = ngx.var.args, always_forward_body=true }},
                                {"/mongo".. reqUri , { method = ngx.HTTP_POST, body = reqBody , args = ngx.var.args, always_forward_body=true }}
                            })
                ngx.status = resPg.status
                for k, v in pairs(resPg.header) do
                    ngx.header[k] = v
                end
                ngx.print(resPg.body)
            }
        }

        location /pgsql
        {
            if ($request_method = 'OPTIONS') {
                add_header 'Access-Control-Allow-Origin' '*';
                add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
                add_header 'Access-Control-Allow-Headers' '*';
                add_header 'Access-Control-Expose-Headers' '*';
                add_header 'Content-Type' 'text/plain charset=UTF-8';
                add_header 'Content-Length' 0;
                return 204;
            }

            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            add_header 'Access-Control-Allow-Headers' '*';
            add_header 'Access-Control-Expose-Headers' '*';

            rewrite ^/pgsql/test$ $original_path break;
            proxy_pass http://xxx:xxx/test;
            proxy_http_version 1.1;
            proxy_set_header   Connection       "";
        }


        location /mongo
        {
            if ($request_method = 'OPTIONS') {
                add_header 'Access-Control-Allow-Origin' '*';
                add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
                add_header 'Access-Control-Allow-Headers' '*';
                add_header 'Access-Control-Expose-Headers' '*';
                add_header 'Content-Type' 'text/plain charset=UTF-8';
                add_header 'Content-Length' 0;
                return 204;
            }

            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            add_header 'Access-Control-Allow-Headers' '*';
            add_header 'Access-Control-Expose-Headers' '*';

            rewrite ^/mongo/test$ $original_path break;
            proxy_pass http://xxx:xxx/test;
            proxy_http_version 1.1;
            proxy_set_header   Connection       "";
        }

......
```

我希望nginx能以流式的方式转发请求（收到请求就转发，收到响应就转发），所以我配置了proxy_buffering off，proxy_request_buffering off，always_forward_body=true。但是抓包后发现 ：

- /pgsql 和 /mongo 的子请求只有在收到父请求的 body 后才会转发；
- 响应结果也只有在收到 /pgsql 的 response 后才会转发；



我看到文档描述：`always_forward_body` when set to true，...... The request body read by either [ngx.req.read_body()](https://github.com/openresty/lua-nginx-module#ngxreqread_body) or [lua_need_request_body on](https://github.com/openresty/lua-nginx-module#lua_need_request_body) will be directly forwarded to the subrequest without copying the whole request body data when creating the subrequest (no matter the request body data is buffered in memory buffers or temporary files).

是否是我的配置问题。


版本信息
```bash
/usr/local/openresty/nginx/sbin/nginx -V
nginx version: openresty/1.19.9.1
built by gcc 9.3.1 20200408 (Red Hat 9.3.1-2) (GCC)
built with OpenSSL 1.1.1k  25 Mar 2021 (running with OpenSSL 1.1.1w  11 Sep 2023)
TLS SNI support enabled
configure arguments: --prefix=/usr/local/openresty/nginx --with-cc-opt='-O2 -DNGX_LUA_ABORT_AT_PANIC -I/usr/local/openresty/zlib/include -I/usr/local/openresty/pcre/include -I/usr/local/openresty/openssl111/include' --add-module=../ngx_devel_kit-0.3.1 --add-module=../echo-nginx-module-0.62 --add-module=../xss-nginx-module-0.06 --add-module=../ngx_coolkit-0.2 --add-module=../set-misc-nginx-module-0.32 --add-module=../form-input-nginx-module-0.12 --add-module=../encrypted-session-nginx-module-0.08 --add-module=../srcache-nginx-module-0.32 --add-module=../ngx_lua-0.10.20 --add-module=../ngx_lua_upstream-0.07 --add-module=../headers-more-nginx-module-0.33 --add-module=../array-var-nginx-module-0.05 --add-module=../memc-nginx-module-0.19 --add-module=../redis2-nginx-module-0.15 --add-module=../redis-nginx-module-0.3.7 --add-module=../ngx_stream_lua-0.0.10 --with-ld-opt='-Wl,-rpath,/usr/local/openresty/luajit/lib -L/usr/local/openresty/zlib/lib -L/usr/local/openresty/pcre/lib -L/usr/local/openresty/openssl111/lib -Wl,-rpath,/usr/local/openresty/zlib/lib:/usr/local/openresty/pcre/lib:/usr/local/openresty/openssl111/lib' --with-cc='ccache gcc -fdiagnostics-color=always' --with-pcre-jit --with-stream --with-stream_ssl_module --with-stream_ssl_preread_module --with-http_v2_module --without-mail_pop3_module --without-mail_imap_module --without-mail_smtp_module --with-http_stub_status_module --with-http_realip_module --with-http_addition_module --with-http_auth_request_module --with-http_secure_link_module --with-http_random_index_module --with-http_gzip_static_module --with-http_sub_module --with-http_dav_module --with-http_flv_module --with-http_mp4_module --with-http_gunzip_module --with-threads --with-compat --with-stream --with-http_ssl_module

```