Connection leak in ngx.location.capture()

The following settings lead to connections leak in nginx():

server {
  listen 8081;
  server_name  localhost;
  proxy_buffering on;
  proxy_buffers 8 128k;
  proxy_max_temp_file_size 0;
  location /a/ {
    content_by_lua_block {
      local data = ngx.location.capture('/b/')
      ngx.say("ok")
    }
  }
  location /b/ {
    proxy_pass http://127.0.0.1:8082/get/;
  }
  location /s/ {
    stub_status;
  }
}

If http://127.0.0.1:8082/get/ returned more than 1m (8 * 128k) of data then the upstream connection hangs. It still hangs even if the client connection breaks. You can easily see the leakage by configuring nginx with --with-http_stub_status_module option and by requesting http://127.0.0.1:8081/s/. Normal case (before requests):

Active connections: 1 
server accepts handled requests
 1 1 1 
Reading: 0 Writing: 1 Waiting: 0 

After 3 requests:

Active connections: 4 
server accepts handled requests
 5 5 5 
Reading: 0 Writing: 4 Waiting: 0 

You can setup a server for http://127.0.0.1:8082/get/ with the following configuration:

server {
  listen 8082;
  server_name  localhost;

  location /get/ {
    content_by_lua_block {
      ngx.say(string.rep(" ", 8*1024*1024))
    }
  }
}

Note that all works fine when proxy_buffering off; is set.

Reproduce the leak with the provided nginx configurations, a response larger than 1 MB, and proxy_buffering on; inspect ngx.location.capture() behavior around upstream responses and client disconnects. Use the /s/ stub_status endpoint to verify that connections return to the baseline after requests complete, including when proxy_buffering is disabled for comparison.