http_send() blocks at connect when using ngx.socket

[RealHacker]

When I test DSN with lua command line, I can successfully send message to Sentry server. But when running in open-resty, I cannot send any message to Sentry. Adding logs to raven.lua, I found the code always blocks when calling sock:connect:

ngx.log(ngx.ERR, "after tcp, before connect", self.host, self.port)
   ok, err = sock:connect(self.host, self.port)
   if not ok then
      return nil, err
   end
ngx.log(ngx.ERR,` "after connect, before send")

The second log never gots printed.

What could be wrong?