[erlang-questions] Sending file descriptors and sockets to another beam

[Loïc Hoguin]

That's pretty cool.

Here's one very real use case using this. Evidently the same could be 
done with Erlang with this patch:

   https://blog.cloudflare.com/know-your-scm_rights/

On 24/09/2019 17:33, Danil Zagoskin wrote:
> Hi!
> 
> As you may know, OTP 22 has a new low-level socket API: 
> http://erlang.org/doc/man/socket.html
> It allows to communicate over UNIX sockets,
> and UNIX sockets have sendmsg feature, which has an option to pass a 
> file descriptor.
> 
> I tried to pass the FD with a fresh OTP, and this didn't just work.
> After a bit of hacking I've made a patch for socket NIF and a demo.
>    Patch: https://github.com/erlang/otp/pull/2400
>    Demo: https://gist.github.com/stolen/303c30d4edbb8835f9bec3fad0d75ede
> 
> Example: in four different shells execute:
> 
>   * |./sendsock.escript proxy|
>   * |./sendsock.escript worker|
>   * |telnet localhost 13456|
>   * |telnet localhost 13456|
> 
> See the listen and established sockets are owned by different 
> |beam.smp| processes:
> 
> |$ lsof -i:13456 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME 
> beam.smp 82931 stolen 32u IPv4 0x3c2e6eba43bdf26f 0t0 TCP *:13456 
> (LISTEN) beam.smp 82952 stolen 32u IPv4 0x3c2e6eba47fa726f 0t0 TCP 
> localhost:13456->localhost:62163 (ESTABLISHED) beam.smp 82952 stolen 33u 
> IPv4 0x3c2e6eba41348e8f 0t0 TCP localhost:13456->localhost:62185 
> (ESTABLISHED) telnet 82976 stolen 5u IPv4 0x3c2e6eba4f19452f 0t0 TCP 
> localhost:62163->localhost:13456 (ESTABLISHED) telnet 82993 stolen 5u 
> IPv4 0x3c2e6eba4786052f 0t0 TCP localhost:62185->localhost:13456 
> (ESTABLISHED)|
> 
> 
> What this may be useful for?
>    * Hot upgrades with BEAM upgrade/restart
>    * Load-balancing TCP load between unreliable workers without proxying
>    * Single-connection distribution (see https://github.com/stolen/webdist)
>    * Maybe anything else
> 
> Have fun!
> 
> -- 
> Danil Zagoskin | z@REDACTED <mailto:z@REDACTED>
> 
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-- 
Loïc Hoguin
https://ninenines.eu