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

[Tristan Sloughter]

Oh nice, maybe this is why I could never get the systemd socket activation to work! It relies on using the file descriptor 3 for the listen socket.

On Tue, Sep 24, 2019, at 09:33, 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
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