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

[Danil Zagoskin]

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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