abstract_tcp_server2: fix busy calling of idle IO service

[moneromooo-monero]

This would make monerod use 100% CPU when running with torsocks
without Tor running

[garlicgambit]

Thanks for looking into this issue.

The issue still persists. Tested on Tails with the local Tor proxy with the following command:
DNS_PUBLIC=tcp torsocks ./monerod --p2p-bind-ip 127.0.0.1 --no-igd --rpc-bind-ip 127.0.0.1

We have some new/improved info. If you configure iptables to drop monerod traffic to Tor and allow it again monerod will establish new connections with Tor and will have normal cpu use. However if you do a simple restart the of the Tor process monerod won't create new connections to Tor and will keep using 100% of the cpu.

[anonimal]

Tested on Tails with the local Tor proxy with the following command:
DNS_PUBLIC=tcp torsocks ./monerod --p2p-bind-ip 127.0.0.1 --no-igd --rpc-bind-ip 127.0.0.1

There's no need to torsocks behind a local transproxy.

If you configure iptables to drop monerod traffic to Tor, and allow it again, monerod will establish new connections with Tor (and monero will have normal cpu use). However, if you do a simple restart of the Tor process, monerod won't create new connections to Tor and will continue to use 100% of the cpu.

Don't play with iptables rules unless you know what you're doing. Flushing and then restarting rules on a local transproxy leaves a potential window for application leaks. If you must, use a sandboxed internal network machine with gateway transproxy.

[moneromooo-monero]

Does #3997 help ?

[garlicgambit]

The issue still persists in #3997.

@anonimal, thanks for the comment. We're using an offline Tails system to troubleshoot this issue. So no worries.

[moneromooo-monero]

OK, then run "sudo perf top -p `pidof monerod`" while it's doing this, let it run for 20 seconds, then paste the state.

[garlicgambit]

Tested on Debian Stretch x64 with PR #3997.

Action: stop Tor process
Command: sudo perf top -p `pidof monerod`
Output:

Overhead  Shared Object         Symbol
  11.99%  [kernel]              [k] do_select
   9.67%  [kernel]              [k] memset_erms
   4.42%  [kernel]              [k] tcp_poll
   4.38%  [kernel]              [k] core_sys_select
   4.10%  [kernel]              [k] __fget
   3.74%  [kernel]              [k] _raw_spin_lock_irqsave
   3.68%  [kernel]              [k] entry_SYSCALL_64
   3.41%  [kernel]              [k] fput
   3.17%  [kernel]              [k] entry_SYSCALL_64_after_swapgs
   2.96%  libpthread-2.24.so    [.] __libc_recv
   2.85%  [kernel]              [k] _raw_spin_lock_bh
   2.74%  [kernel]              [k] tcp_recvmsg
   2.44%  libc-2.24.so          [.] __select
   2.23%  [kernel]              [k] __local_bh_enable_ip
   1.97%  [kernel]              [k] _raw_spin_unlock_irqrestore
   1.88%  [kernel]              [k] __check_object_size
   1.62%  [kernel]              [k] __pollwait
   1.60%  [kernel]              [k] SYSC_recvfrom

Action: stop Tor process
Command: sudo perf top --sort comm,dso
Output:

Overhead  Command          Shared Object
  83.86%  monerod          [kernel]
   5.30%  monerod          libpthread-2.24.so
   4.81%  monerod          libc-2.24.so
   3.82%  monerod          libtorsocks.so.0.0.0

Action: drop traffic to Tor with iptables
Command: sudo perf top -p `pidof monerod`
Output:

Overhead  Shared Object         Symbol
   9.30%  [kernel]              [k] _raw_spin_lock_bh
   8.88%  libc-2.24.so          [.] connect
   7.90%  [kernel]              [k] __fget
   7.66%  [kernel]              [k] __local_bh_enable_ip
   6.51%  [kernel]              [k] entry_SYSCALL_64
   6.08%  [kernel]              [k] SYSC_connect
   5.48%  [kernel]              [k] entry_SYSCALL_64_after_swapgs
   4.45%  [kernel]              [k] __inet_stream_connect
   3.74%  [kernel]              [k] fput

Action: drop traffic to Tor with iptables
Command: sudo perf top --sort comm,dso
Output:

Overhead  Command          Shared Object
  78.25%  monerod          [kernel]
  18.20%  monerod          libc-2.24.so
   1.26%  monerod          libtorsocks.so.0.0.0
   0.48%  monerod          libpthread-2.24.so

Action: run without 'DNS_PUBLIC=tcp', drop traffic to Tor with iptables
Command: sudo perf top -p `pidof monerod`
Output:

Overhead  Shared Object              Symbol
  11.37%  [kernel]                   [k] __local_bh_enable_ip
  10.53%  libc-2.24.so               [.] connect
  10.53%  [kernel]                   [k] _raw_spin_lock_bh
   8.35%  [kernel]                   [k] entry_SYSCALL_64_after_swapgs
   7.64%  [kernel]                   [k] entry_SYSCALL_64
   7.53%  [kernel]                   [k] __fget
   4.84%  [kernel]                   [k] fput

Action: run without 'DNS_PUBLIC=tcp', drop traffic to Tor with iptables
Command: sudo perf top --sort comm,dso
Output:

Overhead  Command          Shared Object
  73.56%  monerod          [kernel]
  20.57%  monerod          libc-2.24.so
   1.34%  monerod          libtorsocks.so.0.0.0
   0.23%  monerod          libpthread-2.24.so

[moneromooo-monero]

What command(s) are you using for "drop traffic to Tor with iptables" ?

[moneromooo-monero]

What command(s) are you using for "drop traffic to Tor with iptables" ?

[moneromooo-monero]

Well, that patch fixes at least one case so let's add it for now.

[fluffypony]

Reviewed