Fix busy loop when FDs are exhausted

[nojima]

We found that once yrmcds exhausts FDs, tcp_server_socket::on_readable() causes a busy loop with logging accept: Too many open files. This PR is to avoid that busy loop.

[nojima]

I found another issue when FDs are exhausted.
yrmcds crashes when it calls cybozu::has_ip_address when no FDs available.

2024-07-24 09:36:27 ERROR [std::system_error] (system:24) getifaddrs: Too many open files

[ymmt2005]

@nojima
Thank you! Can we add a test?

[nojima]

I added the test for FD flood.

[ymmt2005]

@nojima
Thank you, LGTM.

BTW, what about this?

yrmcds crashes when it calls cybozu::has_ip_address when no FDs available.

[nojima]

@ymmt2005

BTW, what about this?

yrmcds crashes when it calls cybozu::has_ip_address when no FDs available.

I am not sure what to do.
Perhaps we should first define how yrmcds should behave when FDs are exhausted.

In any case, I think it is outside the scope of this PR.

[ymmt2005]

OK. Please create an issue for the problem so that we can revisit it later.

[nojima]

I added issue #92.

[ymmt2005]

Thanks.