Describe the bug

I have below TLS listener which works fine.

 <source>
      @type forward
          port 20001
          bind 0.0.0.0
          <transport tls>
                ca_path /opt/tls/ca.crt
                cert_path /opt/tls/tls.crt
                private_key_path /opt/tls/tls.key
                client_cert_auth true
          </transport>
  </source>

Recently clients can no longer connect. It turned out the /opt/tls/tls.crt on the server got an empty file. fluentd came up with no errors, but of course won't accept any TLS connections with empty cert file. I would expect fluentd should fail to start, and give errors about the empty cert file tls.crt.

To Reproduce

See above in description

Expected behavior

fluentd should fail during starting up, instead of starting up but fail to work.

Your Environment

• Fluentd or td-agent version: fluentd --version or td-agent --version

fluentd --version
fluentd 1.11.1

• Operating system: cat /etc/os-release

cat /etc/os-release 
NAME="CentOS Linux"
VERSION="8 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="8"
PLATFORM_ID="platform:el8"
PRETTY_NAME="CentOS Linux 8 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:8"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-8"
CENTOS_MANTISBT_PROJECT_VERSION="8"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="8"

• Kernel version: uname -r

uname -r
4.18.0-193.6.3.el8_2.x86_64

If you hit the problem with older fluentd version, try latest version first.

Your Configuration

See above in description

Your Error Log

No errors during starting up.

Additional context