set SO_LINGER=0 to TCP connection

[Sniper91]

Closing TCP socket after every probe may cause large numbers of connections sitting in the TIME_WAIT state
Relevant Issue: #794

set SO_LINGER=0 to avoid problem above
Reference: https://stackoverflow.com/questions/3757289/when-is-tcp-option-so-linger-0-required

[mem]

I tried to reproduce the situation reported in the issue and I couldn't. I don't see connections in the TIME_WAIT state. Would it be possible for you to provide more details as to how to reproduce this?

I'm hesitant to simply reset TCP connections from the client side instead of closing them in an orderly fashion.

[Sniper91]

1. run blackbox_exporter with file blackbox.yml
   version=0.25.0, branch=HEAD, revision=ef3ff4fef195333fb8ee0039fb487b2f5007908f

docker run -d  --rm   -p 9115:9115   --name blackbox_exporter   -v $(pwd):/config   quay.io/prometheus/blackbox-exporter:latest --config.file=/config/blackbox.yml

2. run prometheus

docker run -d --network host --rm --name prometheus   -v $(pwd)/prometheus.yml:/etc/prometheus/prometheus.yml prom/prometheus

configuration file:

global:
  scrape_interval: 5s
scrape_configs:
  - job_name: 'blackbox'
    metrics_path: /probe
    params:
      module: [http_2xx] 
    static_configs:
      - targets:
          - https://prometheus.io
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_target
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: 127.0.0.1:9115

3. wait several minutes, check tcp connection state of blackbox_exporter container

  docker exec blackbox_exporter netstat -tnp