Data race in ProbeHTTP

[ozon2]

Hi, with blackbox_exporter 0.21.0 I get data races in ProbeHTTP, caused by the new caser used here https://github.com/prometheus/blackbox_exporter/blob/master/prober/http.go#L355.

==================
WARNING: DATA RACE
Write at 0x00c000468c80 by goroutine 28:
  golang.org/x/text/cases.(*titleCaser).Transform()
      /home/tpillot/dev/pkg/mod/golang.org/x/[email protected]/cases/map.go:336 +0x144
  golang.org/x/text/transform.String()
      /home/tpillot/dev/pkg/mod/golang.org/x/[email protected]/transform/transform.go:601 +0x1f8
  golang.org/x/text/cases.Caser.String()
      /home/tpillot/dev/pkg/mod/golang.org/x/[email protected]/cases/cases.go:51 +0x8475
  github.com/prometheus/blackbox_exporter/prober.ProbeHTTP()
      /home/tpillot/dev/pkg/mod/github.com/prometheus/[email protected]/prober/http.go:355 +0x842b

Previous write at 0x00c000468c80 by goroutine 29:
  golang.org/x/text/cases.(*titleCaser).Transform()
      /home/tpillot/dev/pkg/mod/golang.org/x/[email protected]/cases/map.go:336 +0x144
  golang.org/x/text/transform.String()
      /home/tpillot/dev/pkg/mod/golang.org/x/[email protected]/transform/transform.go:601 +0x1f8
  golang.org/x/text/cases.Caser.String()
      /home/tpillot/dev/pkg/mod/golang.org/x/[email protected]/cases/cases.go:51 +0x3075
  github.com/prometheus/blackbox_exporter/prober.ProbeHTTP()
      /home/tpillot/dev/pkg/mod/github.com/prometheus/[email protected]/prober/http.go:428 +0x302b

Here is an example to reproduce the issue:

import (
	"sync"

	"github.com/go-kit/log"
	"github.com/prometheus/blackbox_exporter/config"
        "github.com/prometheus/blackbox_exporter/prober"
)

func TestBlackBox(t *testing.T) {
	logger := log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
	mod := config.DefaultModule
	mod.HTTP.Headers = map[string]string{"a": "b"}

	var wg sync.WaitGroup

	probe := func() {
		defer wg.Done()

		prober.ProbeHTTP(context.Background(), "example.com", mod, prometheus.NewRegistry(), logger)
	}

	for i := 0; i < 2; i++ {
		wg.Add(1)

		go probe()
	}

	wg.Wait()
}

[mem]

I just ran into this.

It's because the caser is reused. The docs say not to do that.

I'll upload a fix.

[mem]

For the record, my own build of BBE doesn't use -race, but this manifests itself as a panic, because one caser overrides the indices of the other and it ends up e.g. with an out of bounds slice access.