Make caser a local varialbe, not a global one.

The cases.Caser returned by calling cases.Title *cannot* be shared among
goroutines. This might happen when Prometheus tries to scrape multiple
targets at the same time. From the docs:

A Caser may be stateful and should therefore not be shared between
goroutines.

Fixes: #922

Signed-off-by: Marcelo E. Magallon <marcelo.magallon@grafana.com>

prober/http.go

@@ -46,8 +46,6 @@ import (
 	"github.com/prometheus/blackbox_exporter/config"
 )
 
-var caser = cases.Title(language.Und)
-
 func matchRegularExpressions(reader io.Reader, httpConfig config.HTTPProbe, logger log.Logger) bool {
 	body, err := ioutil.ReadAll(reader)
 	if err != nil {
@@ -342,6 +340,16 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		return false
 	}
 
+	// Do not move the following variable to global scope. The cases.Caser returned by
+	// calling cases.Title *cannot* be shared among goroutines. This might happen when
+	// Prometheus tries to scrape multiple targets at the same time. From the docs:
+	//
+	// A Caser may be stateful and should therefore not be shared between goroutines.
+	//
+	// Issue: https://github.com/prometheus/blackbox_exporter/issues/922
+
+	caser := cases.Title(language.Und)
+
 	httpClientConfig := module.HTTP.HTTPClientConfig
 	if len(httpClientConfig.TLSConfig.ServerName) == 0 {
 		// If there is no `server_name` in tls_config, use

prober/http_test.go

@@ -35,6 +35,8 @@ import (
 	"github.com/go-kit/log"
 	"github.com/prometheus/client_golang/prometheus"
 	pconfig "github.com/prometheus/common/config"
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
 
 	"github.com/prometheus/blackbox_exporter/config"
 )
@@ -1068,6 +1070,7 @@ func TestHTTPHeaders(t *testing.T) {
 		"Accept-Language": "en-US",
 	}
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		caser := cases.Title(language.Und)
 		for key, value := range headers {
 			if caser.String(key) == "Host" {
 				if r.Host != value {