Fix segfault in x509_cert (#4874)

plugins/inputs/x509_cert/README.md

@@ -10,7 +10,7 @@ file or network connection.
 # Reads metrics from a SSL certificate
 [[inputs.x509_cert]]
   ## List certificate sources
-  sources = ["/etc/ssl/certs/ssl-cert-snakeoil.pem", "https://example.org"]
+  sources = ["/etc/ssl/certs/ssl-cert-snakeoil.pem", "https://example.org:443"]
 
   ## Timeout for SSL connection
   # timeout = "5s"

plugins/inputs/x509_cert/dev/telegraf.conf

@@ -0,0 +1,5 @@
+[[inputs.x509_cert]]
+    sources = ["https://www.influxdata.com:443"]
+
+[[outputs.file]]
+  files = ["stdout"]

plugins/inputs/x509_cert/x509_cert.go

@@ -80,7 +80,10 @@ func (c *X509Cert) getCert(location string, timeout time.Duration) ([]*x509.Cert
 		}
 		defer ipConn.Close()
 
-		tlsCfg.ServerName = u.Host
+		if tlsCfg == nil {
+			tlsCfg = &tls.Config{}
+		}
+		tlsCfg.ServerName = u.Hostname()
 		conn := tls.Client(ipConn, tlsCfg)
 		defer conn.Close()
 

plugins/inputs/x509_cert/x509_cert_test.go

@@ -4,6 +4,8 @@ import (
 	"crypto/tls"
 	"encoding/base64"
 	"fmt"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"io/ioutil"
 	"os"
 	"testing"
@@ -203,3 +205,19 @@ func TestStrings(t *testing.T) {
 		})
 	}
 }
+
+func TestGatherCert(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping integration test in short mode")
+	}
+
+	m := &X509Cert{
+		Sources: []string{"https://www.influxdata.com:443"},
+	}
+
+	var acc testutil.Accumulator
+	err := m.Gather(&acc)
+	require.NoError(t, err)
+
+	assert.True(t, acc.HasMeasurement("x509_cert"))
+}