Fix segmentation violation on connection failed (influxdata#7593)

plugins/inputs/http_response/http_response.go

@@ -271,14 +271,6 @@ func (h *HTTPResponse) httpGather(u string) (map[string]interface{}, map[string]
 	resp, err := h.client.Do(request)
 	response_time := time.Since(start).Seconds()
 
-	// Add the response headers
-	for headerName, tag := range h.HTTPHeaderTags {
-		headerValues, foundHeader := resp.Header[headerName]
-		if foundHeader && len(headerValues) > 0 {
-			tags[tag] = headerValues[0]
-		}
-	}
-
 	// If an error in returned, it means we are dealing with a network error, as
 	// HTTP error codes do not generate errors in the net/http library
 	if err != nil {
@@ -306,6 +298,14 @@ func (h *HTTPResponse) httpGather(u string) (map[string]interface{}, map[string]
 	// required by the net/http library
 	defer resp.Body.Close()
 
+	// Add the response headers
+	for headerName, tag := range h.HTTPHeaderTags {
+		headerValues, foundHeader := resp.Header[headerName]
+		if foundHeader && len(headerValues) > 0 {
+			tags[tag] = headerValues[0]
+		}
+	}
+
 	// Set log the HTTP response code
 	tags["status_code"] = strconv.Itoa(resp.StatusCode)
 	fields["http_response_code"] = resp.StatusCode

plugins/inputs/http_response/http_response_test.go

@@ -287,6 +287,33 @@ func TestHTTPHeaderTags(t *testing.T) {
 		"result":      "success",
 	}
 	checkOutput(t, &acc, expectedFields, expectedTags, absentFields, nil)
+
+	// Connection failed
+	h = &HTTPResponse{
+		Log:             testutil.Logger{},
+		Address:         "https:/nonexistent.nonexistent", // Any non-routable IP works here
+		Body:            "",
+		Method:          "GET",
+		ResponseTimeout: internal.Duration{Duration: time.Second * 5},
+		HTTPHeaderTags:  map[string]string{"Server": "my_server", "Content-Type": "content_type"},
+		FollowRedirects: false,
+	}
+
+	acc = testutil.Accumulator{}
+	err = h.Gather(&acc)
+	require.NoError(t, err)
+
+	expectedFields = map[string]interface{}{
+		"result_type": "connection_failed",
+		"result_code": 3,
+	}
+	expectedTags = map[string]interface{}{
+		"server": nil,
+		"method": "GET",
+		"result": "connection_failed",
+	}
+	absentFields = []string{"http_response_code", "response_time", "content_length", "response_string_match"}
+	checkOutput(t, &acc, expectedFields, expectedTags, absentFields, nil)
 }
 
 func findInterface() (net.Interface, error) {