Fixes #4090. Marathon service discovery for 5XX http response (#4091)

Signed-off-by: rohit01 <hello@rohit.io>

discovery/marathon/marathon.go

@@ -336,6 +336,10 @@ func fetchApps(client *http.Client, url string) (*AppList, error) {
 		return nil, err
 	}
 
+	if (resp.StatusCode < 200) || (resp.StatusCode >= 300) {
+		return nil, fmt.Errorf("Non 2xx status '%v' response during marathon service discovery", resp.StatusCode)
+	}
+
 	body, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err

discovery/marathon/marathon_test.go

@@ -16,7 +16,9 @@ package marathon
 import (
 	"context"
 	"errors"
+	"io"
 	"net/http"
+	"net/http/httptest"
 	"testing"
 	"time"
 
@@ -326,6 +328,33 @@ func TestMarathonZeroTaskPorts(t *testing.T) {
 	}
 }
 
+func Test500ErrorHttpResponseWithValidJSONBody(t *testing.T) {
+	var (
+		ch     = make(chan []*targetgroup.Group, 1)
+		client = fetchApps
+	)
+	// Simulate 500 error with a valid JSON response.
+	respHandler := func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Header().Set("Content-Type", "application/json")
+		io.WriteString(w, `{}`)
+	}
+	// Create a test server with mock HTTP handler.
+	ts := httptest.NewServer(http.HandlerFunc(respHandler))
+	defer ts.Close()
+	// Backup conf for future tests.
+	backupConf := conf
+	defer func() {
+		conf = backupConf
+	}()
+	// Setup conf for the test case.
+	conf = SDConfig{Servers: []string{ts.URL}}
+	// Execute test case and validate behaviour.
+	if err := testUpdateServices(client, ch); err == nil {
+		t.Fatalf("Expected error for 5xx HTTP response from marathon server")
+	}
+}
+
 func marathonTestAppListWithoutPortMappings(labels map[string]string, runningTasks int) *AppList {
 	var (
 		task = Task{