Add test for #2003 (#2115)

This adds a test for cleanup in c349446aSigned-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/setup_test.go

 package forward
 
 import (
+	"io/ioutil"
+	"os"
 	"reflect"
 	"strings"
 	"testing"
@@ -118,3 +120,55 @@ func TestSetupTLS(t *testing.T) {
 		}
 	}
 }
+
+func TestSetupResolvconf(t *testing.T) {
+	const resolv = "resolv.conf"
+	if err := ioutil.WriteFile(resolv,
+		[]byte(`nameserver 10.10.255.252
+nameserver 10.10.255.253`), 0666); err != nil {
+		t.Fatalf("Failed to write resolv.conf file: %s", err)
+	}
+	defer os.Remove(resolv)
+
+	tests := []struct {
+		input         string
+		shouldErr     bool
+		expectedErr   string
+		expectedNames []string
+	}{
+		// pass
+		{`forward . ` + resolv, false, "", []string{"10.10.255.252:53", "10.10.255.253:53"}},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		f, err := parseForward(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: expected error but found %s for input %s", i, err, test.input)
+			continue
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: expected no error but found one for input %s, got: %v", i, test.input, err)
+			}
+
+			if !strings.Contains(err.Error(), test.expectedErr) {
+				t.Errorf("Test %d: expected error to contain: %v, found error: %v, input: %s", i, test.expectedErr, err, test.input)
+			}
+		}
+
+		if !test.shouldErr {
+			for j, n := range test.expectedNames {
+				addr := f.proxies[j].addr
+				if n != addr {
+					t.Errorf("Test %d, expected %q, got %q", j, n, addr)
+				}
+			}
+		}
+		for _, p := range f.proxies {
+			p.health.Check(p) // this should almost always err, we don't care it shoulnd't crash
+		}
+	}
+}