CLOUDP-226171: handle safely pointers of slices in the output templates (#2569)

Author: Ciprian Tibulca

internal/templatewriter/templatewriter.go

@@ -16,6 +16,7 @@ package templatewriter
 
 import (
 	"io"
+	"reflect"
 	"text/tabwriter"
 	"text/template"
 )
@@ -27,6 +28,23 @@ const (
 	tabwriterPadChar  = ' '
 )
 
+var funcMap = template.FuncMap{
+	"valueOrEmptySlice": valueOrEmptySlice,
+}
+
+func valueOrEmptySlice(slice interface{}) (result interface{}) {
+	if slice == nil {
+		return result
+	}
+
+	k := reflect.TypeOf(slice).Kind()
+	if (k == reflect.Slice || k == reflect.Ptr) && reflect.ValueOf(slice).IsNil() {
+		return result
+	}
+
+	return slice
+}
+
 // newTabWriter returns a tabwriter that handles tabs(`\t) to space columns evenly.
 func newTabWriter(output io.Writer) *tabwriter.Writer {
 	return tabwriter.NewWriter(output, tabwriterMinWidth, tabwriterWidth, tabwriterPadding, tabwriterPadChar, 0)
@@ -37,7 +55,7 @@ func newTabWriter(output io.Writer) *tabwriter.Writer {
 // this template will be handled with a tabwriter so you can use tabs (\t)
 // and new lines (\n) to space your content evenly.
 func Print(writer io.Writer, t string, v interface{}) error {
-	tmpl, err := template.New("output").Parse(t)
+	tmpl, err := template.New("output").Funcs(funcMap).Parse(t)
 	if err != nil {
 		return err
 	}

internal/templatewriter/templatewriter_test.go

@@ -0,0 +1,95 @@
+// Copyright 2024 MongoDB Inc
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package templatewriter
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type printTest struct {
+	name     string
+	template string
+	data     interface{}
+	expected string
+	wantErr  require.ErrorAssertionFunc
+}
+
+func Test_Print(t *testing.T) {
+	var buf bytes.Buffer
+
+	tests := []printTest{
+		{
+			name:     "primitive data",
+			template: "name: {{.}}",
+			data:     "Jane",
+			expected: "name: Jane",
+			wantErr:  require.NoError,
+		},
+		{
+			name:     "nil data",
+			template: "name: {{.}}",
+			data:     nil,
+			expected: "name: <no value>",
+			wantErr:  require.NoError,
+		},
+		{
+			name:     "pointer of non empty slice",
+			template: "items: {{range .Items}}{{.}} {{end}}",
+			data:     struct{ Items *[]string }{Items: &[]string{"AWS", "GCP", "Azure"}},
+			expected: "items: AWS GCP Azure ",
+			wantErr:  require.NoError,
+		},
+		{
+			name:     "nil pointer of slice",
+			template: "items: {{range .Items}}{{.}} {{end}}",
+			data:     struct{ Items *[]string }{Items: nil},
+			expected: "",
+			wantErr:  require.Error, // expected to fail, as Items is nil
+		},
+		{
+			name:     "nil pointer of slice",
+			template: "items: {{range valueOrEmptySlice .Items}}{{.}} {{end}}",
+			data:     struct{ Items *[]string }{Items: nil},
+			expected: "items: ",
+			wantErr:  require.NoError,
+		},
+		{
+			name:     "non empty slice",
+			template: "items: {{range valueOrEmptySlice .Items}}{{.}} {{end}}",
+			data:     struct{ Items []string }{Items: []string{"AWS", "GCP", "Azure"}},
+			expected: "items: AWS GCP Azure ",
+			wantErr:  require.NoError,
+		},
+		{
+			name:     "empty slice",
+			template: "items: {{range valueOrEmptySlice .Items}}{{.}} {{end}}",
+			data:     struct{ Items []string }{Items: []string{}},
+			expected: "items: ",
+			wantErr:  require.NoError,
+		},
+	}
+
+	for _, conf := range tests {
+		t.Run(conf.name, func(t *testing.T) {
+			conf.wantErr(t, Print(&buf, conf.template, conf.data))
+			assert.Equal(t, conf.expected, buf.String())
+		})
+		buf.Reset()
+	}
+}