Support comma-delimited string as labels in issue template (#21831)

The [labels in issue YAML
templates](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms#top-level-syntax)
can be a string array or a comma-delimited string, so a single string
should be valid labels.

The old codes committed in #20987 ignore this, that's why the warning is
displayed:

<img width="618" alt="image"
src="https://user-images.githubusercontent.com/9418365/202112642-93dc72d0-71c3-40a2-9720-30fc2d48c97c.png">

Fixes #17877.

2 files changed, 108 insertions, 8 deletions

diff --git a/modules/structs/issue.go b/modules/structs/issue.go
index 25c6251fbf..45c3f6294a 100644
--- a/modules/structs/issue.go
+++ b/modules/structs/issue.go
@@ -5,8 +5,12 @@
 package structs
 
 import (
+	"fmt"
 	"path"
+	"strings"
 	"time"
+
+	"gopkg.in/yaml.v3"
 )
 
 // StateType issue state type
@@ -143,14 +147,47 @@ type IssueFormField struct {
 // IssueTemplate represents an issue template for a repository
 // swagger:model
 type IssueTemplate struct {
-	Name     string            `json:"name" yaml:"name"`
-	Title    string            `json:"title" yaml:"title"`
-	About    string            `json:"about" yaml:"about"` // Using "description" in a template file is compatible
-	Labels   []string          `json:"labels" yaml:"labels"`
-	Ref      string            `json:"ref" yaml:"ref"`
-	Content  string            `json:"content" yaml:"-"`
-	Fields   []*IssueFormField `json:"body" yaml:"body"`
-	FileName string            `json:"file_name" yaml:"-"`
+	Name     string              `json:"name" yaml:"name"`
+	Title    string              `json:"title" yaml:"title"`
+	About    string              `json:"about" yaml:"about"` // Using "description" in a template file is compatible
+	Labels   IssueTemplateLabels `json:"labels" yaml:"labels"`
+	Ref      string              `json:"ref" yaml:"ref"`
+	Content  string              `json:"content" yaml:"-"`
+	Fields   []*IssueFormField   `json:"body" yaml:"body"`
+	FileName string              `json:"file_name" yaml:"-"`
+}
+
+type IssueTemplateLabels []string
+
+func (l *IssueTemplateLabels) UnmarshalYAML(value *yaml.Node) error {
+	var labels []string
+	if value.IsZero() {
+		*l = labels
+		return nil
+	}
+	switch value.Kind {
+	case yaml.ScalarNode:
+		str := ""
+		err := value.Decode(&str)
+		if err != nil {
+			return err
+		}
+		for _, v := range strings.Split(str, ",") {
+			if v = strings.TrimSpace(v); v == "" {
+				continue
+			}
+			labels = append(labels, v)
+		}
+		*l = labels
+		return nil
+	case yaml.SequenceNode:
+		if err := value.Decode(&labels); err != nil {
+			return err
+		}
+		*l = labels
+		return nil
+	}
+	return fmt.Errorf("line %d: cannot unmarshal %s into IssueTemplateLabels", value.Line, value.ShortTag())
 }
 
 // IssueTemplateType defines issue template type
diff --git a/modules/structs/issue_test.go b/modules/structs/issue_test.go
index 5312585d0f..72b40f7cf2 100644
--- a/modules/structs/issue_test.go
+++ b/modules/structs/issue_test.go
@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"gopkg.in/yaml.v3"
 )
 
 func TestIssueTemplate_Type(t *testing.T) {
@@ -41,3 +42,65 @@ func TestIssueTemplate_Type(t *testing.T) {
 		})
 	}
 }
+
+func TestIssueTemplateLabels_UnmarshalYAML(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		tmpl    *IssueTemplate
+		want    *IssueTemplate
+		wantErr string
+	}{
+		{
+			name:    "array",
+			content: `labels: ["a", "b", "c"]`,
+			tmpl: &IssueTemplate{
+				Labels: []string{"should_be_overwrote"},
+			},
+			want: &IssueTemplate{
+				Labels: []string{"a", "b", "c"},
+			},
+		},
+		{
+			name:    "string",
+			content: `labels: "a,b,c"`,
+			tmpl: &IssueTemplate{
+				Labels: []string{"should_be_overwrote"},
+			},
+			want: &IssueTemplate{
+				Labels: []string{"a", "b", "c"},
+			},
+		},
+		{
+			name:    "empty",
+			content: `labels:`,
+			tmpl: &IssueTemplate{
+				Labels: []string{"should_be_overwrote"},
+			},
+			want: &IssueTemplate{
+				Labels: nil,
+			},
+		},
+		{
+			name: "error",
+			content: `
+labels:
+  a: aa
+  b: bb
+`,
+			tmpl:    &IssueTemplate{},
+			wantErr: "line 3: cannot unmarshal !!map into IssueTemplateLabels",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := yaml.Unmarshal([]byte(tt.content), tt.tmpl)
+			if tt.wantErr != "" {
+				assert.EqualError(t, err, tt.wantErr)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, tt.tmpl)
+			}
+		})
+	}
+}