tpl/data: Misc header improvements, tests, allow multiple headers of same key

Closes #5617

common/types/convert.go

@@ -15,21 +15,52 @@ package types
 
 import (
 	"encoding/json"
+	"fmt"
 	"html/template"
+	"reflect"
 
 	"github.com/spf13/cast"
 )
 
-// ToStringSlicePreserveString converts v to a string slice.
-// If v is a string, it will be wrapped in a string slice.
+// ToStringSlicePreserveString is the same as ToStringSlicePreserveStringE,
+// but it never fails.
 func ToStringSlicePreserveString(v interface{}) []string {
+	vv, _ := ToStringSlicePreserveStringE(v)
+	return vv
+}
+
+// ToStringSlicePreserveStringE converts v to a string slice.
+// If v is a string, it will be wrapped in a string slice.
+func ToStringSlicePreserveStringE(v interface{}) ([]string, error) {
 	if v == nil {
-		return nil
+		return nil, nil
 	}
 	if sds, ok := v.(string); ok {
-		return []string{sds}
+		return []string{sds}, nil
 	}
-	return cast.ToStringSlice(v)
+	result, err := cast.ToStringSliceE(v)
+	if err == nil {
+		return result, nil
+	}
+
+	// Probably []int or similar. Fall back to reflect.
+	vv := reflect.ValueOf(v)
+
+	switch vv.Kind() {
+	case reflect.Slice, reflect.Array:
+		result = make([]string, vv.Len())
+		for i := 0; i < vv.Len(); i++ {
+			s, err := cast.ToStringE(vv.Index(i).Interface())
+			if err != nil {
+				return nil, err
+			}
+			result[i] = s
+		}
+		return result, nil
+	default:
+		return nil, fmt.Errorf("failed to convert %T to a string slice", v)
+	}
+
 }
 
 // TypeToString converts v to a string if it's a valid string type.

common/types/convert_test.go

@@ -24,7 +24,9 @@ func TestToStringSlicePreserveString(t *testing.T) {
 	c := qt.New(t)
 
 	c.Assert(ToStringSlicePreserveString("Hugo"), qt.DeepEquals, []string{"Hugo"})
+	c.Assert(ToStringSlicePreserveString(qt.Commentf("Hugo")), qt.DeepEquals, []string{"Hugo"})
 	c.Assert(ToStringSlicePreserveString([]interface{}{"A", "B"}), qt.DeepEquals, []string{"A", "B"})
+	c.Assert(ToStringSlicePreserveString([]int{1, 3}), qt.DeepEquals, []string{"1", "3"})
 	c.Assert(ToStringSlicePreserveString(nil), qt.IsNil)
 }