Fix "concurrent map iteration and map write" in pages from data

Fixes gohugoio#13254

Author: bep

common/maps/maps_test.go

@@ -73,10 +73,14 @@ func TestPrepareParams(t *testing.T) {
 	for i, test := range tests {
 		t.Run(fmt.Sprint(i), func(t *testing.T) {
 			// PrepareParams modifies input.
+			prepareClone := PrepareParamsClone(test.input)
 			PrepareParams(test.input)
 			if !reflect.DeepEqual(test.expected, test.input) {
 				t.Errorf("[%d] Expected\n%#v, got\n%#v\n", i, test.expected, test.input)
 			}
+			if !reflect.DeepEqual(test.expected, prepareClone) {
+				t.Errorf("[%d] Expected\n%#v, got\n%#v\n", i, test.expected, prepareClone)
+			}
 		})
 	}
 }

common/maps/params.go

@@ -303,7 +303,7 @@ func toMergeStrategy(v any) ParamsMergeStrategy {
 }
 
 // PrepareParams
-// * makes all the keys in the given map lower cased and will do so
+// * makes all the keys in the given map lower cased and will do so recursively.
 // * This will modify the map given.
 // * Any nested map[interface{}]interface{}, map[string]interface{},map[string]string  will be converted to Params.
 // * Any _merge value will be converted to proper type and value.
@@ -343,3 +343,42 @@ func PrepareParams(m Params) {
 		}
 	}
 }
+
+// PrepareParamsClone is like PrepareParams, but it does not modify the input.
+func PrepareParamsClone(m Params) Params {
+	m2 := make(Params)
+	for k, v := range m {
+		var retyped bool
+		lKey := strings.ToLower(k)
+		if lKey == MergeStrategyKey {
+			v = toMergeStrategy(v)
+			retyped = true
+		} else {
+			switch vv := v.(type) {
+			case map[any]any:
+				var p Params = cast.ToStringMap(v)
+				v = PrepareParamsClone(p)
+				retyped = true
+			case map[string]any:
+				var p Params = v.(map[string]any)
+				v = PrepareParamsClone(p)
+				retyped = true
+			case map[string]string:
+				p := make(Params)
+				for k, v := range vv {
+					p[k] = v
+				}
+				v = p
+				PrepareParams(p)
+				retyped = true
+			}
+		}
+
+		if retyped || k != lKey {
+			m2[lKey] = v
+		} else {
+			m2[k] = v
+		}
+	}
+	return m2
+}

resources/page/pagemeta/page_frontmatter.go

@@ -158,8 +158,11 @@ func (p *PageConfig) Compile(basePath string, pagesFromData bool, ext string, lo
 
 	if p.Params == nil {
 		p.Params = make(maps.Params)
+	} else if pagesFromData {
+		p.Params = maps.PrepareParamsClone(p.Params)
+	} else {
+		maps.PrepareParams(p.Params)
 	}
-	maps.PrepareParams(p.Params)
 
 	if p.Content.Markup == "" && p.Content.MediaType == "" {
 		if ext == "" {