mapstr: fix M.Clone behaviour for arrays of M (#262)

The current implementation does not clone in branches of slices in a
mapstr.M, resulting in cases where the clone will share values with the
original and allowing unexpected mutation of the original. This change
fixes that by adding slices type handling. Note that this will not fix
cases of unexpected sharing where the node value type is not a mapstr.M
or map[string]any, so structs that are included with pointer fields will
still show this effect.

mapstr/mapstr.go

@@ -154,19 +154,44 @@ func (m M) CopyFieldsTo(to M, key string) error {
 }
 
 // Clone returns a copy of the M. It recursively makes copies of inner
-// maps.
+// maps. Nested arrays and non-map types are not cloned.
 func (m M) Clone() M {
 	result := make(M, len(m))
+	cloneMap(result, m)
+	return result
+}
 
-	for k := range m {
-		if innerMap, ok := tryToMapStr(m[k]); ok {
-			result[k] = innerMap.Clone()
-		} else {
-			result[k] = m[k]
+func cloneMap(dst, src M) {
+	for k, v := range src {
+		switch v := v.(type) {
+		case M:
+			d := make(M, len(v))
+			dst[k] = d
+			cloneMap(d, v)
+		case map[string]interface{}:
+			d := make(map[string]interface{}, len(v))
+			dst[k] = d
+			cloneMap(d, v)
+		case []M:
+			a := make([]M, 0, len(v))
+			for _, m := range v {
+				d := make(M, len(m))
+				cloneMap(d, m)
+				a = append(a, d)
+			}
+			dst[k] = a
+		case []map[string]interface{}:
+			a := make([]map[string]interface{}, 0, len(v))
+			for _, m := range v {
+				d := make(map[string]interface{}, len(m))
+				cloneMap(d, m)
+				a = append(a, d)
+			}
+			dst[k] = a
+		default:
+			dst[k] = v
 		}
 	}
-
-	return result
 }
 
 // HasKey returns true if the key exist. If an error occurs then false is

mapstr/mapstr_test.go

@@ -348,6 +348,7 @@ func TestClone(t *testing.T) {
 			"c31": 1,
 			"c32": 2,
 		},
+		"c4": []M{{"c41": 1}},
 	}
 
 	// Clone the original mapstr and then increment every value in it. Ensures the test will fail if
@@ -364,6 +365,7 @@ func TestClone(t *testing.T) {
 				"c31": 1,
 				"c32": 2,
 			},
+			"c4": []M{{"c41": 1}},
 		},
 		cloned,
 	)
@@ -377,6 +379,10 @@ func incrementMapstrValues(m M) {
 			m[k] = v + 1
 		case M:
 			incrementMapstrValues(m[k].(M))
+		case []M:
+			for _, c := range v {
+				incrementMapstrValues(c)
+			}
 		}
 
 	}