Starlark: dict.setdefault should fail if frozen

... even if `setdefault` call did not update the dict.

This is similar to bazelbuild#12519.

Closes bazelbuild#12642.

RELNOTES: dict.setdefault(key, ...) now fails if dict is frozen, even if it already contains key. This is an incompatible API change.
PiperOrigin-RevId: 346093037

src/main/java/net/starlark/java/eval/Dict.java

@@ -265,15 +265,12 @@ public Tuple popitem() throws EvalException {
             named = true,
             doc = "a default value if the key is absent."),
       })
-  @SuppressWarnings("unchecked") // Cast of value to V
-  public Object setdefault(K key, Object defaultValue) throws EvalException {
-    // TODO(adonovan): opt: use putIfAbsent to avoid hashing twice.
-    Object value = get(key);
-    if (value != null) {
-      return value;
-    }
-    putEntry(key, (V) defaultValue);
-    return defaultValue;
+  public V setdefault(K key, V defaultValue) throws EvalException {
+    Starlark.checkMutable(this);
+    Starlark.checkHashable(key);
+
+    V prev = contents.putIfAbsent(key, defaultValue); // see class doc comment
+    return prev != null ? prev : defaultValue;
   }
 
   @StarlarkMethod(

src/test/java/net/starlark/java/eval/testdata/dict.star

@@ -38,6 +38,10 @@ assert_eq(foo, bar)
 assert_eq(foo.pop('a', 0), 0)
 assert_eq(foo.popitem(), ('b', [1, 2]))
 
+---
+d = {1: 2}
+freeze(d)
+d.setdefault(1, 2) ### trying to mutate a frozen dict value
 ---
 dict().popitem() ### empty dictionary
 ---