starlark: fix ArrayStoreException crash in list.append

...by establishing the invariant that StarlarkList.elems.getClass() == Object[].class.

+ Test

PiperOrigin-RevId: 343114456

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

@@ -244,7 +244,7 @@ public static Iterable<?> toIterable(Object x) throws EvalException {
   public static Object[] toArray(Object x) throws EvalException {
     // Specialize Sequence and Dict to avoid allocation and/or indirection.
     if (x instanceof Sequence) {
-      return ((Sequence<?>) x).toArray();
+      return ((Sequence<?>) x).toArray(new Object[((Sequence) x).size()]);
     } else if (x instanceof Dict) {
       return ((Dict<?, ?>) x).keySet().toArray();
     } else {

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

@@ -78,6 +78,7 @@ public final class StarlarkList<E> extends AbstractList<E>
   // but without the extra indirection of using ArrayList.
 
   // elems[0:size] holds the logical elements, and elems[size:] are not used.
+  // elems.getClass() == Object[].class. This is necessary to avoid ArrayStoreException.
   private int size;
   private int iteratorCount; // number of active iterators (unused once frozen)
   private Object[] elems = EMPTY_ARRAY; // elems[i] == null  iff  i >= size
@@ -88,15 +89,16 @@ public final class StarlarkList<E> extends AbstractList<E>
   private static final Object[] EMPTY_ARRAY = {};
 
   private StarlarkList(@Nullable Mutability mutability, Object[] elems, int size) {
+    Preconditions.checkArgument(elems.getClass() == Object[].class);
     this.elems = elems;
     this.size = size;
     this.mutability = mutability == null ? Mutability.IMMUTABLE : mutability;
   }
 
   /**
-   * Takes ownership of the supplied array and returns a new StarlarkList instance that initially
-   * wraps the array. The caller must not subsequently modify the array, but the StarlarkList
-   * instance may do so.
+   * Takes ownership of the supplied array of class Object[].class, and returns a new StarlarkList
+   * instance that initially wraps the array. The caller must not subsequently modify the array, but
+   * the StarlarkList instance may do so.
    */
   static <T> StarlarkList<T> wrap(@Nullable Mutability mutability, Object[] elems) {
     return new StarlarkList<>(mutability, elems, elems.length);
@@ -184,13 +186,13 @@ public static <T> StarlarkList<T> immutableCopyOf(Iterable<? extends T> elems) {
    */
   public static <T> StarlarkList<T> of(@Nullable Mutability mutability, T... elems) {
     checkElemsValid(elems);
-    return wrap(mutability, Arrays.copyOf(elems, elems.length));
+    return wrap(mutability, Arrays.copyOf(elems, elems.length, Object[].class));
   }
 
   /** Returns an immutable {@code StarlarkList} with the given items. */
   public static <T> StarlarkList<T> immutableOf(T... elems) {
     checkElemsValid(elems);
-    return wrap(null, Arrays.copyOf(elems, elems.length));
+    return wrap(null, Arrays.copyOf(elems, elems.length, Object[].class));
   }
 
   @Override

src/test/java/net/starlark/java/eval/StarlarkListTest.java

@@ -321,12 +321,20 @@ public void testCopyOfTakesCopy() throws EvalException {
 
   @Test
   public void testWrapTakesOwnershipOfArray() throws EvalException {
-    String[] wrapped = {"hello"};
+    Object[] wrapped = {"hello"};
     Mutability mutability = Mutability.create("test");
-    StarlarkList<String> mutableList = StarlarkList.wrap(mutability, wrapped);
+    StarlarkList<Object> mutableList = StarlarkList.wrap(mutability, wrapped);
 
     // Big no-no, but we're proving a point.
     wrapped[0] = "goodbye";
-    assertThat((List<String>) mutableList).containsExactly("goodbye");
+    assertThat((List<?>) mutableList).containsExactly("goodbye");
+  }
+
+  @Test
+  public void testOfReturnsListWhoseArrayElementTypeIsObject() throws EvalException {
+    Mutability mu = Mutability.create("test");
+    StarlarkList<Object> list = StarlarkList.of(mu, "a", "b");
+    list.addElement(StarlarkInt.of(1)); // no ArrayStoreException
+    assertThat(list.toString()).isEqualTo("[\"a\", \"b\", 1]");
   }
 }