Use most of the main AbstractFuture implementation from J2KT and fr…

…om GWT/J2CL.

This CL introduces a superclass, `AbstractFutureState`, following the pattern of [`AggregateFutureState`](https://github.com/google/guava/blob/master/guava/src/com/google/common/util/concurrent/AggregateFutureState.java). That superclass contains platform-specific operations.

Fixes #2934

RELNOTES=n/a
PiperOrigin-RevId: 726495822

android/guava-testlib/src/com/google/common/testing/NullPointerTester.java

@@ -18,6 +18,9 @@
 
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
+import static java.util.Arrays.stream;
+import static java.util.Objects.requireNonNull;
+import static java.util.stream.Stream.concat;
 
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
@@ -33,6 +36,7 @@
 import com.google.common.reflect.Parameter;
 import com.google.common.reflect.Reflection;
 import com.google.common.reflect.TypeToken;
+import com.google.common.util.concurrent.AbstractFuture;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Constructor;
@@ -88,8 +92,28 @@ public NullPointerTester() {
        */
       ignoredMembers.add(Converter.class.getMethod("apply", Object.class));
     } catch (NoSuchMethodException shouldBeImpossible) {
-      // OK, fine: If it doesn't exist, then there's chance that we're going to be asked to test it.
+      // Fine: If it doesn't exist, then there's no chance that we're going to be asked to test it.
     }
+
+    /*
+     * These methods "should" call checkNotNull. However, I'm wary of accidentally introducing
+     * anything that might slow down execution on such a hot path. Given that the methods are only
+     * package-private, I feel OK with just not testing them for NPE.
+     *
+     * Note that testing casValue is particularly dangerous because it uses Unsafe under some
+     * versions of Java, and apparently Unsafe can cause SIGSEGV instead of NPE—almost as if it's
+     * not safe.
+     */
+    concat(
+            stream(AbstractFuture.class.getDeclaredMethods()),
+            stream(requireNonNull(AbstractFuture.class.getSuperclass()).getDeclaredMethods()))
+        .filter(
+            m ->
+                m.getName().equals("getDoneValue")
+                    || m.getName().equals("casValue")
+                    || m.getName().equals("casListeners")
+                    || m.getName().equals("gasListeners"))
+        .forEach(ignoredMembers::add);
   }
 
   /**

android/guava-tests/test/com/google/common/util/concurrent/AbstractFutureFallbackAtomicHelperTest.java

@@ -110,8 +110,8 @@ private void runTestMethod(ClassLoader classLoader) throws Exception {
   private void checkHelperVersion(ClassLoader classLoader, String expectedHelperClassName)
       throws Exception {
     // Make sure we are actually running with the expected helper implementation
-    Class<?> abstractFutureClass = classLoader.loadClass(AbstractFuture.class.getName());
-    Field helperField = abstractFutureClass.getDeclaredField("ATOMIC_HELPER");
+    Class<?> abstractFutureStateClass = classLoader.loadClass(AbstractFutureState.class.getName());
+    Field helperField = abstractFutureStateClass.getDeclaredField("ATOMIC_HELPER");
     helperField.setAccessible(true);
     assertEquals(expectedHelperClassName, helperField.get(null).getClass().getSimpleName());
   }

android/guava-tests/test/com/google/common/util/concurrent/AggregateFutureStateFallbackAtomicHelperTest.java

@@ -112,8 +112,8 @@ private void runTestMethod(ClassLoader classLoader) throws Exception {
   private void checkHelperVersion(ClassLoader classLoader, String expectedHelperClassName)
       throws Exception {
     // Make sure we are actually running with the expected helper implementation
-    Class<?> abstractFutureClass = classLoader.loadClass(AggregateFutureState.class.getName());
-    Field helperField = abstractFutureClass.getDeclaredField("ATOMIC_HELPER");
+    Class<?> abstractFutureStateClass = classLoader.loadClass(AggregateFutureState.class.getName());
+    Field helperField = abstractFutureStateClass.getDeclaredField("ATOMIC_HELPER");
     helperField.setAccessible(true);
     assertEquals(expectedHelperClassName, helperField.get(null).getClass().getSimpleName());
   }