[res] Better native pointer tracking in Java

Make sure we clear the native pointers when freeing them,
so any race condition that calls into it gets a null instead
of calling into already freed object

Bug: 197260547
Test: build + unit tests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7c2f195cfc8c02403d61a394213398d13584a5de)
Merged-In: I817dc2f5ef24e1fafeba3ce4c1c440abe70ad3ed
Change-Id: I817dc2f5ef24e1fafeba3ce4c1c440abe70ad3ed

core/java/android/content/res/AssetManager.java

@@ -528,6 +528,10 @@ public final class AssetManager implements AutoCloseable {
         if (!mOpen) {
             throw new RuntimeException("AssetManager has been closed");
         }
+        // Let's still check if the native object exists, given all the memory corruptions.
+        if (mObject == 0) {
+            throw new RuntimeException("AssetManager is open but the native object is gone");
+        }
     }
 
     /**
@@ -1153,6 +1157,7 @@ public final class AssetManager implements AutoCloseable {
     int[] getAttributeResolutionStack(long themePtr, @AttrRes int defStyleAttr,
             @StyleRes int defStyleRes, @StyleRes int xmlStyle) {
         synchronized (this) {
+            ensureValidLocked();
             return nativeAttributeResolutionStack(
                     mObject, themePtr, xmlStyle, defStyleAttr, defStyleRes);
         }

core/java/android/content/res/StringBlock.java

@@ -62,7 +62,7 @@ public final class StringBlock implements Closeable {
     private static final String TAG = "AssetManager";
     private static final boolean localLOGV = false;
 
-    private final long mNative;
+    private long mNative;   // final, but gets modified when closed
     private final boolean mUseSparse;
     private final boolean mOwnsNative;
 
@@ -207,6 +207,7 @@ public final class StringBlock implements Closeable {
                 if (mOwnsNative) {
                     nativeDestroy(mNative);
                 }
+                mNative = 0;
             }
         }
     }

core/java/android/content/res/XmlBlock.java

@@ -73,7 +73,9 @@ public final class XmlBlock implements AutoCloseable {
     private void decOpenCountLocked() {
         mOpenCount--;
         if (mOpenCount == 0) {
+            mStrings.close();
             nativeDestroy(mNative);
+            mNative = 0;
             if (mAssets != null) {
                 mAssets.xmlBlockGone(hashCode());
             }
@@ -621,7 +623,7 @@ public final class XmlBlock implements AutoCloseable {
     }
 
     private @Nullable final AssetManager mAssets;
-    private final long mNative;
+    private long mNative;   // final, but gets reset on close
     /*package*/ final StringBlock mStrings;
     private boolean mOpen = true;
     private int mOpenCount = 1;