Fix concurrency issue in PropertyInvalidatedCache

Bug: 253063488

Ensure the global lock is held while fetching the list of active
caches.  The global lock is necessary while the list is being fetched;
it is not necessary while each cache in the list is cleared.

Added a new test for PropertyInvalidatedCache.onTrimMemory().  This
test will not catch race conditions but does verify that
onTrimMemory() behaves as expected.

Test: atest
 * FrameworksCoreTests:PropertyInvalidatedCacheTests
Change-Id: I5022620cd4f2561179af709246a9bf149423143f

core/java/android/app/PropertyInvalidatedCache.java

@@ -1405,6 +1405,17 @@ public class PropertyInvalidatedCache<Query, Result> {
         return isDisabled();
     }
 
+    /**
+     * Return the number of entries in the cache.  This is used for testing and has package-only
+     * visibility.
+     * @hide
+     */
+    public int size() {
+        synchronized (mLock) {
+            return mCache.size();
+        }
+    }
+
     /**
      * Returns a list of caches alive at the current time.
      */
@@ -1612,8 +1623,12 @@ public class PropertyInvalidatedCache<Query, Result> {
      * @hide
      */
     public static void onTrimMemory() {
-        for (PropertyInvalidatedCache pic : getActiveCaches()) {
-            pic.clear();
+        ArrayList<PropertyInvalidatedCache> activeCaches;
+        synchronized (sGlobalLock) {
+            activeCaches = getActiveCaches();
+        }
+        for (int i = 0; i < activeCaches.size(); i++) {
+            activeCaches.get(i).clear();
         }
     }
 }

core/tests/coretests/src/android/app/PropertyInvalidatedCacheTests.java

@@ -368,4 +368,20 @@ public class PropertyInvalidatedCacheTests {
             PropertyInvalidatedCache.MODULE_BLUETOOTH, "getState");
         assertEquals(n1, "cache_key.bluetooth.get_state");
     }
+
+    @Test
+    public void testOnTrimMemory() {
+        TestCache cache = new TestCache(MODULE, "trimMemoryTest");
+        // The cache is not active until it has been invalidated once.
+        cache.invalidateCache();
+        // Populate the cache with six entries.
+        for (int i = 0; i < 6; i++) {
+            cache.query(i);
+        }
+        // The maximum number of entries in TestCache is 4, so even though six entries were
+        // created, only four are retained.
+        assertEquals(4, cache.size());
+        PropertyInvalidatedCache.onTrimMemory();
+        assertEquals(0, cache.size());
+    }
 }