Implement reference counting for frozen realms

Author: alecbarber

realm/realm-library/src/androidTest/java/io/realm/FrozenObjectsTests.java

@@ -93,6 +93,34 @@ public void freezeRealm() {
         frozenRealm.close();
     }
 
+    @Test
+    public void freezeRealmTwice_atSameVersion() {
+        // Freeze the realm twice at the same version
+        assertFalse(realm.isFrozen());
+        Realm frozenRealm1 = realm.freeze();
+        Realm frozenRealm2 = realm.freeze();
+
+        // If we close one frozen instance, the other instance should be unaffected
+        frozenRealm2.close();
+        assertFalse(frozenRealm1.isClosed());
+        frozenRealm1.close();
+    }
+
+    @Test
+    public void freezeRealmTwice_atDifferentVersions() {
+        // Freeze the realm twice at different versions
+        assertFalse(realm.isFrozen());
+        Realm frozenRealm1 = realm.freeze();
+        realm.executeTransaction((Realm) -> realm.copyToRealm(new Dog("Woof", 3)));
+        Realm frozenRealm2 = realm.freeze();
+
+        // If we close one frozen instance, the other instance should be unaffected
+        frozenRealm1.close();
+        assertTrue(frozenRealm1.isClosed());
+        assertFalse(frozenRealm2.isClosed());
+        frozenRealm2.close();
+    }
+
     @Test
     public void freezeDynamicRealm() {
         DynamicRealm dynamicRealm = DynamicRealm.getInstance(realmConfig);

realm/realm-library/src/main/java/io/realm/RealmCache.java

@@ -70,19 +70,14 @@ interface Callback0 {
 
     private abstract static class ReferenceCounter {
 
-        // How many references to this Realm instance in this thread.
-        protected final ThreadLocal<Integer> localCount = new ThreadLocal<>();
         // How many threads have instances refer to this configuration.
         protected AtomicInteger globalCount = new AtomicInteger(0);
 
         // Returns `true` if an instance of the Realm is available on the caller thread.
         abstract boolean hasInstanceAvailableForThread();
 
         // Increment how many times an instance has been handed out for the current thread.
-        public void incrementThreadCount(int increment) {
-            Integer currentCount = localCount.get();
-            localCount.set(currentCount != null ? currentCount + increment : increment);
-        }
+        public abstract void incrementThreadCount(int increment);
 
         // Returns the Realm instance for the caller thread
         abstract BaseRealm getRealmInstance();
@@ -97,11 +92,9 @@ public void incrementThreadCount(int increment) {
         abstract int getThreadLocalCount();
 
         // Updates the number of references handed out for a given thread
-        public void setThreadCount(int refCount) {
-            localCount.set(refCount);
-        }
+        public abstract void setThreadCount(int refCount);
 
-        // Returns the number of gloal instances handed out. This is roughly equivalent
+        // Returns the number of global instances handed out. This is roughly equivalent
         // to the number of threads currently using the Realm as each thread also does
         // reference counting of Realm instances.
         public int getGlobalCount() {
@@ -118,6 +111,11 @@ boolean hasInstanceAvailableForThread() {
             return cachedRealm != null;
         }
 
+        @Override
+        public void incrementThreadCount(int increment) {
+            globalCount.addAndGet(increment);
+        }
+
         @Override
         BaseRealm getRealmInstance() {
             return cachedRealm;
@@ -128,19 +126,17 @@ void onRealmCreated(BaseRealm realm) {
             // The Realm instance has been created without exceptions. Cache and reference count can be updated now.
             cachedRealm = realm;
 
-            localCount.set(0);
-            // This is the first instance in current thread, increase the global count.
-            globalCount.incrementAndGet();
+            // The global count for a frozen Realm has the same role as the local count for a live
+            // Realm, so initialise it to 0.
+            globalCount.set(0);
 
         }
 
         @Override
         public void clearThreadLocalCache() {
             String canonicalPath = cachedRealm.getPath();
 
-            // The last instance in this thread.
-            // Clears local ref & counter.
-            localCount.set(null);
+            // The last instance of this frozen Realm. Clear reference.
             cachedRealm = null;
 
             // Clears global counter.
@@ -156,18 +152,33 @@ int getThreadLocalCount() {
             // of a thread local count doesn't make sense. Just return the global count instead.
             return globalCount.get();
         }
+
+        @Override
+        public void setThreadCount(int refCount) {
+            // Since the concept of a thread local count doesn't make sense, we instead set the
+            // global count here.
+            globalCount.set(refCount);
+        }
     }
 
     // Reference counter for Realms that are thread confined
     private static class ThreadConfinedReferenceCounter extends ReferenceCounter {
         // The Realm instance in this thread.
         private final ThreadLocal<BaseRealm> localRealm = new ThreadLocal<>();
+        // How many references to this Realm instance in this thread.
+        private final ThreadLocal<Integer> localCount = new ThreadLocal<>();
 
         @Override
         public boolean hasInstanceAvailableForThread() {
             return localRealm.get() != null;
         }
 
+        @Override
+        public void incrementThreadCount(int increment)  {
+            Integer currentCount = localCount.get();
+            localCount.set(currentCount != null ? currentCount + increment : increment);
+        }
+
         @Override
         public BaseRealm getRealmInstance() {
             return localRealm.get();
@@ -203,6 +214,11 @@ public int getThreadLocalCount() {
             Integer refCount = localCount.get();
             return (refCount != null) ? refCount : 0;
         }
+
+        @Override
+        public void setThreadCount(int refCount)  {
+            localCount.set(refCount);
+        }
     }
 
     private enum RealmCacheType {