Fix StackOverflowError when merging ParseObject from JSON (#925)

* Add test for StackOverflowError during mergeFromServer

This test shows the #896 bug.

* Make a synchronized copy of availableKeys

`synchronizedSet(s)` ends up making a new wrapper for the supplied set, which
means on subsequent calls, it becomes a SynchronizedSet, wrapped in a
SynchronizedSet, wrapped ... etc.

When using the LocalDataStore, the ParseObject.State is reused for every
matching ClassName+ObjectId pair, so every time a ParseObject is parsed from
JSON data, the existing object is "refreshed" by making a copy of its State,
and then merging with the new data. Every call to State.newBuilder() is
therefore adding a new layer of SynchronizedSet to the availableKeys Set.

Eventually that nested hierarchy of sets becomes so large that it causes a
StackOverflowError for any operation on the collection.

By making a copy of the set before wrapping it in synchronizedSet(), that
nested hierarchy becomes severed, and we end up with just one level of
wrappers.

* Accept the updated Android Build-Tools 27 license

Author: Joe

.travis.yml

@@ -11,7 +11,7 @@ jdk:
 before_install:
     - pip install --user codecov
     - mkdir "$ANDROID_HOME/licenses" || true
-    - echo "d56f5187479451eabf01fb78af6dfcb131a6481e" > "$ANDROID_HOME/licenses/android-sdk-license"
+    - echo "24333f8a63b6825ea9c5514f83c2829b004d1fee" >> "$ANDROID_HOME/licenses/android-sdk-license"
 
 script:
   - ./gradlew clean testDebugUnitTest jacocoTestReport

parse/src/main/java/com/parse/ParseObject.java

@@ -4076,7 +4076,7 @@ public Init(String className) {
                 objectId = state.objectId();
                 createdAt = state.createdAt();
                 updatedAt = state.updatedAt();
-                availableKeys = Collections.synchronizedSet(state.availableKeys());
+                availableKeys = Collections.synchronizedSet(new HashSet<>(state.availableKeys()));
                 for (String key : state.keySet()) {
                     serverData.put(key, state.get(key));
                     availableKeys.add(key);

parse/src/test/java/com/parse/ParseObjectTest.java

@@ -44,6 +44,7 @@
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Matchers.any;
 import static org.mockito.Matchers.anyString;
+import static org.mockito.Matchers.eq;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
@@ -128,7 +129,7 @@ public void testFromJSONPayload() throws JSONException {
 
     //endregion
 
-    //region testGetter
+    //region testFromJson
 
     @Test
     public void testFromJSONPayloadWithoutClassname() throws JSONException {
@@ -137,6 +138,30 @@ public void testFromJSONPayloadWithoutClassname() throws JSONException {
         assertNull(parseObject);
     }
 
+    @Test
+    public void testFromJsonWithLdsStackOverflow() throws JSONException {
+        ParseObject localObj = ParseObject.createWithoutData("GameScore", "TT1ZskATqS");
+        OfflineStore lds = mock(OfflineStore.class);
+        Parse.setLocalDatastore(lds);
+
+        when(lds.getObject(eq("GameScore"), eq("TT1ZskATqS"))).thenReturn(localObj);
+
+        JSONObject json = new JSONObject("{" +
+                "\"className\":\"GameScore\"," +
+                "\"createdAt\":\"2015-06-22T21:23:41.733Z\"," +
+                "\"objectId\":\"TT1ZskATqS\"," +
+                "\"updatedAt\":\"2015-06-22T22:06:18.104Z\"" +
+                "}");
+        ParseObject obj;
+        for (int i = 0; i < 50000; i++) {
+            obj = ParseObject.fromJSON(json, "GameScore", ParseDecoder.get(), Collections.<String>emptySet());
+        }
+    }
+
+    //endregion
+
+    //region testGetter
+
     @Test
     public void testRevert() throws ParseException {
         List<Task<Void>> tasks = new ArrayList<>();