Making closed flag in Store and Query volatile (#818)

This way checkOpen() is more accurate in multithreaded scenarios.
Note that putting "synchronized" on checkOpen() would not help much, as the main race is between calling checkOpen() and actually using the handle.
Alternatively, we could lock during the entire time the handle is in use, but this might already be overkill as the current behavior should uncover bad usage patterns already without sacrificing performance and risking new locking issues.


(cherry picked from commit 38be697f774e395359b8717d4991f2fa707bd18c)

Author: greenrobot

objectbox-java/src/main/java/io/objectbox/BoxStore.java

@@ -232,7 +232,8 @@ public static boolean isSyncServerAvailable() {
     /** Set when running inside TX */
     final ThreadLocal<Transaction> activeTx = new ThreadLocal<>();
 
-    private boolean closed;
+    // volatile so checkOpen() is more up-to-date (no need for synchronized; it's a race anyway)
+    volatile private boolean closed;
 
     final Object txCommitCountLock = new Object();
 

objectbox-java/src/main/java/io/objectbox/query/Query.java

@@ -110,7 +110,8 @@ native void nativeSetParameter(long handle, int entityId, int propertyId, @Nulla
     private final int queryAttempts;
     private static final int INITIAL_RETRY_BACK_OFF_IN_MS = 10;
 
-    long handle;
+    // volatile so checkOpen() is more up-to-date (no need for synchronized; it's a race anyway)
+    volatile long handle;
 
     Query(Box<T> box, long queryHandle, @Nullable List<EagerRelation<T, ?>> eagerRelations, @Nullable  QueryFilter<T> filter,
           @Nullable Comparator<T> comparator) {