Invalidate structure after ownership transferred to promise in Promise.reply (#178)

If we don't invalidate after Promise.reply, we may sit in situations:

1. structure got freed while it is owned by promise, we can get hit from
   gst log:

   ```
   (PromiseTest:13221): GStreamer-CRITICAL **: 10:28:01.594: gst_structure_free: assertion 'GST_STRUCTURE_REFCOUNT (structure) == NULL' failed
   ```
2. structure got freed after its owning promise freed, we got
   double-free.

3. structure got used after freed due to interrupted before reply or
   owning promise freed.

Author: kezhuw

src/org/freedesktop/gstreamer/Promise.java

@@ -19,6 +19,7 @@
 
 import static org.freedesktop.gstreamer.lowlevel.GstPromiseAPI.GSTPROMISE_API;
 
+import org.freedesktop.gstreamer.glib.NativeObject;
 import org.freedesktop.gstreamer.lowlevel.GstAPI.GstCallback;
 
 import com.sun.jna.Pointer;
@@ -86,7 +87,9 @@ public PromiseResult waitResult() {
      * {@link PromiseResult} state. If the promise has already been interrupted
      * than the replied will not be visible to any waiters
      *
-     * @param structure the {@link Structure} to reply the promise with
+     * @param structure the {@link Structure} to reply the promise with, caller
+     * should not use this structure afterward as it is invalidated through
+     * {@link NativeObject#invalidate()}
      */
     public void reply(final Structure structure) {
         GSTPROMISE_API.gst_promise_reply(this, structure);

src/org/freedesktop/gstreamer/glib/Natives.java

@@ -187,6 +187,26 @@ public static GPointer getPointer(NativeObject obj) {
         return obj.getPointer();
     }
 
+    /**
+     * Return whether underlying native pointer is owned by this object.
+     *
+     * @param obj native object which may hold a reference to native pointer
+     * @return whether underlying native pointer is owned by this object
+     */
+    public static boolean ownsReference(NativeObject obj) {
+        return obj.handle.ownsReference();
+    }
+
+    /**
+     * Returns whether this object is valid or not.
+     *
+     * @param obj native object
+     * @return whether this object is valid or not
+     */
+    public static boolean validReference(NativeObject obj) {
+        return obj.handle.getPointer() != null;
+    }
+
     /**
      * Increase the reference count of a {@link RefCountedObject}
      *

src/org/freedesktop/gstreamer/lowlevel/GstPromiseAPI.java

@@ -28,6 +28,7 @@
 import org.freedesktop.gstreamer.lowlevel.annotations.CallerOwnsReturn;
 
 import com.sun.jna.Pointer;
+import org.freedesktop.gstreamer.lowlevel.annotations.Invalidate;
 
 /**
  * GstPromise methods and structures
@@ -65,7 +66,7 @@ protected List<String> getFieldOrder() {
 
   PromiseResult gst_promise_wait(Promise promise);
 
-  void gst_promise_reply(Promise promise, Structure s);
+  void gst_promise_reply(Promise promise, @Invalidate Structure s);
   void gst_promise_interrupt(Promise promise);
   void gst_promise_expire(Promise promise);
 

test/org/freedesktop/gstreamer/PromiseTest.java

@@ -19,8 +19,11 @@
 package org.freedesktop.gstreamer;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
+import org.freedesktop.gstreamer.glib.Natives;
+import org.freedesktop.gstreamer.lowlevel.GPointer;
 import org.freedesktop.gstreamer.lowlevel.GType;
 import org.junit.BeforeClass;
 import org.junit.AfterClass;
@@ -82,19 +85,34 @@ public void testExpire() {
         assertEquals("promise reply state not correct", promiseStatus, PromiseResult.EXPIRED);
     }
 
+    @Test
+    public void testInvalidateReply() {
+        if (!Gst.testVersion(1, 14)) {
+            return;
+        }
+        Promise promise = new Promise();
+        Structure data = new Structure("data");
+
+        assertTrue(Natives.ownsReference(data));
+        promise.reply(data);
+        assertFalse(Natives.ownsReference(data));
+        assertFalse(Natives.validReference(data));
+    }
+
     @Test
     public void testReplyData() {
         if (!Gst.testVersion(1, 14)) {
             return;
         }
         Promise promise = new Promise();
         Structure data = new Structure("data", "test", GType.UINT, 1);
+        GPointer pointer = Natives.getPointer(data);
 
         promise.reply(data);
         assertEquals("promise state not in replied", promise.waitResult(), PromiseResult.REPLIED);
 
         Structure result = promise.getReply();
-        assertTrue("result of promise does not match reply", result.isEqual(data));
+        assertEquals("result of promise does not match reply", pointer, Natives.getPointer(result));
     }
 
     @Test