JBR-9642 Wayland: Call wl_data_offer.finish()

This patch adds a call to wl_data_offer.finish() upon a successful
completion of a drag-and-drop operation, as well as more synchronized
annotations in WLDataOffer, to match the existing ones.

Calling finish() doesn't seem to be required by most compositors when
performing drag-and-drop, at least within the same window. Other
toolkits, such as Qt, only call it when dealing with cross-application
drag-and-drop. In the interest of maximal compatibility, this patch
implements calling finish() always when a drag-and-drop operation
succeeds.

src/java.desktop/unix/classes/sun/awt/wl/WLDataOffer.java

@@ -68,7 +68,7 @@ public class WLDataOffer {
         }
     }
 
-    public byte[] receiveData(String mime) throws IOException  {
+    public synchronized byte[] receiveData(String mime) throws IOException  {
         int fd;
 
         if (nativePtr == 0) {
@@ -86,7 +86,7 @@ public class WLDataOffer {
         }
     }
 
-    public void accept(long serial, String mime) {
+    public synchronized void accept(long serial, String mime) {
         if (nativePtr == 0) {
             throw new IllegalStateException("nativePtr is 0");
         }
@@ -94,15 +94,17 @@ public class WLDataOffer {
         acceptImpl(nativePtr, serial, mime);
     }
 
-    public void finishDnD() {
+    public synchronized void finishDnD() {
         if (nativePtr == 0) {
             throw new IllegalStateException("nativePtr is 0");
         }
 
-        finishDnDImpl(nativePtr);
+        if (selectedAction != 0) {
+            finishDnDImpl(nativePtr);
+        }
     }
 
-    public void setDnDActions(int actions, int preferredAction) {
+    public synchronized void setDnDActions(int actions, int preferredAction) {
         if (nativePtr == 0) {
             throw new IllegalStateException("nativePtr is 0");
         }

src/java.desktop/unix/classes/sun/awt/wl/WLDropTargetContextPeer.java

@@ -106,7 +106,10 @@ public class WLDropTargetContextPeer extends SunDropTargetContextPeer {
     }
 
     @Override
-    protected void doDropDone(boolean success, int dropAction, boolean isLocal) {
+    protected synchronized void doDropDone(boolean success, int dropAction, boolean isLocal) {
+        if (success && currentOffer != null) {
+            currentOffer.finishDnD();
+        }
         reset();
     }