ElementTree#includes: relax synchronized access

lookupCache is declared volatile and it's type DataTreeLookup has all
fields final, so they can be used outside the synchronized block.
Sadly same pattern does not work for getElementData() as the returned
Object is not guaranteed to be thread-safe but some checks can be moved
outside the synchronized block.

ElementTree.includes() is heavily used for example during file search.

Author: EcljpseB0T

resources/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/watson/ElementTree.java

@@ -385,15 +385,20 @@ public DeltaDataTree getDataTree() {
 	 * Returns the element data for the given element identifier.
 	 * The given element must be present in this tree.
 	 */
-	public synchronized Object getElementData(IPath key) {
+	public Object getElementData(IPath key) {
 		/* don't allow modification of the implicit root */
-		if (key.isRoot())
+		if (key.isRoot()) {
 			return null;
-		DataTreeLookup lookup = lookupCache; // Grab it in case it's replaced concurrently.
-		if (lookup == null || lookup.key != key)
-			lookupCache = lookup = tree.lookup(key);
-		if (lookup.isPresent)
-			return lookup.data;
+		}
+		synchronized (this) {
+			DataTreeLookup lookup = lookupCache; // Grab it in case it's replaced concurrently.
+			if (lookup == null || lookup.key != key) {
+				lookupCache = lookup = tree.lookup(key);
+			}
+			if (lookup.isPresent) {
+				return lookup.data;
+			}
+		}
 		throw createElementNotFoundException(key);
 	}
 
@@ -545,10 +550,12 @@ public synchronized void immutable() {
 	 * Returns true if this element tree includes an element with the given
 	 * key, false otherwise.
 	 */
-	public synchronized boolean includes(IPath key) {
+	public boolean includes(IPath key) {
 		DataTreeLookup lookup = lookupCache; // Grab it in case it's replaced concurrently.
 		if (lookup == null || lookup.key != key) {
-			lookupCache = lookup = tree.lookup(key);
+			synchronized (this) {
+				lookupCache = lookup = tree.lookup(key);
+			}
 		}
 		return lookup.isPresent;
 	}