Add optimistic locking to OpenableElementInfo add/delete/set operations

I have an impression that JavaModel.getJavaProjects() that uses
OpenableElementInfo to maintain children may sometimes return stale data
because OpenableElementInfo is not synchronized and probably could be
accessed from different threads (the volatile children field indicates
that the original authors assumed multi-threaded access to that data).

This could explain why we sometimes see absolutely fully unexpected test
failures related to the Java projects state.

See eclipse-jdt#2716

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/model/AbstractJavaModelTests.java

@@ -2418,6 +2418,10 @@ public void run(IProgressMonitor monitor) throws CoreException {
 		assertTrue("Not Java project: " + project, project.hasNature(JavaCore.NATURE_ID));
 		IProject sameProject = getWorkspaceRoot().getProject(projectName);
 		assertEquals("Returned project doesn't match same project from workspace: " + sameProject + " vs " + project, sameProject, project);
+
+		List<IJavaProject> javaProjects = List.of(getJavaModel().getJavaProjects());
+		boolean foundInModel = javaProjects.stream().anyMatch(p -> projectName.equals(p.getElementName()));
+		assertTrue("Project '" + projectName + "' should be present in JavaModel, but we found only: " + javaProjects, foundInModel);
 		return javaProject;
 	}
 	protected IJavaProject importJavaProject(String projectName, String[] sourceFolders, String[] libraries, String output) throws CoreException {

org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/OpenableElementInfo.java

@@ -42,20 +42,43 @@ public class OpenableElementInfo extends JavaElementInfo {
 	 */
 	protected Object[] nonJavaResources;
 
-	public void addChild(IJavaElement child) {
+	public void addChild(final IJavaElement child) {
 		IJavaElement[] oldChildren = this.children;
 		int length = oldChildren.length;
 		if (length == 0) {
-			this.children = new IJavaElement[] {child};
+			synchronized (this) {
+				if (oldChildren == this.children) {
+					this.children = new IJavaElement[] {child};
+				} else {
+					// try again, holding a lock
+					addChild(child);
+				}
+			}
 		} else {
 			for (int i = 0; i < length; i++) {
-				if (oldChildren[i].equals(child))
-					return; // already included
+				if (oldChildren[i].equals(child)) {
+					synchronized (this) {
+						if (oldChildren == this.children){
+							return; // already included
+						} else {
+							// try again, holding a lock
+							addChild(child);
+							return;
+						}
+					}
+				}
 			}
 			IJavaElement[] newChildren = new IJavaElement[length+1];
 			System.arraycopy(oldChildren, 0, newChildren, 0, length);
 			newChildren[length] = child;
-			this.children = newChildren;
+			synchronized (this) {
+				if (oldChildren == this.children) {
+					this.children = newChildren;
+				} else {
+					// try again, holding a lock
+					addChild(child);
+				}
+			}
 		}
 	}
 
@@ -71,26 +94,44 @@ public boolean isStructureKnown() {
 		return this.isStructureKnown;
 	}
 
-	public void removeChild(IJavaElement child) {
+	public void removeChild(final IJavaElement child) {
 		IJavaElement[] oldChildren = this.children;
 		for (int i = 0, length = oldChildren.length; i < length; i++) {
 			if (oldChildren[i].equals(child)) {
 				if (length == 1) {
-					this.children = JavaElement.NO_ELEMENTS;
+					synchronized (this) {
+						if (oldChildren == this.children) {
+							this.children = JavaElement.NO_ELEMENTS;
+						} else {
+							// try again, holding a lock
+							removeChild(child);
+							return;
+						}
+					}
 				} else {
 					IJavaElement[] newChildren = new IJavaElement[length-1];
 					System.arraycopy(oldChildren, 0, newChildren , 0, i);
 					if (i < length-1)
 						System.arraycopy(oldChildren, i+1, newChildren, i, length-1-i);
-					this.children = newChildren;
+					synchronized (this) {
+						if (oldChildren == this.children) {
+							this.children = newChildren;
+						} else {
+							// try again, holding a lock
+							removeChild(child);
+							return;
+						}
+					}
 				}
 				break;
 			}
 		}
 	}
 
 	public void setChildren(IJavaElement[] children) {
-		this.children= (children.length > 0) ? children : JavaElement.NO_ELEMENTS;
+		synchronized (this) {
+			this.children= (children.length > 0) ? children : JavaElement.NO_ELEMENTS;
+		}
 	}
 
 	public void setModule(IModuleDescription module) {