### Eclipse Workspace Patch 1.0
#P org.eclipse.jdt.core
Index: model/org/eclipse/jdt/internal/core/ProjectReferenceChange.java
===================================================================
RCS file: /cvsroot/eclipse/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/ProjectReferenceChange.java,v
retrieving revision 1.3
diff -u -r1.3 ProjectReferenceChange.java
--- model/org/eclipse/jdt/internal/core/ProjectReferenceChange.java	6 Mar 2007 02:38:49 -0000	1.3
+++ model/org/eclipse/jdt/internal/core/ProjectReferenceChange.java	11 Jan 2008 11:32:28 -0000
@@ -15,8 +15,12 @@
 
 import org.eclipse.core.resources.IProject;
 import org.eclipse.core.resources.IProjectDescription;
+import org.eclipse.core.resources.IWorkspace;
 import org.eclipse.core.resources.IWorkspaceRoot;
+import org.eclipse.core.resources.IWorkspaceRunnable;
 import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.IProgressMonitor;
+import org.eclipse.core.runtime.jobs.ISchedulingRule;
 import org.eclipse.jdt.core.IClasspathEntry;
 import org.eclipse.jdt.core.JavaModelException;
 import org.eclipse.jdt.core.compiler.CharOperation;
@@ -37,61 +41,69 @@
 	 */
 	public void updateProjectReferencesIfNecessary() throws JavaModelException {
 		
-		String[] oldRequired = this.oldResolvedClasspath == null ? CharOperation.NO_STRINGS : this.project.projectPrerequisites(this.oldResolvedClasspath);
+		final String[] oldRequired = this.oldResolvedClasspath == null ? CharOperation.NO_STRINGS : this.project.projectPrerequisites(this.oldResolvedClasspath);
 		IClasspathEntry[] newResolvedClasspath = this.project.getResolvedClasspath();
-		String[] newRequired = this.project.projectPrerequisites(newResolvedClasspath);
-		try {
-			IProject projectResource = this.project.getProject();
-			IProjectDescription description = projectResource.getDescription();
-			 
-			IProject[] projectReferences = description.getDynamicReferences();
-			
-			HashSet oldReferences = new HashSet(projectReferences.length);
-			for (int i = 0; i < projectReferences.length; i++){
-				String projectName = projectReferences[i].getName();
-				oldReferences.add(projectName);
-			}
-			HashSet newReferences = (HashSet)oldReferences.clone();
-	
-			for (int i = 0; i < oldRequired.length; i++){
-				String projectName = oldRequired[i];
-				newReferences.remove(projectName);
-			}
-			for (int i = 0; i < newRequired.length; i++){
-				String projectName = newRequired[i];
-				newReferences.add(projectName);
-			}
-	
-			Iterator iter;
-			int newSize = newReferences.size();
-			
-			checkIdentity: {
-				if (oldReferences.size() == newSize){
-					iter = newReferences.iterator();
-					while (iter.hasNext()){
-						if (!oldReferences.contains(iter.next())){
-							break checkIdentity;
+		final String[] newRequired = this.project.projectPrerequisites(newResolvedClasspath);
+		final IProject projectResource = this.project.getProject();
+		IWorkspaceRunnable runnable = new IWorkspaceRunnable() {
+			public void run(IProgressMonitor monitor) throws CoreException {
+				IProjectDescription description = projectResource.getDescription();
+				 
+				IProject[] projectReferences = description.getDynamicReferences();
+				
+				HashSet oldReferences = new HashSet(projectReferences.length);
+				for (int i = 0; i < projectReferences.length; i++){
+					String projectName = projectReferences[i].getName();
+					oldReferences.add(projectName);
+				}
+				HashSet newReferences = (HashSet)oldReferences.clone();
+		
+				for (int i = 0; i < oldRequired.length; i++){
+					String projectName = oldRequired[i];
+					newReferences.remove(projectName);
+				}
+				for (int i = 0; i < newRequired.length; i++){
+					String projectName = newRequired[i];
+					newReferences.add(projectName);
+				}
+		
+				Iterator iter;
+				int newSize = newReferences.size();
+				
+				checkIdentity: {
+					if (oldReferences.size() == newSize){
+						iter = newReferences.iterator();
+						while (iter.hasNext()){
+							if (!oldReferences.contains(iter.next())){
+								break checkIdentity;
+							}
 						}
+						return;
 					}
-					return;
 				}
+				String[] requiredProjectNames = new String[newSize];
+				int index = 0;
+				iter = newReferences.iterator();
+				while (iter.hasNext()){
+					requiredProjectNames[index++] = (String)iter.next();
+				}
+				Util.sort(requiredProjectNames); // ensure that if changed, the order is consistent
+				
+				IProject[] requiredProjectArray = new IProject[newSize];
+				IWorkspaceRoot wksRoot = projectResource.getWorkspace().getRoot();
+				for (int i = 0; i < newSize; i++){
+					requiredProjectArray[i] = wksRoot.getProject(requiredProjectNames[i]);
+				}
+				description.setDynamicReferences(requiredProjectArray);
+				projectResource.setDescription(description, null);
 			}
-			String[] requiredProjectNames = new String[newSize];
-			int index = 0;
-			iter = newReferences.iterator();
-			while (iter.hasNext()){
-				requiredProjectNames[index++] = (String)iter.next();
-			}
-			Util.sort(requiredProjectNames); // ensure that if changed, the order is consistent
-			
-			IProject[] requiredProjectArray = new IProject[newSize];
-			IWorkspaceRoot wksRoot = projectResource.getWorkspace().getRoot();
-			for (int i = 0; i < newSize; i++){
-				requiredProjectArray[i] = wksRoot.getProject(requiredProjectNames[i]);
-			}
-			description.setDynamicReferences(requiredProjectArray);
-			projectResource.setDescription(description, null);
-	
+		};
+		try {
+			// ensure that a sheduling rule is used so that the project description is not modified by another thread while we update it
+			// see https://bugs.eclipse.org/bugs/show_bug.cgi?id=214981
+			IWorkspace workspace = projectResource.getWorkspace();
+			ISchedulingRule rule = workspace.getRuleFactory().modifyRule(projectResource); // sheduling rule for modifying the project
+			workspace.run(runnable, rule, IWorkspace.AVOID_UPDATE, null);
 		} catch(CoreException e){
 			if (!ExternalJavaProject.EXTERNAL_PROJECT_NAME.equals(this.project.getElementName()))
 				throw new JavaModelException(e);