Do not serialize out container cache when locked

Author: briandealwis

plugins/com.google.cloud.tools.eclipse.appengine.libraries/src/com/google/cloud/tools/eclipse/appengine/libraries/repository/LibraryClasspathContainerResolverService.java

@@ -170,7 +170,13 @@ public IStatus resolveContainer(
             new IJavaProject[] {javaProject},
             new IClasspathContainer[] {container},
             subMonitor.newChild(1));
-        serializer.saveContainer(javaProject, container);
+        if (!javaProject.getProject().getWorkspace().isTreeLocked()) {
+          // The classpath container update may have been triggered as part of a resource change
+          // in which case the workspace is locked and so we cannot serialize out our
+          // container cache.  There will be other opportunities.
+          // https://github.com/GoogleCloudPlatform/google-cloud-eclipse/issues/3181
+          serializer.saveContainer(javaProject, container);
+        }
         for (Job job : sourceAttacherJobs) {
           job.schedule();
         }