Defer initialization of classpath to the background if called from main

Currently there are some bad behaving UI components in the eclipse IDE
that trigger resolving of the classpath containers in the UI, this leads
to very bad startup performance and even deadlocks in startup.

This now detects the issue, logs a warning of the offending component
and defer the initialization of classpath to a background job, this
currently increase time from starting eclipse until UI is shown
noticeable.

Fix #1481

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/RequiredPluginsInitializer.java

@@ -13,20 +13,32 @@
  *******************************************************************************/
 package org.eclipse.pde.internal.core;
 
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicBoolean;
+
 import org.eclipse.core.resources.IProject;
 import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.ILog;
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.IStatus;
 import org.eclipse.core.runtime.Status;
+import org.eclipse.core.runtime.jobs.IJobChangeEvent;
 import org.eclipse.core.runtime.jobs.Job;
+import org.eclipse.core.runtime.jobs.JobChangeAdapter;
 import org.eclipse.jdt.core.ClasspathContainerInitializer;
 import org.eclipse.jdt.core.IClasspathContainer;
 import org.eclipse.jdt.core.IJavaProject;
 import org.eclipse.jdt.core.JavaCore;
+import org.eclipse.jdt.core.JavaModelException;
 import org.eclipse.pde.core.plugin.IPluginModelBase;
 
 public class RequiredPluginsInitializer extends ClasspathContainerInitializer {
 
+	private static final AtomicBoolean WARNING_LOGGED = new AtomicBoolean();
+
+	private static final Map<IJavaProject, Job> JOB_MAP = new ConcurrentHashMap<>();
+
 	private static final Job initPDEJob = Job.create(PDECoreMessages.PluginModelManager_InitializingPluginModels,
 			monitor -> {
 				if (!PDECore.getDefault().getModelManager().isInitialized()) {
@@ -36,6 +48,48 @@ public class RequiredPluginsInitializer extends ClasspathContainerInitializer {
 
 	@Override
 	public void initialize(IPath containerPath, IJavaProject javaProject) throws CoreException {
+		if ("main".equals(Thread.currentThread().getName())) { //$NON-NLS-1$
+			// See https://github.com/eclipse-pde/eclipse.pde/issues/1481
+			if (WARNING_LOGGED.compareAndSet(false, true)) {
+				ILog.get().warn(
+						"RequiredPluginsInitializer called from within the UI thread this will badly impact your IDE performance!", //$NON-NLS-1$
+						new RuntimeException("Called from main thread here")); //$NON-NLS-1$
+			}
+			JOB_MAP.compute(javaProject, (jp, oldjob) -> {
+				if (oldjob != null) {
+					oldjob.cancel();
+				}
+				Job job = Job.create(PDECoreMessages.PluginModelManager_InitializingPluginModels, m -> {
+					if (oldjob != null) {
+						try {
+							oldjob.join();
+						} catch (InterruptedException e) {
+						}
+					}
+					setClasspath(jp);
+				});
+				job.addJobChangeListener(new JobChangeAdapter() {
+					@Override
+					public void done(IJobChangeEvent event) {
+						JOB_MAP.remove(jp);
+					}
+				});
+				job.schedule();
+				return job;
+			});
+			return;
+		}
+		Job job = JOB_MAP.get(javaProject);
+		if (job != null) {
+			try {
+				job.join();
+			} catch (InterruptedException e) {
+			}
+		}
+		setClasspath(javaProject);
+	}
+
+	protected void setClasspath(IJavaProject javaProject) throws JavaModelException {
 		IProject project = javaProject.getProject();
 		// The first project to be built may initialize the PDE models, potentially long running, so allow cancellation
 		PluginModelManager manager = PDECore.getDefault().getModelManager();