disallow modal indexing during PSI/VFS events

platform/platform-impl/src/com/intellij/openapi/project/DumbServiceImpl.java

@@ -385,6 +385,7 @@ public class DumbServiceImpl extends DumbService implements Disposable, Modifica
   }
 
   private void showModalProgress() {
+    NoAccessDuringPsiEvents.checkCallContext();
     try {
       ((ApplicationImpl)ApplicationManager.getApplication()).executeSuspendingWriteAction(myProject, IdeBundle.message("progress.indexing"), () ->
         runBackgroundProcess(ProgressManager.getInstance().getProgressIndicator()));

platform/platform-impl/src/com/intellij/openapi/project/NoAccessDuringPsiEvents.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2000-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.intellij.openapi.project;
+
+import com.intellij.openapi.application.Application;
+import com.intellij.openapi.application.ApplicationManager;
+import com.intellij.openapi.diagnostic.Logger;
+import com.intellij.openapi.vfs.VirtualFileManager;
+import com.intellij.psi.impl.DebugUtil;
+import com.intellij.psi.util.PsiModificationTracker;
+import com.intellij.util.messages.MessageBus;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * @author peter
+ */
+public class NoAccessDuringPsiEvents {
+  private static final Logger LOG = Logger.getInstance("#com.intellij.openapi.project.NoAccessDuringPsiEvents");
+  private static final Set<String> ourReportedTraces = new HashSet<>();
+
+  public static void checkCallContext() {
+    Application application = ApplicationManager.getApplication();
+    if (!application.isWriteAccessAllowed()) return;
+
+    MessageBus bus = application.getMessageBus();
+    if (bus.hasUndeliveredEvents(VirtualFileManager.VFS_CHANGES) || bus.hasUndeliveredEvents(PsiModificationTracker.TOPIC)) {
+      if (ourReportedTraces.add(DebugUtil.currentStackTrace())) {
+        LOG.error("It's prohibited to access index during event dispatching");
+      }
+    }
+  }
+}