introduce ManagingFS.getCheapFileSystemStructureModificationCount that increments only on structural VFS changes to save VirtualFilePointer-s from being recalculated on every workspace.xml change

platform/platform-api/src/com/intellij/openapi/vfs/newvfs/ManagingFS.java

@@ -43,9 +43,25 @@ public abstract class ManagingFS implements FileSystemInterface {
 
   public abstract int getModificationCount(@NotNull VirtualFile fileOrDirectory);
 
-  // Only counts modifications done in current IDEA session
+  /**
+   * @return a number that's incremented every time something changes in the VFS, i.e. file hierarchy, names, flags, attributes, contents. 
+   * This only counts modifications done in current IDE session.
+   * @see #getCheapFileSystemStructureModificationCount()
+   * @see #getFilesystemModificationCount() 
+   */
   public abstract int getCheapFileSystemModificationCount();
 
+  /**
+   * @return a number that's incremented every time something changes in the VFS structure, i.e. file hierarchy or names. 
+   * This only counts modifications done in current IDE session.
+   * @see #getCheapFileSystemModificationCount() 
+   */
+  public abstract int getCheapFileSystemStructureModificationCount();
+
+  /**
+   * @return a number that's incremented every time something changes in the VFS, i.e. file hierarchy, names, flags, attributes, contents.
+   * This number is persisted between IDE sessions and so it'll always increase. This method invocation means disk access, so it's not terribly cheap. 
+   */
   public abstract int getFilesystemModificationCount();
 
   public abstract long getCreationTimestamp();

platform/platform-impl/src/com/intellij/openapi/vfs/impl/FilePointerPartNode.java

@@ -22,6 +22,7 @@ import com.intellij.openapi.util.SystemInfo;
 import com.intellij.openapi.util.text.StringUtil;
 import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.openapi.vfs.VirtualFileManager;
+import com.intellij.openapi.vfs.newvfs.ManagingFS;
 import com.intellij.util.ArrayUtil;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
@@ -43,6 +44,7 @@ class FilePointerPartNode {
 
   private int pointersUnder = 1;   // number of alive pointers in this node plus all nodes beneath
   private static final VirtualFileManager ourFileManager = VirtualFileManager.getInstance();
+  private static final ManagingFS ourManagingFS = ManagingFS.getInstance();
 
   FilePointerPartNode(@NotNull String part, FilePointerPartNode parent, Pair<VirtualFile,String> fileAndUrl) {
     this.part = part;
@@ -242,7 +244,7 @@ class FilePointerPartNode {
   Pair<VirtualFile, String> update() {
     long lastUpdated = myLastUpdated;
     Pair<VirtualFile, String> fileAndUrl = myFileAndUrl;
-    long fsModCount = ourFileManager.getModificationCount();
+    long fsModCount = ourManagingFS.getCheapFileSystemStructureModificationCount();
     if (lastUpdated == fsModCount) return fileAndUrl;
     VirtualFile file = fileAndUrl.first;
     String url = fileAndUrl.second;

platform/platform-impl/src/com/intellij/openapi/vfs/newvfs/impl/VirtualDirectoryImpl.java

@@ -31,6 +31,7 @@ import com.intellij.openapi.vfs.newvfs.RefreshQueue;
 import com.intellij.openapi.vfs.newvfs.events.VFileCreateEvent;
 import com.intellij.openapi.vfs.newvfs.persistent.FSRecords;
 import com.intellij.openapi.vfs.newvfs.persistent.PersistentFS;
+import com.intellij.openapi.vfs.newvfs.persistent.PersistentFSImpl;
 import com.intellij.util.ArrayUtil;
 import com.intellij.util.Function;
 import com.intellij.util.UriUtil;
@@ -434,6 +435,7 @@ public class VirtualDirectoryImpl extends VirtualFileSystemEntry {
     assert appended[i] > 0 : file;
     System.arraycopy(array, i, appended, i + 1, array.length - i);
     myData.myChildrenIds = appended;
+    ((PersistentFSImpl)PersistentFS.getInstance()).incStructuralModificationCount();
   }
 
   public void removeChild(@NotNull VirtualFile file) {
@@ -447,6 +449,7 @@ public class VirtualDirectoryImpl extends VirtualFileSystemEntry {
 
   private void removeFromArray(int index) {
     myData.myChildrenIds = ArrayUtil.remove(myData.myChildrenIds, index);
+    ((PersistentFSImpl)PersistentFS.getInstance()).incStructuralModificationCount();
   }
 
   public boolean allChildrenLoaded() {

platform/platform-impl/src/com/intellij/openapi/vfs/newvfs/impl/VirtualFileSystemEntry.java

@@ -29,6 +29,7 @@ import com.intellij.openapi.vfs.encoding.EncodingManager;
 import com.intellij.openapi.vfs.encoding.EncodingRegistry;
 import com.intellij.openapi.vfs.newvfs.NewVirtualFile;
 import com.intellij.openapi.vfs.newvfs.persistent.PersistentFS;
+import com.intellij.openapi.vfs.newvfs.persistent.PersistentFSImpl;
 import com.intellij.psi.SingleRootFileViewProvider;
 import com.intellij.util.LocalTimeCounter;
 import com.intellij.util.text.CharArrayUtil;
@@ -325,6 +326,7 @@ public abstract class VirtualFileSystemEntry extends NewVirtualFile {
     VirtualDirectoryImpl parent = getParent();
     parent.removeChild(this);
     mySegment.setNameId(myId, FileNameCache.storeName(newName));
+    ((PersistentFSImpl)PersistentFS.getInstance()).incStructuralModificationCount();
     parent.addChild(this);
   }
 

platform/platform-impl/src/com/intellij/openapi/vfs/newvfs/persistent/PersistentFSImpl.java

@@ -74,6 +74,7 @@ public class PersistentFSImpl extends PersistentFS implements ApplicationCompone
       clearIdCache();
     }
   });
+  private volatile int myStructureModificationCount;
 
   public PersistentFSImpl(@NotNull MessageBus bus) {
     myEventBus = bus;
@@ -266,6 +267,15 @@ public class PersistentFSImpl extends PersistentFS implements ApplicationCompone
     return FSRecords.getLocalModCount();
   }
 
+  @Override
+  public int getCheapFileSystemStructureModificationCount() {
+    return myStructureModificationCount;
+  }
+
+  public void incStructuralModificationCount() {
+    myStructureModificationCount++;
+  }
+
   @Override
   public int getFilesystemModificationCount() {
     return FSRecords.getModCount();
@@ -910,6 +920,7 @@ public class PersistentFSImpl extends PersistentFS implements ApplicationCompone
       if (root != null) return root;
 
       VfsData.initFile(rootId, segment, -1, directoryData);
+      incStructuralModificationCount();
       mark = writeAttributesToRecord(rootId, 0, newRoot, fs, attributes);
 
       myRoots.put(rootUrl, newRoot);
@@ -1172,6 +1183,7 @@ public class PersistentFSImpl extends PersistentFS implements ApplicationCompone
     FSRecords.deleteRecordRecursively(id);
 
     invalidateSubtree(file);
+    incStructuralModificationCount();
   }
 
   private static void invalidateSubtree(@NotNull VirtualFile file) {