don't drop all caches on second completion invocation in the same place after the copy has already been reused

java/java-tests/testSrc/com/intellij/codeInsight/completion/HeavyNormalCompletionTest.groovy

@@ -20,6 +20,7 @@ import com.intellij.openapi.vfs.LocalFileSystem
 import com.intellij.openapi.vfs.VirtualFile
 import com.intellij.psi.JavaPsiFacade
 import com.intellij.psi.PsiClass
+import com.intellij.psi.PsiDocumentManager
 import com.intellij.psi.PsiManager
 import com.intellij.psi.search.GlobalSearchScope
 import com.intellij.psi.util.PsiTreeUtil
@@ -150,4 +151,22 @@ public class Test {
     assert oldCount == PsiManager.getInstance(project).modificationTracker.javaStructureModificationCount
   }
 
+  public void testNoJavaStructureModificationOnSecondInvocationAfterTyping() {
+    myFixture.configureByText 'a.java', 'class Foo { Xxxxx<caret> }'
+
+    def tracker = PsiManager.getInstance(project).modificationTracker
+    def oldCount = tracker.javaStructureModificationCount
+    assert !myFixture.completeBasic()
+    assert oldCount == tracker.javaStructureModificationCount
+
+    myFixture.type 'x'
+    PsiDocumentManager.getInstance(project).commitAllDocuments()
+    assert oldCount != tracker.javaStructureModificationCount
+    oldCount = tracker.javaStructureModificationCount
+    
+    assert !myFixture.completeBasic()
+    assert !myFixture.completeBasic()
+    assert oldCount == tracker.javaStructureModificationCount
+  }
+
 }

platform/lang-impl/src/com/intellij/codeInsight/completion/CodeCompletionHandlerBase.java

@@ -780,6 +780,7 @@ public class CodeCompletionHandlerBase {
         }
         final Document document = cached.second;
         assert document != null;
+        file.putUserData(FILE_COPY_KEY, new SoftReference<Trinity<PsiFile,Document, Long>>(Trinity.create(copy, document, combinedOffsets)));
         document.setText(file.getText());
         return copy;
       }