diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 147a1c0c881..7705b6dac51 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -15,6 +15,11 @@ New features
   create automata from a fixed collection of UTF-8 encoded BytesRef
   (Dawid Weiss, Robert Muir)
 
+Changes in Runtime Behavior
+
+* LUCENE-4190: Don't delete files in the index directory unless they
+  begin with an underscore. (Mike McCandless, Robert Muir)
+
 API Changes
 
 * LUCENE-4138: update of morfologik (Polish morphological analyzer) to 1.5.3.
diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexFileDeleter.java b/lucene/core/src/java/org/apache/lucene/index/IndexFileDeleter.java
index 4447472779c..1d2bdef0d66 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexFileDeleter.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexFileDeleter.java
@@ -149,7 +149,8 @@ final class IndexFileDeleter {
 
     for (String fileName : files) {
 
-      if (!fileName.endsWith("write.lock") && !fileName.equals(IndexFileNames.SEGMENTS_GEN)) {
+      if (!fileName.endsWith("write.lock") && !fileName.equals(IndexFileNames.SEGMENTS_GEN)
+          && (fileName.startsWith("_") || fileName.startsWith(IndexFileNames.SEGMENTS))) {
 
         // Add this file to refCounts with initial count 0:
         getRefCount(fileName);
diff --git a/lucene/core/src/java/org/apache/lucene/index/SegmentInfo.java b/lucene/core/src/java/org/apache/lucene/index/SegmentInfo.java
index e6f2b0ebdc1..4b20d3f0015 100644
--- a/lucene/core/src/java/org/apache/lucene/index/SegmentInfo.java
+++ b/lucene/core/src/java/org/apache/lucene/index/SegmentInfo.java
@@ -244,14 +244,27 @@ public final class SegmentInfo {
   public void setFiles(Set<String> files) {
     setFiles = files;
     sizeInBytes = -1;
+    assert assertSaneFileNames();
   }
 
   public void addFiles(Collection<String> files) {
     setFiles.addAll(files);
+    sizeInBytes = -1;
+    assert assertSaneFileNames();
   }
 
   public void addFile(String file) {
     setFiles.add(file);
+    sizeInBytes = -1;
+    assert assertSaneFileNames();
+  }
+  
+  // used only by assert
+  private boolean assertSaneFileNames() {
+    for (String file : setFiles) {
+      assert file.startsWith("_") : "invalid codec filename: " + file ;
+    }
+    return true;
   }
     
   /**
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestDoc.java b/lucene/core/src/test/org/apache/lucene/index/TestDoc.java
index b345f82b452..8a31910db59 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestDoc.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestDoc.java
@@ -125,13 +125,13 @@ public class TestDoc extends LuceneTestCase {
       printSegment(out, si2);
       writer.close();
 
-      SegmentInfoPerCommit siMerge = merge(directory, si1, si2, "merge", false);
+      SegmentInfoPerCommit siMerge = merge(directory, si1, si2, "_merge", false);
       printSegment(out, siMerge);
 
-      SegmentInfoPerCommit siMerge2 = merge(directory, si1, si2, "merge2", false);
+      SegmentInfoPerCommit siMerge2 = merge(directory, si1, si2, "_merge2", false);
       printSegment(out, siMerge2);
 
-      SegmentInfoPerCommit siMerge3 = merge(directory, siMerge, siMerge2, "merge3", false);
+      SegmentInfoPerCommit siMerge3 = merge(directory, siMerge, siMerge2, "_merge3", false);
       printSegment(out, siMerge3);
       
       directory.close();
@@ -160,13 +160,13 @@ public class TestDoc extends LuceneTestCase {
       printSegment(out, si2);
       writer.close();
 
-      siMerge = merge(directory, si1, si2, "merge", true);
+      siMerge = merge(directory, si1, si2, "_merge", true);
       printSegment(out, siMerge);
 
-      siMerge2 = merge(directory, si1, si2, "merge2", true);
+      siMerge2 = merge(directory, si1, si2, "_merge2", true);
       printSegment(out, siMerge2);
 
-      siMerge3 = merge(directory, siMerge, siMerge2, "merge3", true);
+      siMerge3 = merge(directory, siMerge, siMerge2, "_merge3", true);
       printSegment(out, siMerge3);
       
       directory.close();
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
index 195c455f182..43c03a89530 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
@@ -45,6 +45,7 @@ import org.apache.lucene.search.ScoreDoc;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.store.AlreadyClosedException;
 import org.apache.lucene.store.Directory;
+import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.store.Lock;
 import org.apache.lucene.store.LockFactory;
 import org.apache.lucene.store.LockObtainFailedException;
@@ -1799,4 +1800,23 @@ public class TestIndexWriter extends LuceneTestCase {
     r.close();
     dir.close();
   }
+  
+  //LUCENE-1468 -- make sure opening an IndexWriter with
+  // create=true does not remove non-index files
+  
+  public void testOtherFiles() throws Throwable {
+    Directory dir = newDirectory();
+    try {
+      // Create my own random file:
+      IndexOutput out = dir.createOutput("myrandomfile", newIOContext(random()));
+      out.writeByte((byte) 42);
+      out.close();
+      
+      new IndexWriter(dir, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer(random()))).close();
+      
+      assertTrue(dir.fileExists("myrandomfile"));
+    } finally {
+      dir.close();
+    }
+  }
 }