Index: src/java/org/apache/lucene/index/IndexWriter.java
===================================================================
--- src/java/org/apache/lucene/index/IndexWriter.java	(revision 633410)
+++ src/java/org/apache/lucene/index/IndexWriter.java	(working copy)
@@ -2632,6 +2632,10 @@
    * each input Directory, so it is up to the caller to
    * enforce this.
    *
+   * <p><b>NOTE:</b> while this is running, any attempts to
+   * add or delete documents (with another thread) will be
+   * paused until this method completes.
+   *
    * <p>After this completes, the index is optimized.
    *
    * <p>This method is transactional in how Exceptions are
@@ -2670,11 +2674,16 @@
    * @throws CorruptIndexException if the index is corrupt
    * @throws IOException if there is a low-level IO error
    */
-  public synchronized void addIndexes(Directory[] dirs)
+  public void addIndexes(Directory[] dirs)
     throws CorruptIndexException, IOException {
 
     ensureOpen();
+
+    // Do not allow add docs or deletes while we are running:
+    docWriter.pauseAllThreads();
+
     try {
+
       if (infoStream != null)
         message("flush at addIndexes");
       flush(true, false);
@@ -2684,11 +2693,13 @@
       startTransaction();
 
       try {
-        for (int i = 0; i < dirs.length; i++) {
-          SegmentInfos sis = new SegmentInfos();	  // read infos from dir
-          sis.read(dirs[i]);
-          for (int j = 0; j < sis.size(); j++) {
-            segmentInfos.addElement(sis.info(j));	  // add each info
+        synchronized(this) {
+          for (int i = 0; i < dirs.length; i++) {
+            SegmentInfos sis = new SegmentInfos();	  // read infos from dir
+            sis.read(dirs[i]);
+            for (int j = 0; j < sis.size(); j++) {
+              segmentInfos.addElement(sis.info(j));	  // add each info
+            }
           }
         }
 
@@ -2705,6 +2716,8 @@
     } catch (OutOfMemoryError oom) {
       hitOOM = true;
       throw oom;
+    } finally {
+      docWriter.resumeAllThreads();
     }
   }
 
@@ -2726,6 +2739,10 @@
    * each input Directory, so it is up to the caller to
    * enforce this.
    *
+   * <p><b>NOTE:</b> while this is running, any attempts to
+   * add or delete documents (with another thread) will be
+   * paused until this method completes.
+   *
    * <p>
    * This requires this index not be among those to be added, and the
    * upper bound* of those segment doc counts not exceed maxMergeDocs.
@@ -2737,11 +2754,14 @@
    * @throws CorruptIndexException if the index is corrupt
    * @throws IOException if there is a low-level IO error
    */
-  public synchronized void addIndexesNoOptimize(Directory[] dirs)
+  public void addIndexesNoOptimize(Directory[] dirs)
       throws CorruptIndexException, IOException {
 
     ensureOpen();
 
+    // Do not allow add docs or deletes while we are running:
+    docWriter.pauseAllThreads();
+
     try {
       if (infoStream != null)
         message("flush at addIndexesNoOptimize");
@@ -2753,17 +2773,19 @@
 
       try {
 
-        for (int i = 0; i < dirs.length; i++) {
-          if (directory == dirs[i]) {
-            // cannot add this index: segments may be deleted in merge before added
-            throw new IllegalArgumentException("Cannot add this index to itself");
-          }
+        synchronized(this) {
+          for (int i = 0; i < dirs.length; i++) {
+            if (directory == dirs[i]) {
+              // cannot add this index: segments may be deleted in merge before added
+              throw new IllegalArgumentException("Cannot add this index to itself");
+            }
 
-          SegmentInfos sis = new SegmentInfos(); // read infos from dir
-          sis.read(dirs[i]);
-          for (int j = 0; j < sis.size(); j++) {
-            SegmentInfo info = sis.info(j);
-            segmentInfos.addElement(info); // add each info
+            SegmentInfos sis = new SegmentInfos(); // read infos from dir
+            sis.read(dirs[i]);
+            for (int j = 0; j < sis.size(); j++) {
+              SegmentInfo info = sis.info(j);
+              segmentInfos.addElement(info); // add each info
+            }
           }
         }
 
@@ -2788,18 +2810,30 @@
     } catch (OutOfMemoryError oom) {
       hitOOM = true;
       throw oom;
+    } finally {
+      docWriter.resumeAllThreads();
     }
   }
 
   /* If any of our segments are using a directory != ours
    * then copy them over.  Currently this is only used by
    * addIndexesNoOptimize(). */
-  private synchronized void copyExternalSegments() throws CorruptIndexException, IOException {
-    final int numSegments = segmentInfos.size();
-    for(int i=0;i<numSegments;i++) {
-      SegmentInfo info = segmentInfos.info(i);
-      if (info.dir != directory) {
-        MergePolicy.OneMerge merge = new MergePolicy.OneMerge(segmentInfos.range(i, 1+i), info.getUseCompoundFile());
+  private void copyExternalSegments() throws CorruptIndexException, IOException {
+    while(true) {
+      SegmentInfo info = null;
+      MergePolicy.OneMerge merge = null;
+      synchronized(this) {
+        final int numSegments = segmentInfos.size();
+        for(int i=0;i<numSegments;i++) {
+          info = segmentInfos.info(i);
+          if (info.dir != directory) {
+            merge = new MergePolicy.OneMerge(segmentInfos.range(i, 1+i), info.getUseCompoundFile());
+            break;
+          }
+        }
+      }
+
+      if (merge != null) {
         if (registerMerge(merge)) {
           pendingMerges.remove(merge);
           runningMerges.add(merge);
@@ -2815,7 +2849,9 @@
           // that an IndexReader would fail to load).
           throw new MergePolicy.MergeException("segment \"" + info.name + " exists in external directory yet the MergeScheduler executed the merge in a separate thread",
                                                directory);
-      }
+      } else
+        // No more external segments
+        break;
     }
   }
 
@@ -2823,6 +2859,16 @@
    * <p>After this completes, the index is optimized. </p>
    * <p>The provided IndexReaders are not closed.</p>
 
+   * <p><b>NOTE:</b> the index in each Directory must not be
+   * changed (opened by a writer) while this method is
+   * running.  This method does not acquire a write lock in
+   * each input Directory, so it is up to the caller to
+   * enforce this.
+   *
+   * <p><b>NOTE:</b> while this is running, any attempts to
+   * add or delete documents (with another thread) will be
+   * paused until this method completes.
+   *
    * <p>See {@link #addIndexes(Directory[])} for
    * details on transactional semantics, temporary free
    * space required in the Directory, and non-CFS segments
@@ -2830,10 +2876,14 @@
    * @throws CorruptIndexException if the index is corrupt
    * @throws IOException if there is a low-level IO error
    */
-  public synchronized void addIndexes(IndexReader[] readers)
+  public void addIndexes(IndexReader[] readers)
     throws CorruptIndexException, IOException {
 
     ensureOpen();
+
+    // Do not allow add docs or deletes while we are running:
+    docWriter.pauseAllThreads();
+
     try {
       optimize();					  // start with zero or 1 seg
 
@@ -2844,9 +2894,11 @@
 
       IndexReader sReader = null;
       try {
-        if (segmentInfos.size() == 1){ // add existing index, if any
-          sReader = SegmentReader.get(segmentInfos.info(0));
-          merger.add(sReader);
+        synchronized(this) {
+          if (segmentInfos.size() == 1){ // add existing index, if any
+            sReader = SegmentReader.get(segmentInfos.info(0));
+            merger.add(sReader);
+          }
         }
 
         for (int i = 0; i < readers.length; i++)      // add new indexes
@@ -2864,10 +2916,12 @@
             sReader = null;
           }
 
-          segmentInfos.setSize(0);                      // pop old infos & add new
-          info = new SegmentInfo(mergedName, docCount, directory, false, true,
-                                 -1, null, false);
-          segmentInfos.addElement(info);
+          synchronized(this) {
+            segmentInfos.setSize(0);                      // pop old infos & add new
+            info = new SegmentInfo(mergedName, docCount, directory, false, true,
+                                   -1, null, false);
+            segmentInfos.addElement(info);
+          }
 
           success = true;
 
@@ -2895,7 +2949,9 @@
 
         try {
           merger.createCompoundFile(mergedName + ".cfs");
-          info.setUseCompoundFile(true);
+          synchronized(this) {
+            info.setUseCompoundFile(true);
+          }
         } finally {
           if (!success) {
             if (infoStream != null)
@@ -2910,6 +2966,8 @@
     } catch (OutOfMemoryError oom) {
       hitOOM = true;
       throw oom;
+    } finally {
+      docWriter.resumeAllThreads();
     }
   }