Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 552781)
+++ CHANGES.txt	(working copy)
@@ -1,4 +1,4 @@
-Lucene Change Log
+Lucene Change Log
 
 $Id$
 
@@ -167,6 +167,10 @@
 23. LUCENE-913: Two consecutive score() calls return different 
     scores for Boolean Queries. (Michael Busch, Doron Cohen)
 
+24. LUCENE-948: Fix FNFE exception caused by stale NFS client
+    directory listing caches when writers on different machines are
+    sharing an index over NFS.  (Mike McCandless)
+
 New features
 
  1. LUCENE-759: Added two n-gram-producing TokenFilters.
Index: src/java/org/apache/lucene/index/IndexWriter.java
===================================================================
--- src/java/org/apache/lucene/index/IndexWriter.java	(revision 552781)
+++ src/java/org/apache/lucene/index/IndexWriter.java	(working copy)
@@ -875,6 +875,8 @@
 
       if (commitPending) {
         segmentInfos.write(directory);         // now commit changes
+        if (infoStream != null)
+          infoStream.println("close: wrote segments file \"" + segmentInfos.getCurrentSegmentFileName() + "\"");
         deleter.checkpoint(segmentInfos, true);
         commitPending = false;
         rollbackSegmentInfos = null;
@@ -1362,6 +1364,8 @@
   private void checkpoint() throws IOException {
     if (autoCommit) {
       segmentInfos.write(directory);
+      if (infoStream != null)
+        infoStream.println("checkpoint: wrote segments file \"" + segmentInfos.getCurrentSegmentFileName() + "\"");
     } else {
       commitPending = true;
     }
Index: src/java/org/apache/lucene/index/IndexFileDeleter.java
===================================================================
--- src/java/org/apache/lucene/index/IndexFileDeleter.java	(revision 552781)
+++ src/java/org/apache/lucene/index/IndexFileDeleter.java	(working copy)
@@ -23,6 +23,7 @@
 import org.apache.lucene.store.Directory;
 
 import java.io.IOException;
+import java.io.FileNotFoundException;
 import java.io.PrintStream;
 import java.util.Map;
 import java.util.HashMap;
@@ -120,6 +121,9 @@
     throws CorruptIndexException, IOException {
 
     this.infoStream = infoStream;
+    if (infoStream != null)
+      message("init: current segments file is \"" + segmentInfos.getCurrentSegmentFileName() + "\"");
+
     this.policy = policy;
     this.directory = directory;
 
@@ -153,13 +157,29 @@
               message("init: load commit \"" + fileName + "\"");
             }
             SegmentInfos sis = new SegmentInfos();
-            sis.read(directory, fileName);
-            CommitPoint commitPoint = new CommitPoint(sis);
-            if (sis.getGeneration() == segmentInfos.getGeneration()) {
-              currentCommitPoint = commitPoint;
+            try {
+              sis.read(directory, fileName);
+            } catch (FileNotFoundException e) {
+              // LUCENE-948: on NFS (and maybe others), if
+              // you have writers switching back and forth
+              // between machines, it's very likely that the
+              // dir listing will be stale and will claim a
+              // file segments_X exists when in fact it
+              // doesn't.  So, we catch this and handle it
+              // as if the file does not exist
+              if (infoStream != null) {
+                message("init: hit FileNotFoundException when loading commit \"" + fileName + "\"; skipping this commit point");
+              }
+              sis = null;
             }
-            commits.add(commitPoint);
-            incRef(sis, true);
+            if (sis != null) {
+              CommitPoint commitPoint = new CommitPoint(sis);
+              if (sis.getGeneration() == segmentInfos.getGeneration()) {
+                currentCommitPoint = commitPoint;
+              }
+              commits.add(commitPoint);
+              incRef(sis, true);
+            }
           }
         }
       }