docs/attachments/LUCENE-3445/LUCENE-3445.patch

Index: lucene/contrib/misc/src/java/org/apache/lucene/search/SearcherManager.java
===================================================================
--- lucene/contrib/misc/src/java/org/apache/lucene/search/SearcherManager.java	(revision 1176585)
+++ lucene/contrib/misc/src/java/org/apache/lucene/search/SearcherManager.java	(working copy)
@@ -20,7 +20,7 @@
 import java.io.Closeable;
 import java.io.IOException;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.Semaphore;
 
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.IndexWriter;
@@ -67,7 +67,7 @@
   // Current searcher
   private volatile IndexSearcher currentSearcher;
   private final SearcherWarmer warmer;
-  private final AtomicBoolean reopening = new AtomicBoolean();
+  private final Semaphore reopening = new Semaphore(1);
   private final ExecutorService es;
 
   /** Opens an initial searcher from the Directory.
@@ -136,7 +136,7 @@
 
     // Ensure only 1 thread does reopen at once; other
     // threads just return immediately:
-    if (!reopening.getAndSet(true)) {
+    if (reopening.tryAcquire()) {
       try {
         IndexReader newReader = currentSearcher.getIndexReader().reopen();
         if (newReader != currentSearcher.getIndexReader()) {
@@ -158,7 +158,7 @@
           return false;
         }
       } finally {
-        reopening.set(false);
+        reopening.release();
       }
     } else {
       return false;
@@ -168,12 +168,14 @@
   /** Obtain the current IndexSearcher.  You must match
    *  every call to get with one call to {@link #release};
    *  it's best to do so in a finally clause. */
-  public synchronized IndexSearcher get() {
-    if (currentSearcher == null) {
-      throw new AlreadyClosedException("this SearcherManager is closed");
-    }
-    currentSearcher.getIndexReader().incRef();
-    return currentSearcher;
+  public IndexSearcher acquire() {
+    IndexSearcher searcher;
+    do {
+      if ((searcher = currentSearcher) == null) {
+        throw new AlreadyClosedException("this SearcherManager is closed");
+      }
+    } while (!searcher.getIndexReader().tryIncRef());
+    return searcher;
   }    
 
   /** Release the searcher previously obtained with {@link
@@ -186,7 +188,7 @@
     searcher.getIndexReader().decRef();
   }
 
-  // Replaces old searcher with new one
+  // Replaces old searcher with new one - needs to be synced to make close() work
   private synchronized void swapSearcher(IndexSearcher newSearcher)
     throws IOException {
     IndexSearcher oldSearcher = currentSearcher;
Index: lucene/contrib/misc/src/test/org/apache/lucene/search/TestSearcherManager.java
===================================================================
--- lucene/contrib/misc/src/test/org/apache/lucene/search/TestSearcherManager.java	(revision 1176585)
+++ lucene/contrib/misc/src/test/org/apache/lucene/search/TestSearcherManager.java	(working copy)
@@ -36,7 +36,7 @@
   protected IndexSearcher getFinalSearcher() throws Exception  {
     writer.commit();
     mgr.maybeReopen();
-    return mgr.get();
+    return mgr.acquire();
   }
 
   private SearcherManager mgr;
@@ -94,7 +94,7 @@
       mgr.maybeReopen();
     }
 
-    return mgr.get();
+    return mgr.acquire();
   }
 
   @Override
Index: lucene/src/java/org/apache/lucene/index/IndexReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/IndexReader.java	(revision 1176585)
+++ lucene/src/java/org/apache/lucene/index/IndexReader.java	(working copy)
@@ -200,11 +200,45 @@
    * references.
    *
    * @see #decRef
+   * @see #tryIncRef
    */
   public void incRef() {
     ensureOpen();
     refCount.incrementAndGet();
   }
+  
+  /**
+   * Expert: increments the refCount of this IndexReader
+   * instance only if the IndexReader has not been closed yet
+   * and returns <code>true</code> iff the refCount was
+   * successfully incremented, otherwise <code>false</code>.
+   * If this method returns <code>false</code> the reader is either
+   * already closed or is currently been closed. Either way this
+   * reader instance shouldn't be used by an application unless
+   * <code>true</code> is returned.
+   * <p>
+   * RefCounts are used to determine when a
+   * reader can be closed safely, i.e. as soon as there are
+   * no more references.  Be sure to always call a
+   * corresponding {@link #decRef}, in a finally clause;
+   * otherwise the reader may never be closed.  Note that
+   * {@link #close} simply calls decRef(), which means that
+   * the IndexReader will not really be closed until {@link
+   * #decRef} has been called for all outstanding
+   * references.
+   *
+   * @see #decRef
+   * @see #incRef
+   */
+  public boolean tryIncRef() {
+    int count;
+    while ((count = refCount.get()) > 0) {
+      if(refCount.compareAndSet(count, count+1)) {
+        return true;
+      }
+    }
+    return false;
+  }
 
   /** {@inheritDoc} */
   @Override
Index: lucene/src/test/org/apache/lucene/index/TestIndexReader.java
===================================================================
--- lucene/src/test/org/apache/lucene/index/TestIndexReader.java	(revision 1176585)
+++ lucene/src/test/org/apache/lucene/index/TestIndexReader.java	(working copy)
@@ -27,6 +27,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.HashMap;
+import java.util.Random;
 import java.util.Set;
 import java.util.SortedSet;
 import org.junit.Assume;
@@ -1403,4 +1404,71 @@
     r.close();
     dir.close();
   }
+  
+  public void testTryIncRef() throws CorruptIndexException, LockObtainFailedException, IOException {
+    Directory dir = newDirectory();
+    IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random)));
+    writer.addDocument(new Document());
+    writer.commit();
+    IndexReader r = IndexReader.open(dir);
+    assertTrue(r.tryIncRef());
+    r.decRef();
+    r.close();
+    assertFalse(r.tryIncRef());
+    writer.close();
+    dir.close();
+  }
+  
+  public void testStressTryIncRef() throws CorruptIndexException, LockObtainFailedException, IOException, InterruptedException {
+    Directory dir = newDirectory();
+    IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random)));
+    writer.addDocument(new Document());
+    writer.commit();
+    IndexReader r = IndexReader.open(dir);
+    int numThreads = atLeast(2);
+    
+    IncThread[] threads = new IncThread[numThreads];
+    System.out.println(numThreads);
+    for (int i = 0; i < threads.length; i++) {
+      threads[i] = new IncThread(r, random);
+      threads[i].start();
+    }
+    Thread.sleep(100);
+
+    assertTrue(r.tryIncRef());
+    r.decRef();
+    r.close();
+
+    for (int i = 0; i < threads.length; i++) {
+      threads[i].join();
+      assertNull(threads[i].failed);
+    }
+    assertFalse(r.tryIncRef());
+    writer.close();
+    dir.close();
+  }
+  
+  static class IncThread extends Thread {
+    final IndexReader toInc;
+    final Random random;
+    Throwable failed;
+    
+    IncThread(IndexReader toInc, Random random) {
+      this.toInc = toInc;
+      this.random = random;
+    }
+    
+    @Override
+    public void run() {
+      try {
+        while (toInc.tryIncRef()) {
+          assertFalse(toInc.hasDeletions());
+          toInc.decRef();
+        }
+        assertFalse(toInc.tryIncRef());
+      } catch (Throwable e) {
+        failed = e;
+      }
+    }
+  }
 }