Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 889121)
+++ CHANGES.txt	(working copy)
@@ -42,6 +42,9 @@
 
 * LUCENE-2132: Fix the demo result.jsp to use QueryParser with a 
   Version argument.  (Brian Li via Robert Muir)
+
+* LUCENE-2104: NativeFSLock.release() would silently fail if the lock is held by 
+  another thread/process. (Shai Erera via ?)
   
 New features
 
Index: src/java/org/apache/lucene/store/NativeFSLockFactory.java
===================================================================
--- src/java/org/apache/lucene/store/NativeFSLockFactory.java	(revision 889121)
+++ src/java/org/apache/lucene/store/NativeFSLockFactory.java	(working copy)
@@ -146,7 +146,7 @@
       }
     }
   }
-};
+}
 
 class NativeFSLock extends Lock {
 
@@ -300,6 +300,22 @@
       }
       if (!path.delete())
         throw new LockReleaseFailedException("failed to delete " + path);
+    } else {
+      // if we don't hold the lock, and somebody still called release(), for
+      // example as a result of calling IndexWriter.unlock(), we should attempt
+      // to obtain the lock and release it. If the obtain fails, it means the
+      // lock cannot be released, and we should throw a proper exception rather
+      // than silently failing/not doing anything.
+      boolean obtained = false;
+      try {
+        if ((obtained = obtain()) == false) {
+          throw new LockReleaseFailedException("Lock held by another process and cannot be released: " + path);
+        }
+      } finally {
+        if (obtained) {
+          release();
+        }
+      }
     }
   }
 
Index: src/test/org/apache/lucene/store/TestLockFactory.java
===================================================================
--- src/test/org/apache/lucene/store/TestLockFactory.java	(revision 889121)
+++ src/test/org/apache/lucene/store/TestLockFactory.java	(working copy)
@@ -198,6 +198,26 @@
       assertFalse(l2.isLocked());
     }
 
+    public void testNativeFSLockReleaseByOtherLock() throws IOException {
+
+      NativeFSLockFactory f = new NativeFSLockFactory(System.getProperty("tempDir"));
+
+      f.setLockPrefix("test");
+      Lock l = f.makeLock("commit");
+      Lock l2 = f.makeLock("commit");
+
+      assertTrue("failed to obtain lock", l.obtain());
+      try {
+        assertTrue(l2.isLocked());
+        l2.release();
+        fail("should not have reached here. LockReleaseFailedException should have been thrown");
+      } catch (IOException e) {
+        assertTrue("Unexpected exception", e instanceof LockReleaseFailedException);
+      } finally {
+        l.release();
+      }
+    }
+
     // Verify: NativeFSLockFactory assigns null as lockPrefix if the lockDir is inside directory
     public void testNativeFSLockFactoryPrefix() throws IOException {