Index: lucene/src/test/org/apache/lucene/store/MockIndexOutputWrapper.java
===================================================================
--- lucene/src/test/org/apache/lucene/store/MockIndexOutputWrapper.java	(revision 1026400)
+++ lucene/src/test/org/apache/lucene/store/MockIndexOutputWrapper.java	(working copy)
@@ -131,8 +131,36 @@
 
   @Override
   public void copyBytes(DataInput input, long numBytes) throws IOException {
-    delegate.copyBytes(input, numBytes);
-    // TODO: we may need to check disk full here as well
+    // TODO: somehow factor this hairy disk-full stuff with writeBytes()
+    long freeSpace = dir.maxSize == 0 ? 0 : dir.maxSize - dir.sizeInBytes();
+    long realUsage = 0;
+
+    // If MockRAMDir crashed since we were opened, then
+    // don't write anything:
+    if (dir.crashed)
+      throw new IOException("MockRAMDirectory was crashed; cannot copy to " + name);
+
+    // Enforce disk full:
+    if (dir.maxSize != 0 && freeSpace <= numBytes) {
+      // Compute the real disk free.  This will greatly slow
+      // down our test but makes it more accurate:
+      realUsage = dir.getRecomputedActualSizeInBytes();
+      freeSpace = dir.maxSize - realUsage;
+    }
+
+    if (dir.maxSize != 0 && freeSpace <= numBytes) {
+      if (freeSpace > 0 && freeSpace < numBytes) {
+        realUsage += freeSpace;
+        delegate.copyBytes(input, (int) freeSpace);
+      }
+      if (realUsage > dir.maxUsedSize) {
+        dir.maxUsedSize = realUsage;
+      }
+      throw new IOException("fake disk full at " + dir.getRecomputedActualSizeInBytes() + " bytes when copying " + name);
+    } else {
+      delegate.copyBytes(input, numBytes);
+    }
+    
     dir.maybeThrowDeterministicException();
   }
 }