Beefed up the test case; rewrote the C a little.

branches/bug1734/src/BTrees/MergeTemplate.c

@@ -93,6 +93,13 @@ bucket_merge(Bucket *s1, Bucket *s2, Bucket *s3)
   SetIteration i1 = {0,0,0}, i2 = {0,0,0}, i3 = {0,0,0};
   int cmp12, cmp13, cmp23, mapping, set;
 
+  /* If either "after" bucket is empty, punt. */
+  if (s2->len == 0 || s3->len == 0)
+    {
+      merge_error(-1, -1, -1, 12);
+      goto err;
+    }
+
   if (initSetIteration(&i1, OBJECT(s1), 1) < 0)
       goto err;
   if (initSetIteration(&i2, OBJECT(s2), 1) < 0)
@@ -117,13 +124,6 @@ bucket_merge(Bucket *s1, Bucket *s2, Bucket *s3)
   if (i3.next(&i3) < 0)
       goto err;
 
-  /* If either "after" bucket was empty, punt. */
-  if (i2.position < 0 || i3.position < 0)
-    {
-      merge_error(i1.position, i2.position, i3.position, 12);
-      goto err;
-    }
-
   /* Consult zodb/btrees/interfaces.py for the meaning of the last
    * argument passed to merge_error().
    */

branches/bug1734/src/BTrees/tests/testConflict.py

@@ -812,6 +812,37 @@ class NastyConfict(Base, TestCase):
         else:
             self.fail("expected ConflictError")
 
+        # Same thing, except commit the transactions in the opposite order.
+        b = OOBTree()
+        for i in range(0, 200, 4):
+            b[i] = i
+
+        r1 = self.db.open().root()
+        r1["t"] = b
+        transaction.commit()
+
+        r2 = self.db.open(synch=False).root()
+        copy = r2["t"]
+        # Make sure all of copy is loaded.
+        list(copy.values())
+
+        self.assertEqual(b._p_serial, copy._p_serial)
+
+        # Now one transaction empties the first bucket, and another adds a
+        # key to the first bucket.
+        b[1] = 1
+        transaction.commit()
+
+        for k in range(0, 60, 4):
+            del copy[k]
+        try:
+            transaction.commit()
+        except ConflictError, detail:
+            self.assert_(str(detail).startswith('database conflict error'))
+            transaction.abort()
+        else:
+            self.fail("expected ConflictError")
+
 
 def test_suite():
     suite = TestSuite()