Tentative fix.

branches/bug1734/src/BTrees/MergeTemplate.c

@@ -70,7 +70,9 @@ merge_error(int p1, int p2, int p3, int reason)
  * However, it's not OK for s2 and s3 to, between them, end up deleting all
  * the keys.  This is a higher-level constraint, due to that the caller of
  * bucket_merge() doesn't have enough info to unlink the resulting empty
- * bucket from its BTree correctly.
+ * bucket from its BTree correctly.  It's also not OK if s2 or s3 are empty,
+ * because the transaction that emptied the bucket unlinked the bucket from
+ * the tree, and nothing we do here can get it linked back in again.
  *
  * Key insertion:  s2 or s3 can add a new key, provided the other transaction
  * doesn't insert the same key.  It's not OK even if they insert the same
@@ -115,6 +117,13 @@ 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

@@ -178,7 +178,7 @@ class MappingBase(Base):
 
         test_merge(base, b1, b2, bm, 'merge insert from empty')
 
-    def testMergeEmptyAndFill(self):
+    def testFailMergeEmptyAndFill(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
 
         b1.clear()
@@ -186,7 +186,7 @@ class MappingBase(Base):
         b2.update(e2)
         bm.update(e2)
 
-        test_merge(base, b1, b2, bm, 'merge insert from empty')
+        test_merge(base, b1, b2, bm, 'merge insert from empty', should_fail=1)
 
     def testMergeEmpty(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
@@ -275,7 +275,7 @@ class SetTests(Base):
 
         test_merge(base, b1, b2, bm, 'merge insert from empty')
 
-    def testMergeEmptyAndFill(self):
+    def testFailMergeEmptyAndFill(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
 
         b1.clear()
@@ -283,7 +283,7 @@ class SetTests(Base):
         b2.update(e2)
         bm.update(e2)
 
-        test_merge(base, b1, b2, bm, 'merge insert from empty')
+        test_merge(base, b1, b2, bm, 'merge insert from empty', should_fail=1)
 
     def testMergeEmpty(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
@@ -810,7 +810,6 @@ class NastyConfict(Base, TestCase):
             self.assert_(str(detail).startswith('database conflict error'))
             transaction.abort()
         else:
-            print list(copy.items())
             self.fail("expected ConflictError")
 
 

branches/bug1734/src/ZODB/POSException.py

@@ -187,6 +187,9 @@ class BTreesConflictError(ConflictError):
 
             # 11; conflicting changes in an internal BTree node
             'Conflicting changes in an internal BTree node',
+
+            # 12; i2 or i3 was empty
+            'Empty bucket in a transaction',
             ]
 
     def __init__(self, p1, p2, p3, reason):